1

Detuning effect in quantum dynamics of a strongly coupled single quantum dot-cavity system

International Nuclear Information System (INIS)

[en] The quantum dynamics of a strongly coupled single quantum dot-cavity system with non-zero detuning in a phonon bath is investigated theoretically in terms of a perturbation treatment based on a unitary transformation and an operator displacement. The decoherence due to phonons as a function of the detuning between the cavity mode and exciton is obtained analytically. It is shown that the detuning has a significant impact on the quantum dot exciton lifetime. In realistic experimental conditions, the calculated exciton lifetimes are in good agreement with recent experimental observation (Hennessy et al 2007 Nature 445 896)

2008-08-13

2

Energy Technology Data Exchange (ETDEWEB)

We theoretically investigate the dynamic interaction of a quantum dot in a nanocavity with time-symmetric single-photon pulses. The simulations, based on a wave-function approach, reveal that almost perfect single-photon absorption occurs for quantum-dot-cavity systems operating on the edge between strong- and weak-coupling regimes. The computed maximum absorption probability is close to unity for pulses with a typical length comparable to half of the Rabi period. Furthermore, the dynamic control of the quantum-dot energy via electric fields allows the freezing of the light-matter interaction, leaving the quantum dot in its excited state. Shaping of single-photon wave packets by the electric field control is limited by the occurrence of chirping of the single-photon pulse. This understanding of the interaction of single-photon pulses with the quantum-dot-cavity system provides the basis for the development of advanced protocols for quantum-information processing in the solid state.

Johne, R.; Fiore, A. [COBRA Research Institute, Eindhoven University of Technology, P.O. Box 513, NL-5600 MB Eindhoven (Netherlands)

2011-11-15

3

Phonon Mediated Off-resonant Quantum Dot-Cavity Interaction

Optically controlled quantum dot (QD) spins coupled to semiconductor microcavities constitute a promising platform for robust and scalable quantum information processing devices. In recent experiments on coupled QD optical cavity systems a pronounced interaction between the dot and the cavity has been observed even for detunings of many cavity linewidths. This interaction has been attributed to an incoherent cavity enhanced phonon-mediated scattering process and is absent in atomic systems. We demonstrate that despite its incoherent nature, this process preserves the signatures of coherent interaction between a QD and a strong driving laser, which may be observed via the optical emission from the off-resonant cavity. Under bichromatic driving of the QD, the cavity emission exhibits spectral features consistent with optical dressing of the QD transition, namely Rabi side-bands. These cavity emission measurements are more akin to absorption measurements of a strongly driven QD rather than resonance fluorescence measurements. In addition to revealing new aspects of the off-resonant QD-cavity interaction, this result provides a new, simpler means of coherently probing QDs and opens the possibility of employing off-resonant cavities to optically interface QD-nodes in quantum networks.

Majumdar, Arka; Kim, Erik; Bajcsy, Michal; Rundquist, Armand; Vuckovic, Jelena

2012-02-01

4

Fundamental properties of devices for quantum information technology

DEFF Research Database (Denmark)

This thesis reports a theoretical investigation of the influence of the electronphonon interaction on semiconductor cavity quantum electrodynamical systems, specifically a quantum dot coupled to an optical microcavity. We develop a theoretical description of the decay dynamics of the quantum dot interacting with the cavity and the phonons. It is shown that the presence of the phonon interaction, fundamentally changes the spontaneous emission decay behavior of the quantum dot. Especially in the regime where the quantum dotcavity spectral detuning is significantly larger than any linewidth of the system, the effect of the phonon interaction is very pronounced. A simple approximate analytical expression for the quantum dot decay rate is derived, which predicts a strong asymmetry with respect to the quantum dot-cavity detuning at low temperatures, and allows for a clear interpretation of the physics. Furthermore, a study of the indistinguishability of single photons emitted from the coupled quantum dot-cavity system is performed, with special emphasis on non-Markovian decoherence due to the phonon interaction. We show that common theoretical approaches fail to predict the degree of indistinguishability, on both a qualitative and quantitative level, for experimentally relevant parameters regimes. The important role of non-Markovian effects in the shorttime regime, where virtual processes dominate the decoherence of the quantum dot-cavity system, is emphasized. Importantly, our investigations lead to a maximum achievable degree of indistinguishability, a prediction which eludes common approaches.

Nielsen, Per Kær

2012-01-01

5

Bell-state analysis (BSA) is essential in quantum communication, but it is impossible to distinguish unambiguously the four Bell states in the polarization degree of freedom (DOF) of two-photon systems with only linear optical elements, except for the case in which the BSA is assisted with hyperentangled states, the simultaneous entanglement in more than one DOF. Here, we propose a scheme to distinguish completely the 16 hyperentangled Bell states in both the polarization and the spatial-mode DOFs of two-photon systems, by using the giant nonlinear optics in quantum dot-cavity systems. This scheme can be applied to increase the channel capacity of long-distance quantum communication based on hyperentanglement, such as entanglement swapping, teleportation, and superdense coding. We use hyperentanglement swapping as an example to show the application of this HBSA.

Ren, Bao-Cang; Wei, Hai-Rui; Hua, Ming; Li, Tao; Deng, Fu-Guo

2012-10-01

6

UK PubMed Central (United Kingdom)

Bell-state analysis (BSA) is essential in quantum communication, but it is impossible to distinguish unambiguously the four Bell states in the polarization degree of freedom (DOF) of two-photon systems with only linear optical elements, except for the case in which the BSA is assisted with hyperentangled states, the simultaneous entanglement in more than one DOF. Here, we propose a scheme to distinguish completely the 16 hyperentangled Bell states in both the polarization and the spatial-mode DOFs of two-photon systems, by using the giant nonlinear optics in quantum dot-cavity systems. This scheme can be applied to increase the channel capacity of long-distance quantum communication based on hyperentanglement, such as entanglement swapping, teleportation, and superdense coding. We use hyperentanglement swapping as an example to show the application of this HBSA.

Ren BC; Wei HR; Hua M; Li T; Deng FG

2012-10-01

7

DEFF Research Database (Denmark)

We study the fundamental limit on single-photon indistinguishability imposed by decoherence due to phonon interactions in semiconductor quantum dot-cavity quantum electrodynamics systems. Employing an exact diagonalization approach we find large differences compared to standard methods. An important finding is that short-time non-Markovian effects limit the maximal attainable indistinguishability. The results are explained using a polariton picture that yields valuable insight into the phonon-induced dephasing dynamics.

Nielsen, Per Kær; Lodahl, Peter

2013-01-01

8

Open quantum system identification

Engineering quantum systems offers great opportunities both technologically and scientifically for communication, computation, and simulation. The construction and operation of large scale quantum information devices presents a grand challenge and a major issue is the effective control of coherent dynamics. This is often in the presence of decoherence which further complicates the task of determining the behaviour of the system. Here, we show how to determine open system Markovian dynamics of a quantum system with restricted initialisation and partial output state information.

Schirmer, Sophie G; Zhou, Weiwei; Gong, Erling; Zhang, Ming

2012-01-01

9

So far proposed quantum computers use fragile and environmentally sensitive natural quantum systems. Here we explore the new notion that synthetic quantum systems suitable for quantum computation may be fabricated from smart nanostructures using topological excitations of a stochastic neural-type network that can mimic natural quantum systems. These developments are a technological application of process physics which is an information theory of reality in which space and quantum phenomena are emergent, and so indicates the deep origins of quantum phenomena. Analogous complex stochastic dynamical systems have recently been proposed within neurobiology to deal with the emergent complexity of biosystems, particularly the biodynamics of higher brain function. The reasons for analogous discoveries in fundamental physics and neurobiology are discussed.

Cahill, R T

2002-01-01

10

Models of PT symmetric quantum mechanics provide examples of biorthogonal quantum systems. The latter incorporporate all the structure of PT symmetric models, and allow for generalizations, especially in situations where the PT construction of the dual space fails. The formalism is illustrated by a few exact results for models of the form H=(p+\

Curtright, T; Curtright, Thomas; Mezincescu, Luca

2005-01-01

11

The aim of quantum system identification is to estimate the ingredients inside a black box, in which some quantum-mechanical unitary process takes place, by just looking at its input-output behavior. Here we establish a basic and general framework for quantum system identification, that allows us to classify how much knowledge about the quantum system is attainable, in principle, from a given experimental setup. Prior knowledge on some elements of the black box helps the system identification. We present an example in which a Bell measurement is more efficient to identify the system. When the topology of the system is known, the framework enables us to establish a general criterion for the estimability of the coupling constants in its Hamiltonian.

Burgarth, Daniel

2011-01-01

12

International Nuclear Information System (INIS)

A finite quantum system in which the position and momentum take values in the Galois field GF(pl) is constructed from a smaller quantum system in which the position and momentum take values in Zp, using field extension. The Galois trace is used in the definition of the Fourier transform. The Heisenberg-Weyl group of displacements and the Sp(2, GF(pl)) group of symplectic transformations are studied. A class of transformations inspired by the Frobenius maps in Galois fields is introduced. The relationship of this 'Galois quantum system' with its subsystems in which the position and momentum take values in subfields of GF(pl) is discussed

2005-09-30

13

Quantum states and potentialities of quantum systems

Energy Technology Data Exchange (ETDEWEB)

In a previous article it was shown that in general quantum states represent perspectives on the potentialities of quantum systems, rather than the potentialities themselves. In the present paper the following questions are investigated in the context of this result: How do quantum states which undergo collapse transform under pure translations. Under what conditions do quantum states represent the potentialities themselves. Two alternatives are presented in response to the first question: Quantum states are scalars under translations. The collapse of a quantum state propagates between frames of reference at the speed of light. The advantages and disadvantages of the two alternatives are discussed. The response to the second question is shown to depend on the chosen alternative. In addition, the second alternative is shown to lead to a consistent view of quantum states as ''potential perspectives on potentialities.''

Malin, S.

1986-12-01

14

Energy Technology Data Exchange (ETDEWEB)

A degenerate dynamical system is characterized by a symplectic structure whose rank is not constant throughout phase space. Its phase space is divided into causally disconnected, nonoverlapping regions in each of which the rank of the symplectic matrix is constant, and there are no classical orbits connecting two different regions. Here the question of whether this classical disconnectedness survives quantization is addressed. Our conclusion is that in irreducible degenerate systems-in which the degeneracy cannot be eliminated by redefining variables in the action-the disconnectedness is maintained in the quantum theory: there is no quantum tunnelling across degeneracy surfaces. This shows that the degeneracy surfaces are boundaries separating distinct physical systems, not only classically, but in the quantum realm as well. The relevance of this feature for gravitation and Chern-Simons theories in higher dimensions cannot be overstated.

Micheli, Fiorenza de [Centro de Estudios Cientificos, Arturo Prat 514, Valdivia (Chile); Instituto de Fisica, Pontificia Universidad Catolica de Valparaiso, Casilla 4059, Valparaiso (Chile); Zanelli, Jorge [Centro de Estudios Cientificos, Arturo Prat 514, Valdivia (Chile); Universidad Andres Bello, Av. Republica 440, Santiago (Chile)

2012-10-15

15

International Nuclear Information System (INIS)

A degenerate dynamical system is characterized by a symplectic structure whose rank is not constant throughout phase space. Its phase space is divided into causally disconnected, nonoverlapping regions in each of which the rank of the symplectic matrix is constant, and there are no classical orbits connecting two different regions. Here the question of whether this classical disconnectedness survives quantization is addressed. Our conclusion is that in irreducible degenerate systems—in which the degeneracy cannot be eliminated by redefining variables in the action—the disconnectedness is maintained in the quantum theory: there is no quantum tunnelling across degeneracy surfaces. This shows that the degeneracy surfaces are boundaries separating distinct physical systems, not only classically, but in the quantum realm as well. The relevance of this feature for gravitation and Chern-Simons theories in higher dimensions cannot be overstated.

2012-01-01

16

Energy Technology Data Exchange (ETDEWEB)

A finite quantum system in which the position and momentum take values in the Galois field GF(p{sup l}) is constructed from a smaller quantum system in which the position and momentum take values in Z{sub p}, using field extension. The Galois trace is used in the definition of the Fourier transform. The Heisenberg-Weyl group of displacements and the Sp(2, GF(p{sup l})) group of symplectic transformations are studied. A class of transformations inspired by the Frobenius maps in Galois fields is introduced. The relationship of this 'Galois quantum system' with its subsystems in which the position and momentum take values in subfields of GF(p{sup l}) is discussed.

Vourdas, A [Department of Computing, University of Bradford, Bradford BD7 1DP (United Kingdom)

2005-09-30

17

Quantum Electromechanical Systems

Quantum electromechanical systems are nano-to-micron scale mechanical resonators coupled to electronic devices of comparable dimensions, such that the mechanical resonator behaves in a manifestly quantum manner. Dramatic progress towards realising such systems has been made with the recent demonstration of a GHz mechanical resonator [1], and demonstrated displacement detection close to the quantum limit based on the single electron transistor [2-4]. The latter system, comprising a single electron transistor and micron-scale mechanical resonator electrostatically-coupled to the transistor island, is predicted to exhibit surprisingly rich dynamical behavior [5]. The unprecedented displacement sensitivity of the SET transducer may also enable the observation of quantum 'jumps' in the motion of nanomechanical resonators at milliKelvin temperatures, due to the strain-mediated coupling between the flexing motion and tunneling defects of the resonator [6]. Such investigations are contributing towards a deeper understanding of the transition between quantum and classical dynamics. [1] X. Huang, C. Zorman, M. Mehregany, M. Roukes, Nature 421 (2003) 496; [2] M. Blencowe, M. Wybourne, Appl. Phys. Lett. 77 (2000) 3845; [3] R. Knobel, A. Cleland, Nature 424 (2003), 291; [4] M. LaHaye, O. Buu, B. Camarota, K. Schwab (unpublished); [5] A. Armour, M. Blencowe, Y. Zhang, cond-mat/0307528 (Phys. Rev. B to appear); [6] Y. Tanaka, M. Blencowe (unpublished).

Blencowe, Miles

2004-03-01

18

Quantum circuits for strongly correlated quantum systems

In recent years, we have witnessed an explosion of experimental tools by which quantum systems can be manipulated in a controlled and coherent way. One of the most important goals now is to build quantum simulators, which would open up the possibility of exciting experiments probing various theories in regimes that are not achievable under normal lab circumstances. Here we present a novel approach to gain detailed control on the quantum simulation of strongly correlated quantum many-body systems by constructing the explicit quantum circuits that diagonalize their dynamics. We show that the exact quantum circuits underlying some of the most relevant many-body Hamiltonians only need a finite amount of local gates. As a particularly simple instance, the full dynamics of a one-dimensional Quantum Ising model in a transverse field with four spins is shown to be reproduced using a quantum circuit of only six local gates. This opens up the possibility of experimentally producing strongly correlated states, their tim...

Verstraete, Frank; Latorre, Jose I

2008-01-01

19

Starting from first principles, this book introduces the fundamental concepts and methods of dissipative quantum mechanics and explores related phenomena in condensed matter systems. Major experimental achievements in cooperation with theoretical advances have brightened the field and brought it to the attention of the general community in natural sciences. Nowadays, working knowledge of dissipative quantum mechanics is an essential tool for many physicists. This book - originally published in 1990 and republished in 1999 and and 2008 as enlarged second and third editions - delves significantl

Weiss, Ulrich

2012-01-01

20

International Nuclear Information System (INIS)

[en] Models of PT symmetric quantum mechanics provide examples of biorthogonal quantum systems. The latter incorporate all the structure of PT symmetric models, and allow for generalizations, especially in situations where the PT construction of the dual space fails. The formalism is illustrated by a few exact results for models of the form H=(p+?)2+?k>0?k exp(ikx). In some nontrivial cases, equivalent Hermitian theories are obtained and shown to be very simple: They are just free (chiral) particles. Field theory extensions are briefly considered

2007-01-01

21

Recent advances in the quantum theory of macroscopic systems have brightened up the field and brought it into the focus of a general community in natural sciences. The fundamental concepts, methods and applications including the most recent developments, previously covered for the most part only in the original literature, are presented here in a comprehensive treatment to an audience who is reasonably familiar with quantum-statistical mechanics and has had rudimentary contacts with the path integral formulation.This book deals with the phenomena and theory of decoherence and dissipation in qu

Weiss, U

1999-01-01

22

Quantum Critical Points in Quantum Impurity Systems

The numerical renormalization group method is used to investigate zero temperature phase transitions in quantum impurity systems, in particular in the soft-gap Anderson model, where an impurity couples to a non-trivial fermionic bath. In this case, zero temperature phase transitions occur between two different phases whose fixed points can be built up of non-interacting single-particle states. However, the quantum critical point cannot be described by non-interacting fermionic or bosonic excitations.

Lee, H J

2004-01-01

23

Asymptotic dynamics of quantum discord in open quantum systems

Digital Repository Infrastructure Vision for European Research (DRIVER)

It is well known that quantum entanglement makes possible certain tasks in quantum information theory. However, there are also quantum tasks that display the quantum advantage without entanglement . Distinguishing classical and quantum correlations in quantum systems is therefore of both pr...

24

Three Terminal Quantum Dot System

Digital Repository Infrastructure Vision for European Research (DRIVER)

In this study, the transmission rate for the three terminal quantum dot system is determined using Keldysh nonequilibrium Green’s function technique for interacting and non-interacting cases. The three terminal quantum dot systems consist of three leads and three quantum dots that are arranged i...

Narra Sunil Kumar; N. Chandrasekar; G. Pavan

25

We propose a scheme for the production of hyperentangled photon pairs as well as for the complete differentiation of 16 hyperentangled Bell states in both polarization and spatial-mode degrees of freedom using the quantum-dot cavity systems. This hyperentangled-Bell-state photon generation and the complete hyperentangled-Bell-state-analysis device can serve as crucial components of the high-capacity, long-distance quantum communication. We use the hyperentanglement quantum repeater as an example to show the application of this device. When the hyperentanglement quantum repeater operation is complete, two parties that are far from each other in quantum communication can share two Bell-state spin pairs simultaneously. By using numerical calculations we prove that the present scheme can both work in the weak- and strong-coupling regimes with current technology.

Wang, Tie-Jun; Lu, Yao; Long, Gui Lu

2012-10-01

26

Decoherence in open quantum systems

International Nuclear Information System (INIS)

[en] In the framework of the Lindblad theory for open quantum systems we determine the degree of quantum decoherence of a harmonic oscillator interacting with a thermal bath. In the present paper we have studied QD with the Markovian equation of Lindblad in order to understand the quantum to classical transition for a system consisting of an one-dimensional harmonic oscillator in interaction with a thermal bath in the framework of the theory of open quantum systems based on quantum dynamical semigroups. The role of QD became relevant in many interesting physical problems from field theory, atomic physics, quantum optics and quantum information processing, to which we can add material science, heavy ion collisions, quantum gravity and cosmology, condensed matter physics. Just to mention only a few of them: to understand the way in which QD enhances the quantum to classical transition of density fluctuations; to study systems of trapped and cold atoms (or ions) which may offer the possibility of engineering the environment, like trapped atoms inside cavities, relation between decoherence and other cavity QED effects (such as Casimir effect); on mesoscopic scale, decoherence in the context of Bose-Einstein condensation. In many cases physicists are interested in understanding the specific causes of QD just because they want to prevent decoherence from damaging quantum states and to protect the information stored in quantum states from the degrading effect of the interaction with the environment. Thus, decoherence is responsible for washing out the quantum interference effects which are desirable to be seen as signals in some experiments. QD has a negative influence on many areas relying upon quantum coherence effects, such as quantum computation and quantum control of atomic and molecular processes. The physics of information and computation is such a case, where decoherence is an obvious major obstacle in the implementation of information-processing hardware that takes advantage of the superposition principle. The study of classicality using QD leads to a deeper understanding of the quantum origins of the classical world. Much work has still to be done even to settle the interpretational questions, not to speak about answering them. Nevertheless, as a result of the progress made in the last two decades, the quantum to classical transition has become a subject of experimental investigations, while previously it was mostly a domain of philosophy. The issue of quantum to classical transition points to the necessity of a better understanding of open quantum systems. The Lindblad theory provides a selfconsistent treatment of damping as a general extension of quantum mechanics

2005-01-01

27

Quantum point contacts in quantum wire systems

Energy Technology Data Exchange (ETDEWEB)

Quantum point contacts (QPCs) attract high interest for applications as magnetic focussing, beam splitting (quantum Hall edge states), spin filtering and electron thermometry. Here, we investigate QPCs in complex quantum wire (QWR) systems such as quantum rings. The QPCs were realized by lithographical definition of a short (150 nm) constriction (170 nm width) in (a) a 540 nm wide QWR and (b) 520 nm wide QWR leads of a QWR ring as in. Nanogates on top of the constrictions allow for the control of occupied modes in the QPCs. The devices are based on a GaAs/AlGaAs heterostructure with a 2DEG 55 nm below the surface, patterned by electron beam lithography and wet-chemical etching. Two- and four-terminal conductance measurements at temperatures between 23 mK and 4.2 K were performed using lock-in technique. Our measurements reveal that QPCs in 1D nanostructures can be prepared to show subband separations of 6 meV, clear conductance quantization as well as the 0.7 anomaly. We further show that electron injection across a QPC into a QWR ring allows for electron interference (Aharonov-Bohm effect).

Sternemann, E.; Buchholz, S.S.; Fischer, S.F.; Kunze, U. [Werkstoffe und Nanoelektronik, Ruhr-Universitaet Bochum (Germany); Reuter, D.; Wieck, A.D. [Angewandte Festkoerperphysik, Ruhr-Universitaet Bochum (Germany)

2010-07-01

28

DEFF Research Database (Denmark)

Differences in the confinement of electrons and holes in quantum dots are shown to profoundly impact the magnitude of scattering with acoustic phonons. Using an extensive model that includes the non-Markovian nature of the phonon reservoir, we show how the effect may be addressed by photoluminescence excitation spectroscopy of a single quantum dot. We also investigate the implications for cavity QED, i.e., a coupled quantum dot-cavity system, and demonstrate that the phonon scattering may be strongly quenched. The quenching is explained by a balancing between the deformation potential interaction strengths and the carrier confinement and depends on the quantum dot shape. Numerical examples suggest a route towards engineering the phonon scattering.

Nysteen, Anders; Nielsen, Per Kær

2013-01-01

29

Classical command of quantum systems.

Quantum computation and cryptography both involve scenarios in which a user interacts with an imperfectly modelled or 'untrusted' system. It is therefore of fundamental and practical interest to devise tests that reveal whether the system is behaving as instructed. In 1969, Clauser, Horne, Shimony and Holt proposed an experimental test that can be passed by a quantum-mechanical system but not by a system restricted to classical physics. Here we extend this test to enable the characterization of a large quantum system. We describe a scheme that can be used to determine the initial state and to classically command the system to evolve according to desired dynamics. The bipartite system is treated as two black boxes, with no assumptions about their inner workings except that they obey quantum physics. The scheme works even if the system is explicitly designed to undermine it; any misbehaviour is detected. Among its applications, our scheme makes it possible to test whether a claimed quantum computer is truly quantum. It also advances towards a goal of quantum cryptography: namely, the use of 'untrusted' devices to establish a shared random key, with security based on the validity of quantum physics. PMID:23619692

Reichardt, Ben W; Unger, Falk; Vazirani, Umesh

2013-04-25

30

Decoherence in infinite quantum systems

Energy Technology Data Exchange (ETDEWEB)

We review and discuss a notion of decoherence formulated in the algebraic framework of quantum physics. Besides presenting some sufficient conditions for the appearance of decoherence in the case of Markovian time evolutions we provide an overview over possible decoherence scenarios. The framework for decoherence we establish is sufficiently general to accommodate quantum systems with infinitely many degrees of freedom.

Blanchard, Philippe; Hellmich, Mario [Faculty of Physics, University of Bielefeld, Universitaetsstr. 25, 33615 Bielefeld (Germany); Bundesamt fuer Strahlenschutz (Federal Office for Radiation Protection), Willy-Brandt-Strasse 5, 38226 Salzgitter (Germany)

2012-09-01

31

Decoherence in infinite quantum systems

International Nuclear Information System (INIS)

[en] We review and discuss a notion of decoherence formulated in the algebraic framework of quantum physics. Besides presenting some sufficient conditions for the appearance of decoherence in the case of Markovian time evolutions we provide an overview over possible decoherence scenarios. The framework for decoherence we establish is sufficiently general to accommodate quantum systems with infinitely many degrees of freedom.

2012-09-01

32

International Nuclear Information System (INIS)

The overview of recent developments in the theory of quantum chaos is presented with the special emphasis on a number of unsolved problems and current apparent contradictions. The relation between dynamical quantum chaos and statistical random matrix theory is discussed. 97 refs

1991-01-01

33

Dissipation in quantum systems

Energy Technology Data Exchange (ETDEWEB)

The quantum dynamics of a mechanical subsystem interacting with a thermal bath is studied. A new approach to dissipated quantum mechanics is proposed which is based on the thermodynamic treatment of the energy balance. Simple application of the theory to ideal gases is considered and equations for the evolution of the main average quantities are derived. (author)

Tsekov, R. [Dept. of Phys. Chem., Sofia Univ. (Bulgaria)

1995-11-07

34

Quantum dissipation in unbounded systems.

UK PubMed Central (United Kingdom)

In recent years trajectory based methodologies have become increasingly popular for evaluating the time evolution of quantum systems. A revival of the de Broglie--Bohm interpretation of quantum mechanics has spawned several such techniques for examining quantum dynamics from a hydrodynamic perspective. Using techniques similar to those found in computational fluid dynamics one can construct the wave function of a quantum system at any time from the trajectories of a discrete ensemble of hydrodynamic fluid elements (Bohm particles) which evolve according to nonclassical equations of motion. Until very recently these schemes have been limited to conservative systems. In this paper, we present our methodology for including the effects of a thermal environment into the hydrodynamic formulation of quantum dynamics. We derive hydrodynamic equations of motion from the Caldeira-Leggett master equation for the reduced density matrix and give a brief overview of our computational scheme that incorporates an adaptive Lagrangian mesh. Our applications focus upon the dissipative dynamics of open unbounded quantum systems. Using both the Wigner phase space representation and the linear entropy, we probe the breakdown of the Markov approximation of the bath dynamics at low temperatures. We suggest a criteria for rationalizing the validity of the Markov approximation in open unbound systems and discuss decoherence, energy relaxation, and quantum/classical correspondence in the context of the Bohmian paths.

Maddox JB; Bittner ER

2002-02-01

35

Quantum dissipation in unbounded systems.

In recent years trajectory based methodologies have become increasingly popular for evaluating the time evolution of quantum systems. A revival of the de Broglie--Bohm interpretation of quantum mechanics has spawned several such techniques for examining quantum dynamics from a hydrodynamic perspective. Using techniques similar to those found in computational fluid dynamics one can construct the wave function of a quantum system at any time from the trajectories of a discrete ensemble of hydrodynamic fluid elements (Bohm particles) which evolve according to nonclassical equations of motion. Until very recently these schemes have been limited to conservative systems. In this paper, we present our methodology for including the effects of a thermal environment into the hydrodynamic formulation of quantum dynamics. We derive hydrodynamic equations of motion from the Caldeira-Leggett master equation for the reduced density matrix and give a brief overview of our computational scheme that incorporates an adaptive Lagrangian mesh. Our applications focus upon the dissipative dynamics of open unbounded quantum systems. Using both the Wigner phase space representation and the linear entropy, we probe the breakdown of the Markov approximation of the bath dynamics at low temperatures. We suggest a criteria for rationalizing the validity of the Markov approximation in open unbound systems and discuss decoherence, energy relaxation, and quantum/classical correspondence in the context of the Bohmian paths. PMID:11863623

Maddox, Jeremy B; Bittner, Eric R

2002-01-25

36

Preconditioned quantum linear system algorithm.

UK PubMed Central (United Kingdom)

We describe a quantum algorithm that generalizes the quantum linear system algorithm [Harrow et al., Phys. Rev. Lett. 103, 150502 (2009)] to arbitrary problem specifications. We develop a state preparation routine that can initialize generic states, show how simple ancilla measurements can be used to calculate many quantities of interest, and integrate a quantum-compatible preconditioner that greatly expands the number of problems that can achieve exponential speedup over classical linear systems solvers. To demonstrate the algorithm's applicability, we show how it can be used to compute the electromagnetic scattering cross section of an arbitrary target exponentially faster than the best classical algorithm.

Clader BD; Jacobs BC; Sprouse CR

2013-06-01

37

Smart Nanostructures and Synthetic Quantum Systems

So far proposed quantum computers use fragile and environmentally sensitive natural quantum systems. Here we explore the notion that synthetic quantum systems suitable for quantum computation may be fabricated from smart nanostructures using topological excitations of a neural-type network that can mimic natural quantum systems. These developments are a technological application of process physics which is a semantic information theory of reality in which space and quantum phenomena are emergent.

Cahill, R T

2001-01-01

38

Quantum Geometry and Quantum Mechanics of Integrable Systems

Quantum integrable systems and their classical counterparts are considered. We show that the symplectic structure and invariant tori of the classical system can be deformed by a quantization parameter $\\hbar$ to produce a new (classical) integrable system. The new tori selected by the $\\hbar$-equidistance rule represent the spectrum of the quantum system up to $O(\\hbar^\\infty)$ and are invariant under quantum dynamics in the long-time range $O(\\hbar^{-\\infty})$. The quantum diffusion over the deformed tori is described. The analytic apparatus uses quantum action-angle coordinates explicitly constructed by an $\\hbar$-deformation of the classical action-angles.

Karasev, M V

2009-01-01

39

Introduction to quantum spin systems

Directory of Open Access Journals (Sweden)

Full Text Available This manuscript is the collection of lectures given in the summer school on strongly correlated electron systems held at Isfahan university of technology, June 2007. A short overview on quantum magnetism and spin systems is presented. The numerical exact diagonalization (Lanczos) alghorithm is explained in a pedagogical ground. This is a method to get some ground state properties on finite cluster of lattice models. Two extensions of Lanczos method to get the excited states and also finite temperature properties of quantum models are also explained. The basic notions of quantum phase transition is discussed in term of Ising model in transverse field. Its phase diagram and critical properties are explained using the quantum renormalization group approach. Most of the topics are in tutorial level with hints to recent research activities.

A. Langari

2008-01-01

40

Quantum energy teleportation in a quantum Hall system

We propose an experimental method for a quantum protocol termed quantum energy teleportation (QET), which allows energy transportation to a remote location without physical carriers. Using a quantum Hall system as a realistic model, we discuss the physical significance of QET and estimate the order of energy gain using reasonable experimental parameters.

Yusa, Go; Hotta, Masahiro

2011-01-01

41

Quantum effects in optomechanical systems

The search for experimental demonstrations of the quantum behavior of macroscopic mechanical resonators is a fastly growing field of investigation and recent results suggest that the generation of quantum states of resonators with a mass at the microgram scale is within reach. In this chapter we give an overview of two important topics within this research field: cooling to the motional ground state, and the generation of entanglement involving mechanical, optical and atomic degrees of freedom. We focus on optomechanical systems where the resonator is coupled to one or more driven cavity modes by the radiation pressure interaction. We show that robust stationary entanglement between the mechanical resonator and the output fields of the cavity can be generated, and that this entanglement can be transferred to atomic ensembles placed within the cavity. These results show that optomechanical devices are interesting candidates for the realization of quantum memories and interfaces for continuous variable quantum ...

Genes, C; Vitali, D; Tombesi, P

2009-01-01

42

From EPR to quantum computing: experiments on entangled quantum systems

International Nuclear Information System (INIS)

Einstein, together with Podolski and Rosen (EPR), tried to point out inconsistencies of standard quantum mechanics using an effect which is now called entanglement. The experiments testing EPR's hypothesis laid the basis for the new field of quantum information processing, which in turn gave rise to impressive progress in methods to observe and to analyse the phenomenon of entanglement. Here we give an overview of the various systems useful for the novel applications of quantum communication and quantum computation.

2005-05-14

43

Geometrical structures of multipartite quantum systems

In this paper I will investigate geometrical structures of multipartite quantum systems based on complex projective varieties. These varieties are important in characterization of quantum entangled states. In particular I will establish relation between multi-projective Segre varieties and multip-qubit quantum states. I also will discuss other geometrical approaches such as toric varieties to visualize complex multipartite quantum systems.

Heydari, Hoshang

2011-01-01

44

Quantum Annealing and Quantum Fluctuation Effect in Frustrated Ising Systems

Quantum annealing method has been widely attracted attention in statistical physics and information science since it is expected to be a powerful method to obtain the best solution of optimization problem as well as simulated annealing. The quantum annealing method was incubated in quantum statistical physics. This is an alternative method of the simulated annealing which is well-adopted for many optimization problems. In the simulated annealing, we obtain a solution of optimization problem by decreasing temperature (thermal fluctuation) gradually. In the quantum annealing, in contrast, we decrease quantum field (quantum fluctuation) gradually and obtain a solution. In this paper we review how to implement quantum annealing and show some quantum fluctuation effects in frustrated Ising spin systems.

Tanaka, Shu

2012-01-01

45

Control of the quantum open system via quantum generalized measurement

International Nuclear Information System (INIS)

For any specified pure state of quantum open system, we can construct a kind of quantum generalized measurement (QGM) that the state of the system after measurement will be deterministically collapsed into the specified pure state from any initial state. In other words, any pure state of quantum open system is reachable by QGM. Subsequently, whether the qubit is density matrix controllable is discussed in the case of pure dephasing. Our results reveal that combining QGM with coherent control will enhance the ability of controlling the quantum open system. Furthermore, it is found that the ability to perform QGM on the quantum open system, combined with the ability of coherence control and conditions of decoherence-free subspace, allows us to suppress quantum decoherence.

2006-01-01

46

Entangled systems. New directions in quantum physics

International Nuclear Information System (INIS)

Entangled Systems is an introductory textbook for advanced students of physics, chemistry and computer science which covers an area of physics that has lately witnessed rapid expansion. The topics treated here include foundations of quantum theory, quantum information, quantum communication, quantum computing, quantum teleportation and hidden variables, thus providing not only a solid basis for the study of quantum theory as such, but also a profound foundation of knowledge from which readers can follow the rapid development of the topic or start out into a more specialized branch of research. Commented recommendations for further reading as well as end-of-chapter problems help the reader to access quickly the basic theoretical concepts of future key technologies. Only a basic prior knowledge of quantum theory and the necessary mathematical foundations is assumed, as introductory chapters are provided to present these to the readers. Thus, 'Entangled Systems' can be used both as a course book and for self-study purposes. From the contents: - The Mathematical Framework - Basic Concepts of Quantum Theory - The Simplest Quantum Systems: Qubits - Mixed State and Density Operator - Shannon's Entropy and Classical Information - The von Neumann Entropy and Quantum Information - Composite Systems - Entanglement - Correlations and Non-Local Measurements - There is no (Local-Realistic) Alternative to the Quantum Theory - Working with Entanglement - The Quantum Computer - General Measurements, POVM - The General Evolution of an Open Quantum System and Special Quantum Channels - Decoherence and Approaches to the Description of the Quantum Measurement Process - Two Implementations of Quantum Operations. (orig.)

2007-01-01

47

Quantum information processing A linear systems perspective

In this paper a system-oriented formalism of Quantum Information Processing is presented. Its form resembles that of standard signal processing, although further complexity is added in order to describe pure quantum-mechanical effects and operations. Examples of the application of the formalism to quantum time evolution and quantum measurement are given.

Curty, M; Curty, Marcos; Santos, David J.

2001-01-01

48

DEFF Research Database (Denmark)

We show that Auger processes involving wetting layer transitions mediate emission from a cavity that is detuned from a quantum dot by even tens of meV. The wetting layer thus acts as a reservoir, which by Coulomb scattering can supply or absorb the energy difference between emitter and cavity. We perform microscopic calculations of the effect treating the wetting layer as a non-Markovian reservoir interacting with the coupled quantum dot-cavity system through Coulomb interactions. Experimentally, cavity feeding has been observed in the asymmetric detuning range of -10 to +45 meV. We show that this asymmetry arises naturally from the quasiequilibrium properties of the wetting layer reservoir. Furthermore, we present numerical calculations of both photoluminescence spectra and photon correlations, demonstrating good qualitative agreement with experiments.

Settnes, Mikkel; Nielsen, Per Kær

2013-01-01

49

UK PubMed Central (United Kingdom)

We show that Auger processes involving wetting layer transitions mediate emission from a cavity that is detuned from a quantum dot by even tens of meV. The wetting layer thus acts as a reservoir, which by Coulomb scattering can supply or absorb the energy difference between emitter and cavity. We perform microscopic calculations of the effect treating the wetting layer as a non-Markovian reservoir interacting with the coupled quantum dot-cavity system through Coulomb interactions. Experimentally, cavity feeding has been observed in the asymmetric detuning range of -10 to +45??meV. We show that this asymmetry arises naturally from the quasiequilibrium properties of the wetting layer reservoir. Furthermore, we present numerical calculations of both photoluminescence spectra and photon correlations, demonstrating good qualitative agreement with experiments.

Settnes M; Kaer P; Moelbjerg A; Mork J

2013-08-01

50

Detecting quantum correlations in separable systems

Quantum correlations in composite and separable quantum systems are characterized by non-vanishing quantum discord. We demonstrate the necessary and sufficient conditions for existence of hermitian witness operators for quantum discord, bypassing the problem of non-convexity of the set of all classical states. We propose an example of a linear witness which can detect quantum correlations in large classes of separable bipartite systems. Decomposition of the witness in terms of a small number of locally measurable operators as shown here, paves the way for experimental detection of quantum correlations beyond entanglement.

Adhikari, S; Chakraborty, I

2012-01-01

51

Optimal Control for Open Quantum Systems: Qubits and Quantum Gates

This article provides a review of recent developments in the formulation and execution of optimal control strategies for the dynamics of quantum systems. A brief introduction to the concept of optimal control, the dynamics of of open quantum systems, and quantum information processing is followed by a presentation of recent developments regarding the two main tasks in this context: state-specific and state-independent optimal control. For the former, we present an extension of conventional theory (Pontryagin's principle) to quantum systems which undergo a non-Markovian time-evolution. Owing to its importance for the realization of quantum information processing, the main body of the review, however, is devoted to state-independent optimal control. Here, we address three different approaches: an approach which treats dissipative effects from the environment in lowest-order perturbation theory, a general method based on the time--evolution superoperator concept, as well as one based on the Kraus representation ...

Roloff, Robert; Pötz, Walter

2009-01-01

52

Supersymmetric Biorthogonal Quantum Systems

We discuss supersymmetric biorthogonal systems, with emphasis given to the periodic solutions that occur at spectral singularities of PT symmetric models. For these periodic solutions, the dual functions are associated polynomials that obey inhomogeneous equations. We construct in detail some explicit examples for the supersymmetric pairs of potentials V_{+/-}(z) = -U(z)^2 +/- z(d/(dz))U(z) where U(z) = \\sum_{k>0}u_{k}z^{k}. In particular, we consider the cases generated by U(z) = z and z/(1-z). We also briefly consider the effects of magnetic vector potentials on the partition functions of these systems.

Curtright, T; Schuster, D; Curtright, Thomas; Mezincescu, Luca; Schuster, David

2006-01-01

53

Supersymmetric biorthogonal quantum systems

International Nuclear Information System (INIS)

[en] We discuss supersymmetric biorthogonal systems, with emphasis given to the periodic solutions that occur at spectral singularities of PT symmetric models. For these periodic solutions, the dual functions are associated polynomials that obey inhomogeneous equations. We construct in detail some explicit examples for the supersymmetric pairs of potentials V±(z)=-U(z)2±z(d/dz)U(z) where U(z)??k>0?kzk. In particular, we consider the cases generated by U(z)=z and z/(1-z). We also briefly consider the effects of magnetic vector potentials on the partition functions of these systems

2007-01-01

54

Recurrences in driven quantum systems

We consider an initially bound quantum particle subject to an external time-dependent field. When the external field is large, the particle shows a tendency to repeatedly return to its initial state, irrespective of whether the frequency of the field is sufficient for escape from the well. These recurrences, which are absent in a classical calculation, arise from the system evolving primarily like a free particle in the external field.

Poduri, V; Patil, U; Poduri, V; Browne, D A; Patil, U

1994-01-01

55

Could nanostructure be unspeakable quantum system?

Heisenberg, Bohr and others were forced to renounce on the description of the objective reality as the aim of physics because of the paradoxical quantum phenomena observed on the atomic level. The contemporary quantum mechanics created on the base of their positivism point of view must divide the world into speakable apparatus which amplifies microscopic events to macroscopic consequences and unspeakable quantum system. Examination of the quantum phenomena corroborates the confidence expressed by creators of quantum theory that the renunciation of realism should not apply on our everyday macroscopic world. Nanostructures may be considered for the present as a boundary of realistic description for all phenomena including the quantum one.

Aristov, V V

2010-01-01

56

Quantum Friction: Cooling Quantum Systems with Unitary Time Evolution

We introduce a type of quantum dissipation -- local quantum friction -- by adding to the Hamiltonian a local potential that breaks time-reversal invariance so as to cool the system. Unlike the Kossakowski-Lindblad master equation, local quantum friction directly effects unitary evolution of the wavefunctions rather than the density matrix: it may thus be used to cool fermionic many-body systems with thousands of wavefunctions that must remain orthogonal. In addition to providing an efficient way to simulate quantum dissipation and non-equilibrium dynamics, local quantum friction coupled with adiabatic state preparation significantly speeds up many-body simulations, making the solution of the time-dependent Schr\\"odinger equation significantly simpler than the solution of its stationary counterpart.

Bulgac, Aurel; Roche, Kenneth J; Wlaz?owski, Gabriel

2013-01-01

57

Entangled systems. New directions in quantum physics

Energy Technology Data Exchange (ETDEWEB)

Entangled Systems is an introductory textbook for advanced students of physics, chemistry and computer science which covers an area of physics that has lately witnessed rapid expansion. The topics treated here include foundations of quantum theory, quantum information, quantum communication, quantum computing, quantum teleportation and hidden variables, thus providing not only a solid basis for the study of quantum theory as such, but also a profound foundation of knowledge from which readers can follow the rapid development of the topic or start out into a more specialized branch of research. Commented recommendations for further reading as well as end-of-chapter problems help the reader to access quickly the basic theoretical concepts of future key technologies. Only a basic prior knowledge of quantum theory and the necessary mathematical foundations is assumed, as introductory chapters are provided to present these to the readers. Thus, 'Entangled Systems' can be used both as a course book and for self-study purposes. From the contents: - The Mathematical Framework - Basic Concepts of Quantum Theory - The Simplest Quantum Systems: Qubits - Mixed State and Density Operator - Shannon's Entropy and Classical Information - The von Neumann Entropy and Quantum Information - Composite Systems - Entanglement - Correlations and Non-Local Measurements - There is no (Local-Realistic) Alternative to the Quantum Theory - Working with Entanglement - The Quantum Computer - General Measurements, POVM - The General Evolution of an Open Quantum System and Special Quantum Channels - Decoherence and Approaches to the Description of the Quantum Measurement Process - Two Implementations of Quantum Operations. (orig.)

Audretsch, J. [Konstanz Univ. (Germany). Fachbereich Physik

2007-07-01

58

The QUANTUM Question Answering System

UK PubMed Central (United Kingdom)

We participated to the TREC-X QA main task and listtask with a new system named QUANTUM, which analyzesquestions with shallow parsing techniques and regularexpressions. Instead of using a question classificationbased on entity types, we classify the questions accordingto generic mechanisms (which we call extraction functions)for the extraction of candidate answers. We takeadvantage of the Okapi information retrieval system forone-paragraph-long passage retrieval. We make an extensiveuse of the Alembic named-entity tagger and theWordNet semantic network to extract candidate answersfrom those passages. We deal with the possibility of noanswerquestions (NIL) by looking for a significant scoredrop between the extracted candidate answers.

Luc Plamondon; Guy Lapalme; Leila Kosseim

59

Simulation of n-qubit quantum systems. V. Quantum measurements

The FEYNMAN program has been developed during the last years to support case studies on the dynamics and entanglement of n-qubit quantum registers. Apart from basic transformations and (gate) operations, it currently supports a good number of separability criteria and entanglement measures, quantum channels as well as the parametrizations of various frequently applied objects in quantum information theory, such as (pure and mixed) quantum states, hermitian and unitary matrices or classical probability distributions. With the present update of the FEYNMAN program, we provide a simple access to (the simulation of) quantum measurements. This includes not only the widely-applied projective measurements upon the eigenspaces of some given operator but also single-qubit measurements in various pre- and user-defined bases as well as the support for two-qubit Bell measurements. In addition, we help perform generalized and POVM measurements. Knowing the importance of measurements for many quantum information protocols, e.g., one-way computing, we hope that this update makes the FEYNMAN code an attractive and versatile tool for both, research and education.New version program summaryProgram title: FEYNMAN Catalogue identifier: ADWE_v5_0 Program summary URL:http://cpc.cs.qub.ac.uk/summaries/ADWE_v5_0.html Program obtainable from: CPC Program Library, Queen's University, Belfast, N. Ireland Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed program, including test data, etc.: 27?210 No. of bytes in distributed program, including test data, etc.: 1?960?471 Distribution format: tar.gz Programming language: Maple 12 Computer: Any computer with Maple software installed Operating system: Any system that supports Maple; the program has been tested under Microsoft Windows XP and Linux Classification: 4.15 Catalogue identifier of previous version: ADWE_v4_0 Journal reference of previous version: Comput. Phys. Commun. 179 (2008) 647 Does the new version supersede the previous version?: Yes Nature of problem: During the last decade, the field of quantum information science has largely contributed to our understanding of quantum mechanics, and has provided also new and efficient protocols that are used on quantum entanglement. To further analyze the amount and transfer of entanglement in n-qubit quantum protocols, symbolic and numerical simulations need to be handled efficiently. Solution method: Using the computer algebra system Maple, we developed a set of procedures in order to support the definition, manipulation and analysis of n-qubit quantum registers. These procedures also help to deal with (unitary) logic gates and (nonunitary) quantum operations and measurements that act upon the quantum registers. All commands are organized in a hierarchical order and can be used interactively in order to simulate and analyze the evolution of n-qubit quantum systems, both in ideal and noisy quantum circuits. Reasons for new version: Until the present, the FEYNMAN program supported the basic data structures and operations of n-qubit quantum registers [1], a good number of separability and entanglement measures [2], quantum operations (noisy channels) [3] as well as the parametrizations of various frequently applied objects, such as (pure and mixed) quantum states, hermitian and unitary matrices or classical probability distributions [4]. With the current extension, we here add all necessary features to simulate quantum measurements, including the projective measurements in various single-qubit and the two-qubit Bell basis, and POVM measurements. Together with the previously implemented functionality, this greatly enhances the possibilities of analyzing quantum information protocols in which measurements play a central role, e.g., one-way computation. Running time: Most commands require ?10 seconds of processor time on a Pentium 4 processor with ?2 GHz RAM or newer, if they work with quantum registers with five or less qubits. Moreover, about 5-20 MB of working memory is typica

Radtke, T.; Fritzsche, S.

2010-02-01

60

Entangling transformations in composite finite quantum systems

Energy Technology Data Exchange (ETDEWEB)

Phase space methods are applied in the context of finite quantum systems. 'Galois quantum systems' (with a dimension which is a power of a prime number) are considered, and symplectic Sp(2,Z(d)) transformations are studied. Composite systems comprising two finite quantum systems are also considered. Symplectic Sp(4,Z(d)) transformations are classified into local and entangling ones and the necessary matrices which perform such transformations are calculated numerically.

Vourdas, A [Department of Computing, University of Bradford, Bradford BD7 1DP (United Kingdom)

2003-12-01

61

On quantum Griffiths effects in metallic systems

Elementary analytical extremal statistics arguments are used to analyse the possibility of quantum Griffiths effects in nearly critical systems with overdamped dynamics, such as arise in conventional theories of metallic quantum criticality. The overdamping is found to strongly suppress quantum tunnelling of rare regions, leading to superparamagnetic rather than quantum griffiths behavior. Implications for theories of non-fermi-liquid behavior in heavy fermion materials are discussed.

Millis, A J; Schmalian, J

2002-01-01

62

Quantum gears a simple mechanical system in the quantum regime

The quantum mechanics of a simple mechanical system is considered. A group of gears can serve as a model for several different systems such as an artifically constructed nanomechanical device or a group of ring molecules. It is shown that the classical motion of the gears in which the angular velocities are locked together does not correspond to a solution of the quantum problem. The general solution for several gears is discussed and the correspondence to the classical behaviour established.

MacKinnon, A

2002-01-01

63

Topics in quantum integrable systems

International Nuclear Information System (INIS)

[en] Recent developments in the theory of quantum integrable particle systems in one-dimension with inverse square interactions are reviewed. First the Yangian symmetry is introduced and the energy spectra of the related spin models are discussed. The character of the su(n)1 WZNW theory is shown to be closely related with the Rogers-Szegoe polynomial. Second, the infinite dimensional representation for solutions of the Yang-Baxter equation and the reflection equation is given. Based on the representation, the Dunkl operators associated with the classical root systems are constructed. The Macdonald polynomial and its generalization are discussed in connection with the eigenstates for the trigonometric case. Finally, some results on short-range interacting systems are mentioned

2003-01-01

64

Quantum Speed Limits in Open System Dynamics

Bounds to the speed of evolution of a quantum system are of fundamental interest in quantum metrology, quantum chemical dynamics, and quantum computation. We derive a time-energy uncertainty relation for open quantum systems undergoing a general, completely positive, and trace preserving evolution which provides a bound to the quantum speed limit. When the evolution is of the Lindblad form, the bound is analogous to the Mandelstam-Tamm relation which applies in the unitary case, with the role of the Hamiltonian being played by the adjoint of the generator of the dynamical semigroup. The utility of the new bound is exemplified in different scenarios, ranging from the estimation of the passage time to the determination of precision limits for quantum metrology in the presence of dephasing noise.

del Campo, A.; Egusquiza, I. L.; Plenio, M. B.; Huelga, S. F.

2013-02-01

65

Past Quantum States of a Monitored System

DEFF Research Database (Denmark)

A density matrix ?(t) yields probabilistic information about the outcome of measurements on a quantum system. We introduce here the past quantum state, which, at time T, accounts for the state of a quantum system at earlier times t

Gammelmark, SØren; Julsgaard, Brian

2013-01-01

66

Variational approach for the quantum Zakharov system

International Nuclear Information System (INIS)

The quantum Zakharov system is described in terms of a Lagrangian formalism. A time-dependent Gaussian trial function approach for the envelope electric field and the low-frequency part of the density fluctuation leads to a coupled, nonlinear system of ordinary differential equations. In the semiclassical case, linear stability analysis of this dynamical system shows a destabilizing role played by quantum effects. Arbitrary values of the quantum effects are also considered, yielding the ultimate destruction of the localized, Gaussian trial solution. Numerical simulations are shown for both the semiclassical and the full quantum cases.

2007-01-01

67

Quantum transport from the perspective of quantum open systems

International Nuclear Information System (INIS)

By viewing the non-equilibrium transport setup as a quantum open system, we propose a reduced-density-matrix based quantum transport formalism. At the level of self-consistent Born approximation, it can precisely account for the correlation between tunneling and the system internal many-body interaction, leading to certain novel behavior such as the non-equilibrium Kondo effect. It also opens a new way to construct time-dependent density functional theory for transport through large-scale complex systems.

2006-09-25

68

Simulating Thermalization for an Arbitrary Quantum System

Why it is that quantum systems thermalize has long been a mystery. Although recently there has been groundbreaking progress on this problem, key questions regarding the conditions required for thermalization remain. Further, the problem of obtaining a feasible method to simulate the thermalization of nonlinear quantum systems has also remained unsolved; even for Brownian motion one cannot derive reduced master equations for most bath-coupled nonlinear systems. Simulations of thermal environments are important in many applications, including mesoscopic quantum devices. Here we demonstrate the thermalization of an arbitrary nonlinear quantum system by coupling it to a quantum bath containing up to 10,000 states. The resulting thermalization depends only on the properties of the bath and the bath coupling operator, as required, and can therefore be applied to any system and any system coupling, including Brownian motion. These results not only demonstrate explicit conditions under which a bath will induce therma...

Jacobs, Kurt; Dunjko, Vanja

2009-01-01

69

Geometric Phase in Open Quantum Systems

Geometric phase of an open two-level quantum system with a squeezed, thermal environment is studied for various types of system-environment interactions, both non-dissipative and dissipative. In the former type, we consider quantum non-demolition interaction with a bath of harmonic oscillators as well as of that of two-level systems. In the latter type, we consider the system interacting with a bath of harmonic oscillators in the weak Born-Markov approximation, and further, a simplified Jaynes-Cummings model in a vacuum bath. Our results extend features of geometric phase in open systems reported in the literature to include effects due to squeezing. The Kraus operator representation is employed to connect the open-system effects to quantum noise processes familiar from quantum information theory. This study has some implications for a practical implementation of geometric quantum computation.

Banerjee, S; Banerjee, Subhashish

2006-01-01

70

Dynamical entropy for infinite quantum systems

International Nuclear Information System (INIS)

We review the recent physical application of the so-called Connes-Narnhofer-Thirring entropy, which is the successful quantum mechanical generalization of the classical Kolmogorov-Sinai entropy and, by its very conception, is a dynamical entropy for infinite quantum systems. We thus comparingly review also the physical applications of the classical dynamical entropy for infinite classical systems. 41 refs. (Author)

1990-01-01

71

Controlling the Shannon Entropy of Quantum Systems

This paper proposes a new quantum control method which controls the Shannon entropy of quantum systems. For both discrete and continuous entropies, controller design methods are proposed based on probability density function control, which can drive the quantum state to any target state. To drive the entropy to any target at any prespecified time, another discretization method is proposed for the discrete entropy case, and the conditions under which the entropy can be increased or decreased are discussed. Simulations are done on both two- and three-dimensional quantum systems, where division and prediction are used to achieve more accurate tracking.

Xing, Yifan; Wu, Jun

2013-01-01

72

Quantum information theory with Gaussian systems

International Nuclear Information System (INIS)

This thesis applies ideas and concepts from quantum information theory to systems of continuous-variables such as the quantum harmonic oscillator. The focus is on three topics: the cloning of coherent states, Gaussian quantum cellular automata and Gaussian private channels. Cloning was investigated both for finite-dimensional and for continuous-variable systems. We construct a private quantum channel for the sequential encryption of coherent states with a classical key, where the key elements have finite precision. For the case of independent one-mode input states, we explicitly estimate this precision, i.e. the number of key bits needed per input state, in terms of these parameters. (orig.)

2006-01-01

73

Quantum information theory with Gaussian systems

Energy Technology Data Exchange (ETDEWEB)

This thesis applies ideas and concepts from quantum information theory to systems of continuous-variables such as the quantum harmonic oscillator. The focus is on three topics: the cloning of coherent states, Gaussian quantum cellular automata and Gaussian private channels. Cloning was investigated both for finite-dimensional and for continuous-variable systems. We construct a private quantum channel for the sequential encryption of coherent states with a classical key, where the key elements have finite precision. For the case of independent one-mode input states, we explicitly estimate this precision, i.e. the number of key bits needed per input state, in terms of these parameters. (orig.)

Krueger, O.

2006-04-06

74

Quantum Caustics for Systems with Quadratic Lagrangians

We study caustics in classical and quantum mechanics for systems with quadratic Lagrangians. We derive a closed form of the transition amplitude on caustics and discuss their physical implications in the Gaussian slit (gedanken-)experiment. Application to the quantum mechanical rotor casts doubt on the validilty of Jevicki's correspondence hypothesis which states that in quantum mechanics, stationary points(instantons) arise as simple poles.

Horie, K; Tsutsui, I; Tanimura, S

1999-01-01

75

Interaction between classical and quantum systems

International Nuclear Information System (INIS)

An unconventional approach to the measurement problem in quantum mechanics is considered--the apparatus is treated as a classical system, belonging to the macro-world. In order to have a measurement the apparatus must interact with the quantum system. As a first step, the classical apparatus is embedded into a large quantum mechanical structure, making use of a superselection principle. The apparatus and system are coupled such that the apparatus remains classical (principle of integrity), and unambiguous information of the values of a quantum observable are transferred to the variables of the apparatus. Further measurement of the classical apparatus can be done, causing no problems of principle. Thus interactions causing pointers to move (which are not treated) can be added. The restrictions placed by the principle of integrity on the form of the interaction between classical and quantum systems are examined and illustration is given by means of a simple example in which one sees the principle of integrity at work

1977-01-01

76

Quantum equilibria for macroscopic systems

Energy Technology Data Exchange (ETDEWEB)

Nash equilibria are found for some quantum games with particles with spin-1/2 for which two spin projections on different directions in space are measured. Examples of macroscopic games with the same equilibria are given. Mixed strategies for participants of these games are calculated using probability amplitudes according to the rules of quantum mechanics in spite of the macroscopic nature of the game and absence of Planck's constant. A possible role of quantum logical lattices for the existence of macroscopic quantum equilibria is discussed. Some examples for spin-1 cases are also considered.

Grib, A [Department of Theoretical Physics and Astronomy, Russian State Pedagogical University, St. Petersburg (Russian Federation); Khrennikov, A [Centre for Mathematical Modelling in Physics and Cognitive Sciences Vaexjoe University (Sweden); Parfionov, G [Department of Mathematics, St. Petersburg State University of Economics and Finances (Russian Federation); Starkov, K [Department of Mathematics, St. Petersburg State University of Economics and Finances (Russian Federation)

2006-06-30

77

Quantum equilibria for macroscopic systems

International Nuclear Information System (INIS)

[en] Nash equilibria are found for some quantum games with particles with spin-1/2 for which two spin projections on different directions in space are measured. Examples of macroscopic games with the same equilibria are given. Mixed strategies for participants of these games are calculated using probability amplitudes according to the rules of quantum mechanics in spite of the macroscopic nature of the game and absence of Planck's constant. A possible role of quantum logical lattices for the existence of macroscopic quantum equilibria is discussed. Some examples for spin-1 cases are also considered

2006-06-30

78

Accidental degeneracies in nonlinear quantum deformed systems

We construct a multi-parameter nonlinear deformed algebra for quantum confined systems that includes many other deformed models as particular cases. We demonstrate that such systems exhibit the property of accidental pairwise energy level degeneracies. We also study, as a special case of our multi-parameter deformation formalism, the extension of the Tamm-Dancoff cutoff deformed oscillator and the occurrence of accidental pairwise degeneracy in the energy levels of the deformed system. As an application, we discuss the case of a trigonometric Rosen-Morse potential, which is successfully used in models for quantum confined systems, ranging from electrons in quantum dots to quarks in hadrons.

Aleixo, A. N. F.; Balantekin, A. B.

2011-09-01

79

Dynamical Hysteresis in Bistable Quantum Systems

International Nuclear Information System (INIS)

[en] We are studying bistable quantum systems coupled to a thermal environment in the presence of an external controlling force. The impact of thermal and quantum effects on the switching hysteresis is studied by evaluating a time-dependent real-time double path integral with the recently developed iterative tensor multiplication scheme for the quasiadiabatic path integral propagator method for temperatures ranging from well above to well below quantum-classical crossover temperature. The switching dynamics and its intimate connection to quantum stochastic resonance is studied here for the first time without limitation to a two-state description and/or small external forces. copyright 1997 The American Physical Society

80

Quantum simulation of tunneling in small systems.

A number of quantum algorithms have been performed on small quantum computers; these include Shor's prime factorization algorithm, error correction, Grover's search algorithm and a number of analog and digital quantum simulations. Because of the number of gates and qubits necessary, however, digital quantum particle simulations remain untested. A contributing factor to the system size required is the number of ancillary qubits needed to implement matrix exponentials of the potential operator. Here, we show that a set of tunneling problems may be investigated with no ancillary qubits and a cost of one single-qubit operator per time step for the potential evolution, eliminating at least half of the quantum gates required for the algorithm and more than that in the general case. Such simulations are within reach of current quantum computer architectures. PMID:22916333

Sornborger, Andrew T

2012-08-22

81

Quantum simulation of tunneling in small systems.

UK PubMed Central (United Kingdom)

A number of quantum algorithms have been performed on small quantum computers; these include Shor's prime factorization algorithm, error correction, Grover's search algorithm and a number of analog and digital quantum simulations. Because of the number of gates and qubits necessary, however, digital quantum particle simulations remain untested. A contributing factor to the system size required is the number of ancillary qubits needed to implement matrix exponentials of the potential operator. Here, we show that a set of tunneling problems may be investigated with no ancillary qubits and a cost of one single-qubit operator per time step for the potential evolution, eliminating at least half of the quantum gates required for the algorithm and more than that in the general case. Such simulations are within reach of current quantum computer architectures.

Sornborger AT

2012-01-01

82

Quantum contextuality in N-boson systems

Energy Technology Data Exchange (ETDEWEB)

Quantum contextuality in systems of identical bosonic particles is explicitly exhibited via the maximum violation of a suitable inequality of Clauser-Horne-Shimony-Holt type. Unlike the approaches considered so far, which make use of single-particle observables, our analysis involves collective observables constructed using multiboson operators. An exemplifying scheme to test this violation with a quantum optical setup is also discussed.

Benatti, Fabio [Dipartimento di Fisica, Universita degli Studi di Trieste, I-34151 Trieste (Italy); Istituto Nazionale di Fisica Nucleare, Sezione di Trieste, I-34014 Trieste (Italy); Floreanini, Roberto [Istituto Nazionale di Fisica Nucleare, Sezione di Trieste, I-34014 Trieste (Italy); Genovese, Marco [INRIM, Strada delle Cacce 91, I-10135 Torino (Italy); Olivares, Stefano [Dipartimento di Fisica, Universita degli Studi di Trieste, I-34151 Trieste (Italy)

2011-09-15

83

Lattice construction of quantum integrable systems

International Nuclear Information System (INIS)

The construction of quantum integrable systems is performed using the Quantum Inverse Scattering Method. The R-matrix is preserved which produces a class of integrable models on the lattice whose continuum limit is the original continuum model. The Sine-Gordon model is used as an example.

1993-01-01

84

Universal signature of non-quantum systems

International Nuclear Information System (INIS)

It is shown that 'non-quantum systems', with anomalous statistical properties, would carry a distinctive experimental signature. Such systems can exist in deterministic hidden-variables theories (such as the pilot-wave theory of de Broglie and Bohm). The signature consists of non-additive expectations for non-commuting observables, breaking the sinusoidal modulation of quantum probabilities for two-state systems (Malus' law). This effect is independent of the quantum state (pure or mixed), or of the details of the hidden-variables model. Experiments are proposed, testing polarisation probabilities for single photons.

2004-11-15

85

Universal signature of non-quantum systems

Energy Technology Data Exchange (ETDEWEB)

It is shown that 'non-quantum systems', with anomalous statistical properties, would carry a distinctive experimental signature. Such systems can exist in deterministic hidden-variables theories (such as the pilot-wave theory of de Broglie and Bohm). The signature consists of non-additive expectations for non-commuting observables, breaking the sinusoidal modulation of quantum probabilities for two-state systems (Malus' law). This effect is independent of the quantum state (pure or mixed), or of the details of the hidden-variables model. Experiments are proposed, testing polarisation probabilities for single photons.

Valentini, Antony [Perimeter Institute for Theoretical Physics, 35 King Street North, Waterloo, Ontario N2J 2W9 (Canada) and Theoretical Physics Group, Blackett Laboratory, Imperial College, Prince Consort Road, London SW7 2BZ (United Kingdom); Augustus College, 14 Augustus Road, London SW19 6LN (United Kingdom)]. E-mail: avalentini@perimeterinstitute.ca

2004-11-15

86

Intrinsic and extrinsic properties of quantum systems

The paper attempts to convince that the orthodox interpretation of quantum mechanics does not contradict philosophical realism by throwing light onto certain properties of quantum systems that seem to have escaped attention as yet. The exposition starts with the philosophical notions of realism. Then, the quantum mechanics as it is usually taught is demoted to a mere part of the theory called phenomenology of observations, and the common impression about its contradiction to realism is explained. The main idea of the paper, the physical notion of intrinsic properties, is introduced and many examples thereof are given. It replaces the irritating dichotomy of quantum and classical worlds by a much softer difference between intrinsic and extrinsic properties, which concern equally microscopic and macroscopic systems. Finally, the classicality and the quantum measurement are analyzed and found to present some still unsolved problems. A possible way of dealing with the Schr\\"{o}dinger cat is suggested that is base...

Hajicek, P

2008-01-01

87

Classical and Quantum Discrete Dynamical Systems

We study deterministic and quantum dynamics from a constructive "finite" point of view, since the introduction of a continuum, or other actual infinities in physics poses serious conceptual and technical difficulties, without any need for these concepts to physics as an empirical science. For a consistent description of the symmetries of dynamical systems at different times and the symmetries of the various parts of such systems, we introduce discrete analogs of the gauge connections. Gauge structures are particularly important to describe the quantum behavior. We show that quantum behavior is the result of a fundamental inability to trace the identity of indistinguishable objects in the process of evolution. Information is available only on invariant statements and values, relating to such objects. Using mathematical arguments of a general nature we can show that any quantum dynamics can be reduced to a sequence of permutations. Quantum interferences occur in the invariant subspaces of permutation representa...

Kornyak, Vladimir V

2013-01-01

88

An Application of Quantum Finite Automata to Interactive Proof Systems

Quantum finite automata have been studied intensively since their introduction in late 1990s as a natural model of a quantum computer with finite-dimensional quantum memory space. This paper seeks their direct application to interactive proof systems in which a mighty quantum prover communicates with a quantum-automaton verifier through a common communication cell. Our quantum interactive proof systems are juxtaposed to Dwork-Stockmeyer's classical interactive proof systems whose verifiers are two-way probabilistic automata. We demonstrate strengths and weaknesses of our systems and further study how various restrictions on the behaviors of quantum-automaton verifiers affect the power of quantum interactive proof systems.

Nishimura, H; Nishimura, Harumichi; Yamakami, Tomoyuki

2004-01-01

89

Thermal rectification in quantum graded mass systems

We show the existence of thermal rectification in the graded mass quantum chain of harmonic oscillators with self-consistent reservoirs. Our analytical study allows us to identify the ingredients leading to the effect. The presence of rectification in this effective, simple model (representing graded mass materials, systems that may be constructed in practice) indicates that rectification in graded mass quantum systems may be an ubiquitous phenomenon. Moreover, as the classical version of this model does not present rectification, our results show that, here, rectification is a direct result of the quantum statistics.

Pereira, Emmanuel

2010-01-01

90

The classical capacity of a quantum dense coding system

Energy Technology Data Exchange (ETDEWEB)

Quantum dense coding transmits classical information by sending a quantum system with the assistance of quantum entanglement. The classical information capacity of a quantum dense coding system is obtained, where a sender and receiver share a completely entangled state and a quantum system encoded by applying unitary operators is sent through an arbitrary quantum channel. The result is compared with that obtained in another setting. (letter to the editor)

Ban, Masashi [Advanced Research Laboratory, Hitachi, Ltd, 2520 Akanuma, Hatoyama, Saitama 350-0395 (Japan); Kitajima, Sachiko [Graduate School of Humanities and Sciences, Ochanomizu University, 2-1-1 Ohtsuka, Bunkyo-ku, Tokyo 112-8610 (Japan); Shibata, Fumiaki [Department of Physics, Faculty of Science, Ochanomizu University, 2-1-1 Ohtsuka, Bunkyo-ku, Tokyo 112-8610 (Japan)

2004-09-03

91

Hybrid Impulsive Control for Closed Quantum Systems

The state transfer problem of a class of nonideal quantum systems is investigated. It is known that traditional Lyapunov methods may fail to guarantee convergence for the nonideal case. Hence, a hybrid impulsive control is proposed to accomplish a more accurate convergence. In particular, the largest invariant sets are explicitly characterized, and the convergence of quantum impulsive control systems is analyzed accordingly. Numerical simulation is also presented to demonstrate the improvement of the control performance.

Sun, Jitao; Lin, Hai

2013-01-01

92

Hybrid impulsive control for closed quantum systems.

UK PubMed Central (United Kingdom)

The state transfer problem of a class of nonideal quantum systems is investigated. It is known that traditional Lyapunov methods may fail to guarantee convergence for the nonideal case. Hence, a hybrid impulsive control is proposed to accomplish a more accurate convergence. In particular, the largest invariant sets are explicitly characterized, and the convergence of quantum impulsive control systems is analyzed accordingly. Numerical simulation is also presented to demonstrate the improvement of the control performance.

Zhao S; Sun J; Lin H

2013-01-01

93

Integral formulas for quantum isomonodromic systems

We conisder time-dependent Schr\\"odinger systems, which are quantizations of the Hamiltonian systems obtained from a similarity reduction of the Drinfeld-Sokolov hierarchy by K. Fuji and T. Suzuki, and a similarity reduction of the UC hierarchy by T.Tsuda, independently. These Hamiltonian systems describe isomonodromic deformations for certain Fuchsian systems. Thus, our Schr\\"odinger systems can be regarded as quantum isomonodromic systems. Y. Yamada conjectured that our quantum isomonodromic systems determine instanton partition functions in N=2 SU(L) gauge theory. The main purpose of this paper is to present integral formulas as particular solutions to our quantum isomonodromic systems. These integral formulas are generalizations of the generalized hypergeometric function.

Nagoya, Hajime

2012-01-01

94

Quantum weak chaos in a degenerate system

Quantum weak chaos is studied in a perturbed degenerate system --- a charged particle interacting with a monochromatic wave in a transverse magnetic field. The evolution operator for an arbitrary number of periods of the external field is built and its structure is explored in terms of the QE (quasienergy eigenstates) under resonance condition (wave frequency $=$ cyclotron frequency) in the regime of weak classical chaos. The new phenomenon of diffusion via the quantum separatrices and the influence of chaos on diffusion are investigated and, in the quasi classical limit, compared with its classical dynamics. We determine the crossover from purely quantum diffusion to a diffusion which is the quantum manifestation of classical diffusion along the stochastic web. This crossover results from the non-monotonic dependence of the characteristic localization length of the QE states on the wave amplitude. The width of the quantum separatrices was computed and compared with the width of the classical stochastic web. ...

Demikhovskii, V Y; Luna-Acosta, G A

1998-01-01

95

Isoperiodic classical systems and their quantum counterparts

One-dimensional isoperiodic classical systems have been first analyzed by Abel. Abel's characterization can be extended for singular potentials and potentials which are not defined on the whole real line. The standard shear equivalence of isoperiodic potentials can also be extended by using reflection and inversion transformations. We provide a full characterization of isoperiodic rational potentials showing that they are connected by translations, reflections or Joukowski transformations. Upon quantization many of these isoperiodic systems fail to exhibit identical quantum energy spectra. This anomaly occurs at order O(h^2) because semiclassical corrections of energy levels of order O(h) are identical for all isoperiodic systems. We analyze families of systems where this quantum anomaly occurs and some special systems where the spectral identity is preserved by quantization. Conversely, we point out the existence of isospectral quantum systems which do not correspond to isoperiodic classical systems.

Asorey, M; Marmo, G; Perelomov, A

2007-01-01

96

Quantum Phase Interference for Quantum Tunneling in Spin Systems

The point-particle-like Hamiltonian of a biaxial spin particle with external magnetic field along the hard axis is obtained in terms of the potential field description of spin systems with exact spin-coordinate correspondence. The Zeeman energy term turns out to be an effective gauge potential which leads to a nonintegrable pha se of the Euclidean Feynman propagator. The phase interference between clockwise and anticlockwise under barrier propagations is recognized explicitly as the Aharonov-Bohm effect. An additional phase which is significant for quantum phase interference is discovered with the quantum theory of spin systems besides the known phase obtained with the semiclassical treatment of spin. We also show the energ y dependence of the effect and obtain the tunneling splitting at excited states with the help of periodic instantons.

Liang, J Q; Park, D K; Pu, F C

2000-01-01

97

Classical representation of quantum systems at equilibrium

A classical system has been constructed that reproduces the thermodynamics of a quantum system at equilibrium.The classical system has an effective temperature, local chemical potential, and pair interaction that are defined by requiring equivalence of the pressure, density and pair correlation functions for the classical and quantum systems. The thermodynamic parameters of the classical system are determined such that the ideal gas and weak coupling RPA limits are preserved. The pair correlations predicted from this model are in excellent agreement with Diffusion Monte Carlo results at T=0 and with the finite-temperature results from the Perrot-Dharmawardana model [1]. Systems in harmonic confinement have also been studied to look into the quantum effects on shell formation. [1] M. W. C. Dharma-wardana and F. Perrot, Phys. Rev. Lett. 84, 959 (2000).

Dutta, Sandipan; Dufty, James

2013-03-01

98

Equilibration of quantum systems and subsystems

We unify two recent results concerning equilibration in quantum theory. We first generalise a proof of Reimann [PRL 101,190403 (2008)], that the expectation value of 'realistic' quantum observables will equilibrate under very general conditions, and discuss its implications for the equilibration of quantum systems. We then use this to re-derive an independent result of Linden et. al. [PRE 79, 061103 (2009)], showing that small subsystems generically evolve to an approximately static equilibrium state. Finally, we consider subspaces in which all initial states effectively equilibrate to the same state.

Short, Anthony J

2010-01-01

99

Equilibration of quantum systems and subsystems

International Nuclear Information System (INIS)

[en] We unify two recent results concerning equilibration in quantum theory. We first generalize a proof of Reimann (2008 Phys. Rev. Lett. 101 190403), that the expectation value of 'realistic' quantum observables will equilibrate under very general conditions, and discuss its implications for the equilibration of quantum systems. We then use this to re-derive an independent result of Linden et al (2009 Phys. Rev. E 79 061103), showing that small subsystems generically evolve to an approximately static equilibrium state. Finally, we consider subspaces in which all initial states effectively equilibrate to the same state.

2011-01-01

100

Computational Studies of Quantum Spin Systems

These lecture notes introduce quantum spin systems and several computational methods for studying their ground-state and finite-temperature properties. Symmetry-breaking and critical phenomena are first discussed in the simpler setting of Monte Carlo studies of classical spin systems, to illustrate finite-size scaling at continuous and first-order phase transitions. Exact diagonalization and quantum Monte Carlo (stochastic series expansion) algorithms and their computer implementations are then discussed in detail. Applications of the methods are illustrated by results for some of the most essential models in quantum magnetism, such as the S=1/2 Heisenberg antiferromagnet in one and two dimensions, as well as extended models useful for studying quantum phase transitions between antiferromagnetic and magnetically disordered states.

Sandvik, Anders W

2011-01-01

101

Weakly-coupled systems in quantum control

This paper provides rigorous definitions and analysis of the dynamics of weakly-coupled systems and gives sufficient conditions for an infinite dimensional quantum control system to be weakly-coupled. As an illustration we provide examples chosen among common physical systems.

Boussaid, Nabile; Chambrion, Thomas

2011-01-01

102

Isospectrality for quantum toric integrable systems

We settle affirmatively the isospectral problem for quantum toric integrable systems: the semiclassical joint spectrum of such a system, given by a sequence of commuting Toeplitz operators on a sequence of Hilbert spaces, determines the classical integrable system given by the symplectic manifold and Poisson commuting functions, up to symplectomorphisms. We also give a full description of the semiclassical spectral theory of quantum toric integrable systems. This type of problem belongs to the realm of classical questions in spectral theory going back to pioneer works of Colin de Verdiere, Guillemin, Sternberg and others in the 1970s and 1980s.

Charles, Laurent; Ngoc, San Vu

2011-01-01

103

Symmetric and asymmetric quantum channels in quantum communication systems

Energy Technology Data Exchange (ETDEWEB)

Symmetric and asymmetric quantum channels which act on bipartite bosonic states are considered. The linear dissipative channel and the quantum teleportation channel are applied. The influences of the symmetric and asymmetric quantum channels on bipartite Gaussian states are investigated by means of the inseparability condition. Furthermore, quantum teleportation and quantum dense coding of continuous variables performed by means of two-mode squeezed-vacuum states under the influence of the noisy quantum channels are discussed.

Ban, Masashi [Advanced Research Laboratory, Hitachi, Ltd, 2520 Akanuma, Hatoyama, Saitama 350-0395 (Japan); CREST, Japan Science and Technology Agency, 1-1-9 Yaesu, Chuo-ku, Tokyo 103-0028 (Japan)

2005-04-22

104

Complex quantum systems analysis of large Coulomb systems

This volume is based on lectures given during the program Complex Quantum Systems held at the National University of Singapore's Institute for Mathematical Sciences from 17 February to 27 March 2010. It guides the reader through two introductory expositions on large Coulomb systems to five of the most important developments in the field: derivation of mean field equations, derivation of effective Hamiltonians, alternative high precision methods in quantum chemistry, modern many body methods originating from quantum information, and - the most complex - semirelativistic quantum electrodynamics.

Siedentop, Heinz

2013-01-01

105

Witnessing Quantum Coherence: from solid-state to biological systems

Quantum coherence is one of the primary non-classical features of quantum systems. While protocols such as the Leggett-Garg inequality (LGI) and quantum tomography can be used to test for the existence of quantum coherence and dynamics in a given system, unambiguously detecting inherent ``quantumness'' still faces serious obstacles in terms of experimental feasibility and efficiency, particularly in complex systems. Here we introduce two ``quantum witnesses'' to efficiently verify quantum coherence and dynamics in the time domain, without the expense and burden of non-invasive measurements or full tomographic processes. Using several physical examples, including quantum transport in solid-state nanostructures and in biological organisms, we show that these quantum witnesses are robust and have a much finer resolution in their detection window than the LGI has. These robust quantum indicators may assist in reducing the experimental overhead in unambiguously verifying quantum coherence in complex systems.

Li, Che-Ming; Lambert, Neill; Chen, Yueh-Nan; Chen, Guang-Yin; Nori, Franco

2012-11-01

106

Quantum games in open systems using biophysical Hamiltonians

International Nuclear Information System (INIS)

We analyze the necessary physical conditions to model an open quantum system as a quantum game. By applying the formalism of quantum operations on a particular system, we use Kraus operators as quantum strategies. The physical interpretation is a conflict among different configurations of the environment. The resolution of the conflict displays regimes of minimum loss of information.

2006-09-25

107

Scattering theory for open quantum systems

Energy Technology Data Exchange (ETDEWEB)

Quantum systems which interact with their environment are often modeled by maximal dissipative operators or so-called Pseudo-Hamiltonians. In this paper the scattering theory for such open systems is considered. First it is assumed that a single maximal dissipative operator A{sub D} in a Hilbert space H is used to describe an open quantum system. In this case the minimal self-adjoint dilation K of A{sub D} can be regarded as the Hamiltonian of a closed system which contains the open system {l_brace}A{sub D},h{r_brace}, but since K is necessarily not semibounded from below, this model is difficult to interpret from a physical point of view. In the second part of the paper an open quantum system is modeled with a family {l_brace}A({mu}){r_brace} of maximal dissipative operators depending on energy {mu}, and it is shown that the open system can be embedded into a closed system where the Hamiltonian is semibounded. Surprisingly it turns out that the corresponding scattering matrix can be completely recovered from scattering matrices of single Pseudo-Hamiltonians as in the first part of the paper. The general results are applied to a class of Sturm-Liouville operators arising in dissipative and quantum transmitting Schroedinger-Poisson systems. (orig.)

Behrndt, Jussi [Technische Univ. Berlin (Germany). Inst. fuer Mathematik; Malamud, Mark M. [Donetsk National University (Ukraine). Dept. of Mathematics; Neidhardt, Hagen [Weierstrass-Institut fuer Angewandte Analysis und Stochastik (WIAS) im Forschungsverbund Berlin e.V. (Germany)

2006-07-01

108

Recent advances in quantum integrable systems

Energy Technology Data Exchange (ETDEWEB)

This meeting was dedicated to different aspects of the theory of quantum integrable systems. The organizers have intended to concentrate on topics related to the study of correlation functions, to systems with boundaries and to models at roots of unity. This document gathers the abstracts of 32 contributions, most of the contributions are accompanied by the set of transparencies.

Amico, L.; Belavin, A.; Buffenoir, E.; Castro Alvaredo, A.; Caudrelier, V.; Chakrabarti, A.; Corrig, E.; Crampe, N.; Deguchi, T.; Dobrev, V.K.; Doikou, A.; Doyon, B.; Feher, L.; Fioravanti, D.; Gohmann, F.; Hallnas, M.; Jimbo, M.; Konno, N.C.H.; Korchemsky, G.; Kulish, P.; Lassalle, M.; Maillet, J.M.; McCoy, B.; Mintchev, M.; Pakuliak, S.; Quano, F.Y.Z.; Ragnisco, R.; Ravanini, F.; Rittenberg, V.; Rivasseau, V.; Rossi, M.; Satta, G.; Sedrakyan, T.; Shiraishi, J.; Suzuki, N.C.J.; Yamada, Y.; Zamolodchikov, A.; Ishimoto, Y.; Nagy, Z.; Posta, S.; Sedra, M.B.; Zuevskiy, A.; Gohmann, F

2005-07-01

109

Recent advances in quantum integrable systems

International Nuclear Information System (INIS)

[en] This meeting was dedicated to different aspects of the theory of quantum integrable systems. The organizers have intended to concentrate on topics related to the study of correlation functions, to systems with boundaries and to models at roots of unity. This document gathers the abstracts of 32 contributions, most of the contributions are accompanied by the set of transparencies

2005-01-01

110

Simulation of n-qubit quantum systems. I. Quantum registers and quantum gates

During recent years, quantum computations and the study of n-qubit quantum systems have attracted a lot of interest, both in theory and experiment. Apart from the promise of performing quantum computations, however, these investigations also revealed a great deal of difficulties which still need to be solved in practice. In quantum computing, unitary and non-unitary quantum operations act on a given set of qubits to form (entangled) states, in which the information is encoded by the overall system often referred to as quantum registers. To facilitate the simulation of such n-qubit quantum systems, we present the FEYNMAN program to provide all necessary tools in order to define and to deal with quantum registers and quantum operations. Although the present version of the program is restricted to unitary transformations, it equally supports—whenever possible—the representation of the quantum registers both, in terms of their state vectors and density matrices. In addition to the composition of two or more quantum registers, moreover, the program also supports their decomposition into various parts by applying the partial trace operation and the concept of the reduced density matrix. Using an interactive design within the framework of MAPLE, therefore, we expect the FEYNMAN program to be helpful not only for teaching the basic elements of quantum computing but also for studying their physical realization in the future. Catalogue number:ADWE Program summary URL:http://cpc.cs.qub.ac.uk/summaries/ADWE Program obtainable from:CPC Program Library, Queen's University of Belfast, N. Ireland Licensing provisions:None Computers for which the program is designed:All computers with a license of the computer algebra system MAPLE [Maple is a registered trademark of Waterlo Maple Inc.] Operating systems or monitors under which the program has been tested:Linux, MS Windows XP Programming language used:MAPLE 9.5 (but should be compatible with 9.0 and 8.0, too) Memory and time required to execute with typical data:Storage and time requirements critically depend on the number of qubits, n, in the quantum registers due to the exponential increase of the associated Hilbert space. In particular, complex algebraic operations may require large amounts of memory even for small qubit numbers. However, most of the standard commands (see Section 4 for simple examples) react promptly for up to five qubits on a normal single-processor machine (?1GHz with 512 MB memory) and use less than 10 MB memory. No. of lines in distributed program, including test data, etc.: 8864 No. of bytes in distributed program, including test data, etc.: 493?182 Distribution format: tar.gz Nature of the physical problem:During the last decade, quantum computing has been found to provide a revolutionary new form of computation. The algorithms by Shor [P.W. Shor, SIAM J. Sci. Statist. Comput. 26 (1997) 1484] and Grover [L.K. Grover, Phys. Rev. Lett. 79 (1997) 325. ], for example, gave a first impression how one could solve problems in the future, that are intractable otherwise with all classical computers. Broadly speaking, quantum computing applies quantum logic gates (unitary transformations) on a given set of qubits, often referred to a quantum registers. Although, the theoretical foundation of quantum computing is now well understood, there are still many practical difficulties to be overcome for which (classical) simulations on n-qubit systems may help understand how quantum algorithms work in detail and what kind of physical systems and environments are most suitable for their realization. Method of solution:Using the computer algebra system MAPLE, a set of procedures has been developed to define and to deal with n-qubit quantum registers and quantum logic gates. It provides a hierarchy of commands which can be applied interactively and which is flexible enough to incorporate non-unitary quantum operations and quantum error corrections models in the future. Restrictions on the complexity of the problem:The present version of the program facilitates the set-up and

Radtke, T.; Fritzsche, S.

2005-12-01

111

Tunneling with dissipation in open quantum systems

International Nuclear Information System (INIS)

Based on the general form of the master equation for open quantum systems the tunneling is considered. Using the path integral technique a simple closed form expression for the tunneling rate through a parabolic barrier is obtained. The tunneling in the open quantum systems strongly depends on the coupling with environment. We found the cases when the dissipation prohibits tunneling through the barrier but decreases the crossing of the barrier for the energies above the barrier. As a particular application, the case of decay from the metastable state is considered

1997-01-01

112

System Design for a Long-Line Quantum Repeater

We present a new control algorithm and system design for a network of quantum repeaters, and outline the end-to-end protocol architecture. Such a network will create long-distance quantum states, supporting quantum key distribution as well as distributed quantum computation. Quantum repeaters improve the reduction of quantum-communication throughput with distance from exponential to polynomial. Because a quantum state cannot be copied, a quantum repeater is not a signal amplifier, but rather executes algorithms for quantum teleportation in conjunction with a specialized type of quantum error correction called purification to raise the fidelity of the quantum states. We introduce our banded purification scheme, which is especially effective when the fidelity of coupled qubits is low, improving the prospects for experimental realization of such systems. The resulting throughput is calculated via detailed simulations of a long line composed of shorter hops. Our algorithmic improvements increase throughput by a f...

Van Meter, Rodney; Munro, W J; Nemoto, Kae

2007-01-01

113

The thesis comprises two major themes of quantum statistical dynamics. One is the development of quantum dissipation theory (QDT). It covers the establishment of some basic relations of quantum statistical dynamics, the construction of several nonequivalent complete second-order formulations, and the development of exact QDT. Another is related to the applications of quantum statistical dynamics to a variety of research fields. In particular, unconventional but novel theories of the electron transfer in Debye solvents, quantum transport, and quantum measurement are developed on the basis of QDT formulations. The thesis is organized as follows. In Chapter 1, we present some background knowledge in relation to the aforementioned two themes of this thesis. The key quantity in QDT is the reduced density operator rho(t) ? trBrho T(t); i.e., the partial trace of the total system and bath composite rhoT(t) over the bath degrees of freedom. QDT governs the evolution of reduced density operator, where the effects of bath are treated in a quantum statistical manner. In principle, the reduced density operator contains all dynamics information of interest. However, the conventional quantum transport theory is formulated in terms of nonequilibrium Green's function. The newly emerging field of quantum measurement in relation to quantum information and quantum computing does exploit a sort of QDT formalism. Besides the background of the relevant theoretical development, some representative experiments on molecular nanojunctions are also briefly discussed. In chapter 2, we outline some basic (including new) relations that highlight several important issues on QDT. The content includes the background of nonequilibrium quantum statistical mechanics, the general description of the total composite Hamiltonian with stochastic system-bath interaction, a novel parameterization scheme for bath correlation functions, a newly developed exact theory of driven Brownian oscillator (DBO) systems, and its closely related solvation mode transformation of system-bath coupling Hamiltonian in general. The exact QDT of DBO systems is also used to clarify the validity of conventional QDT formulations that involve Markovian approximation. In Chapter 3, we develop three nonequivalent but all complete second-order QDT (CS-QDT) formulations. Two of them are of the conventional prescriptions in terms of time-local dissipation and memory kernel, respectively. The third one is called the correlated driving-dissipation equations of motion (CODDE). This novel CS-QDT combines the merits of the former two for its advantages in both the application and numerical implementation aspects. Also highlighted is the importance of correlated driving-dissipation effects on the dynamics of the reduced system. In Chapter 4, we construct an exact QDT formalism via the calculus on path integrals. The new theory aims at the efficient evaluation of non-Markovian dissipation beyond the weak system-bath interaction regime in the presence of time-dependent external field. By adopting exponential-like expansions for bath correlation function, hierarchical equations of motion formalism and continued fraction Liouville-space Green's function formalism are established. The latter will soon be used together with the Dyson equation technique for an efficient evaluation of non-perturbative reduced density matrix dynamics. The interplay between system-bath interaction strength, non-Markovian property, and the required level of hierarchy is also studied with the aid of simple spin-boson systems, together with the three proposed schemes to truncate the infinite hierarchy. In Chapter 5, we develop a nonperturbative theory of electron transfer (ET) in Debye solvents. The resulting exact and analytical rate expression is constructed on the basis of the aforementioned continued fraction Liouville-space Green's function formalism, together with the Dyson equation technique. Not only does it recover the celebrated Marcus' inversion and Kramers' turnover behaviors, the new theory also shows some disti

Cui, Ping

114

Feshbach projection formalism for open quantum systems.

UK PubMed Central (United Kingdom)

We provide a new approach to open quantum systems which is based on the Feshbach projection method. Instead of looking for a master equation for the dynamical map acting in the space of density operators we provide the corresponding equation for the evolution in the Hilbert space of the amplitude operators. Its solution enables one to construct a legitimate quantum evolution (completely positive and trace preserving). Our approach, contrary to the standard Nakajima-Zwanzig method, allows for a series of consistent approximations resulting in a legitimate quantum evolution. The new scheme is illustrated by the well-known spin-boson model beyond the rotating wave approximation. It is shown that the presence of counterrotating terms dramatically changes the asymptotic evolution of the system.

Chru?ci?ski D; Kossakowski A

2013-08-01

115

Semiclassical simulation of open quantum systems

Energy Technology Data Exchange (ETDEWEB)

We present an approach for the semiclassical treatment of open quantum systems. An expansion into localized states allows to restrict a simulation to a fraction of the environment that is located within a predefined vicinity of the system. Our approach allows to add and drop environmental particles during the simulation what provides the basis for an effective reduction of the size of the system that is being treated.

Mintert, Florian [Albert-Ludwigs-Universitaet Freiburg, Hermann-Herder-Str. 3, 79104 Freiburg (Germany)]|[Department of Physics, Harvard University, 17 Oxford Street, Cambridge, MA 02138 (United States); Heller, Eric [Department of Physics, Harvard University, 17 Oxford Street, Cambridge, MA 02138 (United States)

2008-07-01

116

Quantum-mechanical aspects of classically chaotic driven systems

Energy Technology Data Exchange (ETDEWEB)

This paper treats atoms and molecules in laser fields as periodically driven quantum systems. The paper concludes by determining that stochastic excitation is possible in quantum systems with quasiperiodic driving. 17 refs. (JDH)

Milonni, P.W.; Ackerhalt, J.R.; Goggin, M.E.

1987-01-01

117

Quantum field theory and multiparticle systems

International Nuclear Information System (INIS)

The use of quantum field theory methods for the investigation of the physical characteristics of the MANY-BODY SYSTEMS is discussed. Mainly discussed is the method of second quantization and the method of the Green functions. Briefly discussed is the method of calculating the Green functions at finite temperatures. (Z.J.)

1981-01-01

118

Economical ontological models for discrete quantum systems

I use the recently proposed framework of ontological models [arXiv:0709.1149v2] to obtain economical ontological models for results of tomographically complete sets of measurements on finite-dimensional quantum systems. I describe how to obtain models with small numbers of ontic states, and present an explicit model with just 34 ontic states for a qutrit.

Galvao, Ernesto F

2009-01-01

119

Black Holes and Nonrelativistic Quantum Systems

Digital Repository Infrastructure Vision for European Research (DRIVER)

We describe black holes in d+3 dimensions, whose thermodynamic properties correspond to those of a scale-invariant nonrelativistic (d+1)-dimensional quantum system with a dynamical exponent z=2. The gravitational model involves a massive Abelian vector field and a scalar field, in addition to the me...

Nickel, Marcel Dominik Johannes; Kovtun, Pavel

120

Wigner quantum systems (Lie superalgebraic approach)

We present three groups of examples of Wigner Quantum Systems related to the Lie superalgebras $osp(1/6n)$, $sl(1/3n)$ and $sl(n/3)$ and discuss shortly their physical features. In the case of $sl(1/3n)$ we indicate that the underlying geometry is noncommutative.

Palev, T D

2002-01-01

121

Hidden supersymmetry in quantum bosonic systems

International Nuclear Information System (INIS)

[en] We show that some simple well-studied quantum mechanical systems without fermion (spin) degrees of freedom display, surprisingly, a hidden supersymmetry. The list includes the bound state Aharonov-Bohm, the Dirac delta and the Poeschl-Teller potential problems, in which the unbroken and broken N = 2 supersymmetry of linear and nonlinear (polynomial) forms is revealed

2007-01-01

122

Hidden supersymmetry in quantum bosonic systems

We show that some related quantum mechanical systems without fermion (spin) degrees of freedom display a hidden supersymmetry. The list includes the bound state Aharonov-Bohm, the Dirac delta and the Poschl-Teller potential problems, in which the unbroken and broken N=2 supersymmetry of linear and nonlinear (polynomial) forms is revealed.

Correa, F; Correa, Francisco; Plyushchay, Mikhail S.

2006-01-01

123

Local Defect in Metallic Quantum Critical Systems

Energy Technology Data Exchange (ETDEWEB)

We present a theory of a single point, line, or plane defect coupling to the square of the order parameter in a metallic system near a quantum critical point at or above its upper critical dimension. At criticality, a spin droplet is nucleated around the defect with its core size determined by the strength of the defect potential. Outside the core a universal slowly decaying tail of the droplet is found, leading to many dissipative channels coupling to the droplet and to a complete suppression of quantum tunneling. We propose an NMR experiment to measure the impurity-induced changes in the local spin susceptibility.

Millis, A. J.; Morr, D. K.; Schmalian, J.

2001-10-15

124

Symmetry and stability of open quantum systems

International Nuclear Information System (INIS)

The presentation of the thesis involves an introduction and six chapters. Chapter 1 presents notions and results used in the other chpaters. Chapters 2-6 present our results which are focused on two notions: generalized observable and dynamic semigroup. These notions characterize a specific research domain (set up during the last 10 years) which is currently called quantum mechanics of open systems. The two notions (generalized observable and dynamic semigroup) are mathematically correlated. They belong to the set of completely positive linear applications among observable algebras. This fact, associated with that formulation of quantum mechanics according to which it is a special case of quantum mechanics namely, that for which the observable algebra is commutative, help to understand the similar essence of the results presented in chapter 2-6. Thus, the natural mathematical background has been achieved for our results; it is represented by that category whose objects are the observable algebras and whose morphisms are completely positive linear contractions generating unity within unity. These ideas are extensively presented in the introduction. The fact that the relations between classical mechanics and quantum mechanics can be rigorously treated as positive linear applications between classical observable algebras commutative and quantum observable algebras non-commutative, which are automatically fully positive, has been initially shown in our paper. (author)

1979-01-01

125

Dynamics of quantum trajectories in chaotic systems

Quantum trajectories defined in the de Broglie--Bohm theory provide a causal way to interpret physical phenomena. In this Letter, we use this formalism to analyze the short time dynamics induced by unstable periodic orbits in a classically chaotic system, a situation in which scars are known to play a very important role. We find that the topologies of the quantum orbits are much more complicated than that of the scarring and associated periodic orbits, since the former have quantum interference built in. Thus scar wave functions are necessary to analyze the corresponding dynamics. Moreover, these topologies imply different return routes to the vicinity of the initial positions, and this reflects in the existence of different contributions in each peak of the survival probability function.

Wisniacki, D A; Benito, R M

2003-01-01

126

Theory of classical and quantum frustration in quantum many-body systems

We present a general scheme for the study of frustration in quantum systems. After introducing a universal measure of frustration for arbitrary quantum systems, we derive for it an exact inequality in terms of a class of entanglement monotones. We then state sufficient conditions for the ground states of quantum spin systems to saturate the inequality and confirm them with extensive numerical tests. These conditions provide a generalization to the quantum domain of the Toulouse criteria for classical frustration-free systems and establish a unified framework for studying the intertwining of geometric and quantum contributions to frustration.

Giampaolo, S M; Monras, A; Illuminati, F

2011-01-01

127

Dissipative effects on quantum glassy systems

We discuss the behavior of a quantum glassy system coupled to a bath of quantum oscillators. We show that the system localizes in the absence of interactions when coupled to a subOhmic bath. When interactions are switched on localization disappears and the system undergoes a phase transition towards a glassy phase. We show that the position of the critical line separating the disordered and the ordered phases strongly depends on the coupling to the bath. For a given type of bath, the ordered glassy phase is favored by a stronger coupling. Ohmic, subOhmic and superOhmic baths lead to different transition lines. We draw our conclusions from the analysis of the partition function using the replicated imaginary-time formalism and from the study of the real-time dynamics of the coupled system using the Schwinger-Keldysh closed time-path formalism.

Cugliandolo, L F; Lozano, G S; Lozza, H; De Silva-Santos, C; Cugliandolo, Leticia F.; Grempel, Daniel; Lozano, Gustavo; Lozza, Homero; Santos, Constantino da Silva

2001-01-01

128

Planar lightwave circuits for quantum cryptographic systems

We propose a quantum cryptographic system based on a planar lightwave circuit (PLC) and report on optical interference experiments using PLC-based unbalanced Mach-Zehnder interferometers (MZIs). The interferometers exhibited high-visibility (>0.98) interference even when the polarisation in the optical fibre connecting the two MZIs was randomly modulated. The results demonstrate that a PLC-based setup is suitable for achieving a polarisation-insensitive phase-coding cryptographic system.

Nambu, Y; Nakamura, K; Nambu, Yoshihiro; Hatanaka, Takaaki; Nakamura, Kazuo

2003-01-01

129

Repeated Interaction Quantum Systems: Deterministic and Random

This paper gives an overview of recent results concerning the long time dynamics of repeated interaction quantum systems in a deterministic and random framework. We describe the non equilibrium steady states (NESS) such systems display and we present, as a macroscopic consequence, a second law of thermodynamics these NESS give rise to. We also explain in some details the analysis of products of certain random matrices underlying this dynamical problem.

Joye, Alain

2008-01-01

130

Entanglement entropy of a simple quantum system

We propose a simple approach to the calculation of the entanglement entropy of a spherically symmetric quantum system composed of two separate regions. We consider bound states of the system described by a wave function that is scale invariant and vanishes exponentially at infinity. Our result is in accordance with the holographic bound on entropy and shows that entanglement entropy scales with the area of the boundary.

Melis, Maurizio

2009-01-01

131

Directory of Open Access Journals (Sweden)

Full Text Available In this article we give a generalization of Hartley's model for the measure of information. We propose a rate of emergence, which is applicable to systems obeying classical or quantum statistics. Quantum sys-tems that obey Fermi-Dirac statistics and Bose-Einstein condensate, as well as classical systems obey-ing the Maxwell-Boltzmann statistics have been con-sidered. We found that the emergence parameter of quantum and classical systems differ as well as the emergence parameter of quantum systems of fermions and bosons. Consequently, the emergence parameter might be used to distinguish the classical system and quantum system, as well as quantum system of fermions and the quantum system of bosons

Lutsenko Y. V.; Trunev A. P.

2013-01-01

132

Geometric description for quantum dynamical systems

International Nuclear Information System (INIS)

[en] A pair of an order-unit space and a base-norm space in spectral duality (in the sense of the geometric spectral theory of E. Alfsen and F. Schultz) is used as a basis for a geometric description of a general quantum system. The geometric spectral theory is combined with an appropriate Lie-algebraic structure in the space of the variables, the positive cone and the other unit being invariant under the adjoint representation of the corresoponding Lie group. The resulting formal structure can be briefly described as invariantly ordered spectral Lie algebra and appears as a generalization of the operator or W*-algebraic models for quantum dynamical systems. In the geometric description, the usual relation between the Hamiltonian and the dynamics of the system is preserved. The geometric description is in principle complete, and at the same time avoids any explicit reference to non-commutative associative multiplication

1985-01-01

133

Quantum phase transitions in finite systems

The aim of this work is to develop a technique for identifying quantum phase transitions which does not rely on the existence of a thermodynamic limit, for studies in finite systems. The approach we adopt exploits an exact mapping of the spectrum of a many-body integrable system, which admits an exact Bethe ansatz solution, into the quasi-exactly solvable spectrum of a one-particle Schrodinger operator. Bifurcations of the minima for the potential of the Schrodinger operator determine critical ground-state couplings. By considering the behaviour of certain ground-state correlation functions, these may be identified as quantum phase transitions in the many-body integrable system with finite particle number. We study two particular examples of bosonic Hamiltonians which admit second-order transitions, and discuss further applications.

Dunning, C; Links, J; Dunning, Clare; Hibberd, Katrina E.; Links, Jon

2006-01-01

134

Nonequilibrium representative ensembles for isolated quantum systems

An isolated quantum system is considered, prepared in a nonequilibrium initial state. In order to uniquely define the system dynamics, one has to construct a representative statistical ensemble. From the principle of least action it follows that the role of the evolution generator is played by a grand Hamiltonian, but not merely by its energy part. A theorem is proved expressing the commutators of field operators with operator products through variational derivatives of these products. A consequence of this theorem is the equivalence of the variational equations for field operators with the Heisenberg equations for the latter. A finite quantum system cannot equilibrate in the strict sense. But it can tend to a quasi-stationary state characterized by ergodic averages and the appropriate representative ensemble depending on initial conditions. Microcanonical ensemble, arising in the eigenstate thermalization, is just a particular case of representative ensembles. Quasi-stationary representative ensembles are de...

Yukalov, V I

2012-01-01

135

Mesoscopic systems: classical irreversibility and quantum coherence.

Mesoscopic physics is a sub-discipline of condensed-matter physics that focuses on the properties of solids in a size range intermediate between bulk matter and individual atoms. In particular, it is characteristic of a domain where a certain number of interacting objects can easily be tuned between classical and quantum regimes, thus enabling studies at the border of the two. In magnetism, such a tuning was first realized with large-spin magnetic molecules called single-molecule magnets (SMMs) with archetype Mn(12)-ac. In general, the mesoscopic scale can be relatively large (e.g. micrometre-sized superconducting circuits), but, in magnetism, it is much smaller and can reach the atomic scale with rare earth (RE) ions. In all cases, it is shown how quantum relaxation can drastically reduce classical irreversibility. Taking the example of mesoscopic spin systems, the origin of irreversibility is discussed on the basis of the Landau-Zener model. A classical counterpart of this model is described enabling, in particular, intuitive understanding of most aspects of quantum spin dynamics. The spin dynamics of mesoscopic spin systems (SMM or RE systems) becomes coherent if they are well isolated. The study of the damping of their Rabi oscillations gives access to most relevant decoherence mechanisms by different environmental baths, including the electromagnetic bath of microwave excitation. This type of decoherence, clearly seen with spin systems, is easily recovered in quantum simulations. It is also observed with other types of qubits such as a single spin in a quantum dot or a superconducting loop, despite the presence of other competitive decoherence mechanisms. As in the molecular magnet V(15), the leading decoherence terms of superconducting qubits seem to be associated with a non-Markovian channel in which short-living entanglements with distributions of two-level systems (nuclear spins, impurity spins and/or charges) leading to 1/f noise induce ?(1)-like relaxation of S(z) with dissipation to the bath of two-level systems with which they interact most. Finally, let us mention that these experiments on quantum oscillations are, most of the time, performed in the classical regime of Rabi oscillations, suggesting that decoherence mechanisms might also be treated classically. PMID:22908339

Barbara, Bernard

2012-09-28

136

Equilibration of quasi-isolated quantum systems

The evolution of a quasi-isolated finite quantum system from a nonequilibrium initial state is considered. The condition of quasi-isolation allows for the description of the system dynamics on the general basis, without specifying the system details and for arbitrary initial conditions. The influence of surrounding results in (at least partial) equilibration and decoherence. The resulting equilibrium state bears information on initial conditions and is characterized by a representative ensemble. It is shown that the system average information with time does not increase. The partial equilibration and non-increase of average information explain the irreversibility of time.

Yukalov, V I

2012-01-01

137

Controllability of multi-partite quantum systems and selective excitation of quantum dots

International Nuclear Information System (INIS)

We consider the degrees of controllability of multi-partite quantum systems, as well as necessary and sufficient criteria for each case. The results are applied to the problem of simultaneous control of an ensemble of quantum dots with a single laser pulse. Finally, we apply optimal control techniques to demonstrate selective excitation of individual dots for a simultaneously controllable ensemble of quantum dots.

2005-10-01

138

Quantum phases and dynamics of geometric phase in a quantum spin chain system under linear quench

We study the quantum phases of anisotropic XY spin chain system in presence and absence of adiabatic quench. A connection between geometric phase and criticality is established from the dynamical behaviour of the geometric phase for a quench induced quantum phase transition in a quantum spin chain. We predict XX criticality associated with a sequence of non-contractible geometric phases.

Sarkar, Sujit

2011-01-01

139

Quantum Correlations Relativity for Continuous-Variables Bipartite Systems

Based on the so-called Entanglement Relativity, we point out relativity of the more general non-classical (quantum) correlations for the continuous-variables bipartite systems. Our observation points out that quantum processing resources based on the non-classical correlations (non-zero quantum discord) are ubiquitous in such systems.

Dugic, M; Jeknic-Dugic, J

2011-01-01

140

Quantum System Engineering (QSE) Frequently Asked Questions (FAQ)

This web-page contains an FAQ offered by the quantum systems engineering group at the University of Washington. The FAQ provides some general information about the utility of quantum microscopy as well as magnetic resonance force microscopy (MRFM).

2005-11-21

141

The Quantum as an Emergent System

Double slit interference is explained with the aid of what we call "21stcentury classical physics". We model a particle as an oscillator ("bouncer") in a thermal context, which is given by some assumed "zero-point" field of the vacuum. In this way, the quantum is understood as an emergent system, i.e., a steady-state system maintained by a constant throughput of (vacuum) energy. To account for the particle's thermal environment, we introduce a "path excitation field", which derives from the thermodynamics of the zero-point vacuum and which represents all possible paths a particle can take via thermal path fluctuations. The intensity distribution on a screen behind a double slit is calculated, as well as the corresponding trajectories and the probability density current. Further, particular features of the relative phase are shown to be responsible for nonlocal effects not only in ordinary quantum theory, but also in our classical approach.

Groessing, Gerhard; Pascasio, Johannes Mesa; Schwabl, Herbert; 10.1088/1742-6596/361/1/012008

2012-01-01

142

Energy Technology Data Exchange (ETDEWEB)

We present a method for probabilistic quantum entanglement swapping between high-dimensional pure entangled systems by introducing only one auxiliary two-level particle. The probability of successful entanglement swapping is just the entanglement of the quantum channel. It can be used for practical long-distance quantum communication efficiently. We present a quantum secret sharing scheme based on quantum entanglement swapping with high-dimensional pure entangled systems. It has the advantage of having high intrinsic qubit efficiency and high capacity. Moreover, it greatly reduces the classical information exchanged for creating the private key.

Zhou Ping; Deng Fuguo; Zhou Hongyu [The Key Laboratory of Beam Technology and Material Modification of Ministry of Education, and Institute of Low Energy Nuclear Physics, Beijing Normal University, Beijing 100875 (China)], E-mail: fgdeng@bnu.edu.cn

2009-03-15

143

International Nuclear Information System (INIS)

We present a method for probabilistic quantum entanglement swapping between high-dimensional pure entangled systems by introducing only one auxiliary two-level particle. The probability of successful entanglement swapping is just the entanglement of the quantum channel. It can be used for practical long-distance quantum communication efficiently. We present a quantum secret sharing scheme based on quantum entanglement swapping with high-dimensional pure entangled systems. It has the advantage of having high intrinsic qubit efficiency and high capacity. Moreover, it greatly reduces the classical information exchanged for creating the private key.

2009-01-01

144

Periodically driven quantum open systems: Tutorial

We present a short derivation and discussion of the master equation for an open quantum system weakly coupled to a heat bath and then its generalization to the case of with periodic external driving based on the Floquet theory. Further, a single heat bath is replaced by several ones. We present also the definition of heat currents which satisfies the second law of thermodynamics and apply the general results to a simple model of periodically modulated qubit.

Alicki, Robert; Kurizki, Gershon

2012-01-01

145

Dynamical Systems and Quantum Bicrossproduct Algebras

We present a unified study of some aspects of quantum bicrossproduct algebras of inhomogeneous Lie algebras, like Poincare, Galilei and Euclidean in N dimensions. The action associated to the bicrossproduct structure allows to obtain a nonlinear action over a new group linked to the translations. This new nonlinear action associates a dynamical system to each generator which is the object of study in this paper.

Arratia, O; Arratia, Oscar; Olmo, Mariano A. del

2002-01-01

146

Quantum chromodynamic evolution of multiquark systems

Energy Technology Data Exchange (ETDEWEB)

We present a new technique which extends the quantum chromodynamic evolution formalism in order to predict the short distance behavior of multiquark wavefunctions. In particular, predictions are given for the deuteron reduced form factor in the high momentum transfer region, and rigorous constraints on the short distance effective force between two baryons are predicted. These new techniques can be generalized in order to analyze the short distance behavior of multibaryon systems.

Ji, C.R.; Brodsky, S.J.

1984-05-01

147

Magnon decay in gapped quantum spin systems

In the O(3) sigma-model description of gapped spin systems, S=1 magnons can only decay into three lower energy magnons. We argue that the symmetry of the quantum spin Hamiltonian often allows decay into two magnons, and compute this decay rate in model systems. Two magnon decay is present in Haldane gap S=1 spin chains, even though it cannot be induced by any allowed term written in powers and gradients of the sigma-model field. We compare our results with recent measurements of Stone et al. (cond-mat/0511266) on a two-dimensional spin system.

Kolezhuk, A; Kolezhuk, Alexei; Sachdev, Subir; 10.1103/.96.087203

2006-01-01

148

Time fractional development of quantum systems

In this study, the effect of time fractionalization on the development of quantum systems is taken under consideration by making use of fractional calculus. In this context, a Mittag-Leffler function is introduced as an important mathematical tool in the generalization of the evolution operator. In order to investigate the time fractional evolution of the quantum (nano) systems, time fractional forms of motion are obtained for a Schrödinger equation and a Heisenberg equation. As an application of the concomitant formalism, the wave functions, energy eigenvalues, and probability densities of the potential well and harmonic oscillator are time fractionally obtained via the fractional derivative order ?, which is a measure of the fractality of time. In the case ?=1, where time becomes homogenous and continuous, traditional physical conclusions are recovered. Since energy and time are conjugate to each other, the fractional derivative order ? is relevant to time. It is understood that the fractionalization of time gives rise to energy fluctuations of the quantum (nano) systems.

Ertik, Hüseyin; Demirhan, Do?an; ?irin, Hüseyin; Büyükk?l?ç, Fevzi

2010-08-01

149

Time fractional development of quantum systems

International Nuclear Information System (INIS)

[en] In this study, the effect of time fractionalization on the development of quantum systems is taken under consideration by making use of fractional calculus. In this context, a Mittag-Leffler function is introduced as an important mathematical tool in the generalization of the evolution operator. In order to investigate the time fractional evolution of the quantum (nano) systems, time fractional forms of motion are obtained for a Schroedinger equation and a Heisenberg equation. As an application of the concomitant formalism, the wave functions, energy eigenvalues, and probability densities of the potential well and harmonic oscillator are time fractionally obtained via the fractional derivative order ?, which is a measure of the fractality of time. In the case ?=1, where time becomes homogenous and continuous, traditional physical conclusions are recovered. Since energy and time are conjugate to each other, the fractional derivative order ? is relevant to time. It is understood that the fractionalization of time gives rise to energy fluctuations of the quantum (nano) systems.

2010-01-01

150

Macroscopic models for quantum systems and computers

International Nuclear Information System (INIS)

We present examples of macroscopic systems entailing a quantum mechanical structure. One of our examples has a structure which is isomorphic to the spin structure for a spin 1/2 and another system entails a structure isomorphic to the structure of two spin 1/2 in the entangled singlet state. We elaborate this system by showing that an arbitrary tensor product state representing two entangled qubits can be described in a complete way by a specific internal constraint between the ray or density states of the two qubits, which describes the behavior of the state of one of the spins if measurements are executed on the other spin. Since any n-qubit unitary operation can be decomposed into 2-qubit gates and unary operations, we argue that our representation of 2-qubit entanglement contributes to a better understanding of the role of n-qubit entanglement in quantum computation. We illustrate our approach on two 2-qubit algorithms proposed by Deutsch, respectively Arvind et al. One of the advantages of the 2-qubit case besides its relative simplicity is that it allows for a nice geometrical representation of entanglement, which contributes to a more intuitive grasp of what is going on in a 2-qubit quantum computation

2007-01-01

151

Macroscopic models for quantum systems and computers

Energy Technology Data Exchange (ETDEWEB)

We present examples of macroscopic systems entailing a quantum mechanical structure. One of our examples has a structure which is isomorphic to the spin structure for a spin 1/2 and another system entails a structure isomorphic to the structure of two spin 1/2 in the entangled singlet state. We elaborate this system by showing that an arbitrary tensor product state representing two entangled qubits can be described in a complete way by a specific internal constraint between the ray or density states of the two qubits, which describes the behavior of the state of one of the spins if measurements are executed on the other spin. Since any n-qubit unitary operation can be decomposed into 2-qubit gates and unary operations, we argue that our representation of 2-qubit entanglement contributes to a better understanding of the role of n-qubit entanglement in quantum computation. We illustrate our approach on two 2-qubit algorithms proposed by Deutsch, respectively Arvind et al. One of the advantages of the 2-qubit case besides its relative simplicity is that it allows for a nice geometrical representation of entanglement, which contributes to a more intuitive grasp of what is going on in a 2-qubit quantum computation.

Aerts, Diederik [Center Leo Apostel, Vrije Universiteit Brussel, Krijgskundestraat 33, 1160 Brussels (Belgium); Czachor, Marek [Katedra Fizyki Teoretycznej i Metod Matematycznych, Politechnika Gdanska, 80-952 Gdansk (Poland); Dehaene, Jeroen [SISTA, Department of Electrical Engineering (ESAT), Faculty of Engineering, Katholieke Universiteit Leuven, 3000 Leuven (Belgium); Moor, Bart De [SISTA, Department of Electrical Engineering (ESAT), Faculty of Engineering, Katholieke Universiteit Leuven, 3000 Leuven (Belgium); D' Hooghe, Bart [Center Leo Apostel, Vrije Universiteit Brussel, Krijgskundestraat 33, 1160 Brussels (Belgium)

2007-05-15

152

Ultracold Quantum Gases and Lattice Systems: Quantum Simulation of Lattice Gauge Theories

Abelian and non-Abelian gauge theories are of central importance in many areas of physics. In condensed matter physics, Abelian U(1) lattice gauge theories arise in the description of certain quantum spin liquids. In quantum information theory, Kitaev's toric code is a Z(2) lattice gauge theory. In particle physics, Quantum Chromodynamics (QCD), the non-Abelian SU(3) gauge theory of the strong interactions between quarks and gluons, is non-perturbatively regularized on a lattice. Quantum link models extend the concept of lattice gauge theories beyond the Wilson formulation, and are well suited for both digital and analog quantum simulation using ultracold atomic gases in optical lattices. Since quantum simulators do not suffer from the notorious sign problem, they open the door to studies of the real-time evolution of strongly coupled quantum systems, which are impossible with classical simulation methods. A plethora of interesting lattice gauge theories suggests itself for quantum simulation, which should al...

Wiese, U -J

2013-01-01

153

Quantum Computing in Condensed Matter Systems.

Specific theoretical calculations of Hamiltonians corresponding to several quantum logic gates, including the NOT gate, quantum signal splitting, and quantum copying, were obtained and prepared for publication. Directions for future work have been identif...

V. Privman

1997-01-01

154

Statistical Thermodynamics of Polymer Quantum Systems

Directory of Open Access Journals (Sweden)

Full Text Available Polymer quantum systems are mechanical models quantized similarly as loop quantum gravity. It is actually in quantizing gravity that the polymer term holds proper as the quantum geometry excitations yield a reminiscent of a polymer material. In such an approach both non-singular cosmological models and a microscopic basis for the entropy of some black holes have arisen. Also important physical questions for these systems involve thermodynamics. With this motivation, in this work, we study the statistical thermodynamics of two one dimensional polymer quantum systems: an ensemble of oscillators that describe a solid and a bunch of non-interacting particles in a box, which thus form an ideal gas. We first study the spectra of these polymer systems. It turns out useful for the analysis to consider the length scale required by the quantization and which we shall refer to as polymer length. The dynamics of the polymer oscillator can be given the form of that for the standard quantum pendulum. Depending on the dominance of the polymer length we can distinguish two regimes: vibrational and rotational. The first occur for small polymer length and here the standard oscillator in Schrödinger quantization is recovered at leading order. The second one, for large polymer length, features dominant polymer effects. In the case of the polymer particles in the box, a bounded and oscillating spectrum that presents a band structure and a Brillouin zone is found. The thermodynamical quantities calculated with these spectra have corrections with respect to standard ones and they depend on the polymer length. When the polymer length is small such corrections resemble those coming from the phenomenological generalized uncertainty relation approach based on the idea of the existence of a minimal length. For generic polymer length, thermodynamics of both systems present an anomalous peak in their heat capacity C_V. In the case of the polymer oscillators this peak separates the vibrational and rotational regimes, while in the ideal polymer gas it reflects the band structure which allows the existence of negative temperatures.

Guillermo Chacón-Acosta; Elisa Manrique; Leonardo Dagdug; Hugo A. Morales-Técotl

2011-01-01

155

Experimental simulation of quantum tunneling in small systems.

UK PubMed Central (United Kingdom)

It is well known that quantum computers are superior to classical computers in efficiently simulating quantum systems. Here we report the first experimental simulation of quantum tunneling through potential barriers, a widespread phenomenon of a unique quantum nature, via NMR techniques. Our experiment is based on a digital particle simulation algorithm and requires very few spin-1/2 nuclei without the need of ancillary qubits. The occurrence of quantum tunneling through a barrier, together with the oscillation of the state in potential wells, are clearly observed through the experimental results. This experiment has clearly demonstrated the possibility to observe and study profound physical phenomena within even the reach of small quantum computers.

Feng GR; Lu Y; Hao L; Zhang FH; Long GL

2013-08-01

156

Experimental simulation of quantum tunneling in small systems

It is well known that quantum computers are superior to classical computers in efficiently simulating quantum systems. Here we report the first experimental simulation of quantum tunneling through potential barriers, a widespread phenomenon of a unique quantum nature, via NMR techniques. Our experiment is based on a digital particle simulation algorithm and requires very few spin-1/2 nuclei without the need of ancillary qubits. The occurrence of quantum tunneling through a barrier, together with the oscillation of the state in potential wells, are clearly observed through the experimental results. This experiment has clearly demonstrated the possibility to observe and study profound physical phenomena within even the reach of small quantum computers.

Feng, Guan-Ru; Lu, Yao; Hao, Liang; Zhang, Fei-Hao; Long, Gui-Lu

2013-01-01

157

Preparing ground States of quantum many-body systems on a quantum computer.

UK PubMed Central (United Kingdom)

Preparing the ground state of a system of interacting classical particles is an NP-hard problem. Thus, there is in general no better algorithm to solve this problem than exhaustively going through all N configurations of the system to determine the one with lowest energy, requiring a running time proportional to N. A quantum computer, if it could be built, could solve this problem in time sqrt[N]. Here, we present a powerful extension of this result to the case of interacting quantum particles, demonstrating that a quantum computer can prepare the ground state of a quantum system as efficiently as it does for classical systems.

Poulin D; Wocjan P

2009-04-01

158

Preparing Ground States of Quantum Many-Body Systems on a Quantum Computer

International Nuclear Information System (INIS)

[en] Preparing the ground state of a system of interacting classical particles is an NP-hard problem. Thus, there is in general no better algorithm to solve this problem than exhaustively going through all N configurations of the system to determine the one with lowest energy, requiring a running time proportional to N. A quantum computer, if it could be built, could solve this problem in time ?(N). Here, we present a powerful extension of this result to the case of interacting quantum particles, demonstrating that a quantum computer can prepare the ground state of a quantum system as efficiently as it does for classical systems

2009-04-03

159

Nature computes: information processing in quantum dynamical systems.

Nature intrinsically computes. It has been suggested that the entire universe is a computer, in particular, a quantum computer. To corroborate this idea we require tools to quantify the information processing. Here we review a theoretical framework for quantifying information processing in a quantum dynamical system. So-called intrinsic quantum computation combines tools from dynamical systems theory, information theory, quantum mechanics, and computation theory. We will review how far the framework has been developed and what some of the main open questions are. On the basis of this framework we discuss upper and lower bounds for intrinsic information storage in a quantum dynamical system. PMID:20887080

Wiesner, Karoline

2010-09-01

160

Nature computes: information processing in quantum dynamical systems.

UK PubMed Central (United Kingdom)

Nature intrinsically computes. It has been suggested that the entire universe is a computer, in particular, a quantum computer. To corroborate this idea we require tools to quantify the information processing. Here we review a theoretical framework for quantifying information processing in a quantum dynamical system. So-called intrinsic quantum computation combines tools from dynamical systems theory, information theory, quantum mechanics, and computation theory. We will review how far the framework has been developed and what some of the main open questions are. On the basis of this framework we discuss upper and lower bounds for intrinsic information storage in a quantum dynamical system.

Wiesner K

2010-09-01

161

Constructing quantum games from a system of Bell's inequalities

We report constructing quantum games directly from a system of Bell's inequalities using Arthur Fine's analysis published in early 1980s. This analysis showed that such a system of inequalities forms a set of both necessary and sufficient conditions required to find a joint distribution function compatible with a given set of joint probabilities, in terms of which the system of Bell's inequalities is usually expressed. Using the setting of a quantum correlation experiment for playing a quantum game, and considering the examples of Prisoners' Dilemma and Matching Pennies, we argue that this approach towards constructing quantum games addresses well known criticism of quantum games.

Iqbal, Azhar

2009-01-01

162

Coherent Nonlinear Feedback of Quantum Systems with Applications to Quantum Optics on Chip

In the control of classical mechanical systems, the feedback has been successfully applied to the production of the desired nonlinear dynamics. However, how much this can be done is still an open problem in quantum mechanical systems. This paper proposes a scheme of generating strong nonlinear quantum effects via the recently developed coherent feedback techniques, which can be shown to outperform the measurement-based quantum feedback scheme that can only generate pseudo-nonlinear quantum effects. Such advancement is demonstrated by two application examples in quantum optics on chip. In the first example, we show that the nonlinear Kerr effect can be generated and amplified to be comparable with the linear effect in a transmission line resonator (TLR). In the second example, we show that by tuning the gains of the quantum amplifiers in a TLR coherent feedback network, non-Gaussian "light" (microwave field) can be generated and manipulated via the nonlinear effects which exhibits fully quantum sub-Poisson pho...

Zhang, Jing; Liu, Yu-xi; Li, Chun-Wen; Tarn, Tzyh-Jong

2011-01-01

163

A kicked quantum system including the continuum

International Nuclear Information System (INIS)

[en] The behaviour of a quantum particle in a separable one-term potential with three-dimensional form factor is investigated under the influence of an external force which alters the potential strength periodically or quasiperiodically. The unperturbed system possesses one bound state and a continuum of scattering states which has treated almost analytically. First numerical results, fully including the emission channel, indicate, for certain parameter combinations with commensurate or incommensurate frequency ratios, either a regular or an irregular dynamical behaviour of the system. 17 refs.; 3 figs

1988-01-01

164

Effective operator formalism for open quantum systems

DEFF Research Database (Denmark)

We present an effective operator formalism for open quantum systems. Employing perturbation theory and adiabatic elimination of excited states for a weakly driven system, we derive an effective master equation which reduces the evolution to the ground-state dynamics. The effective evolution involves a single effective Hamiltonian and one effective Lindblad operator for each naturally occurring decay process. Simple expressions are derived for the effective operators which can be directly applied to reach effective equations of motion for the ground states. We compare our method with the hitherto existing concepts for effective interactions and present physical examples for the application of our formalism, including dissipative state preparation by engineered decay processes.

Reiter, Florentin; SØrensen, Anders SØndberg

2012-01-01

165

Many-body Wigner quantum systems

International Nuclear Information System (INIS)

We present examples of many-body Wigner quantum systems. The position and the momentum operators RA and PA, A = 1, ..., n + 1, of the particles are noncanonical and are chosen so that Heisenberg and the Hamiltonian equations are identical. The spectrum of the energy with respect to the centre of mass is equidistant and has finite number of energy levels. The composite system is spread in a small volume around the centre of mass and within it the geometry is noncommutative. The underlying statistics is an exclusion statistics. (author). 23 refs.

1996-01-01

166

Dynamical Localization in Disordered Quantum Spin Systems

We say that a quantum spin system is dynamically localized if the time-evolution of local observables satisfies a zero-velocity Lieb-Robinson bound. In terms of this definition we have the following main results: First, for general systems with short range interactions, dynamical localization implies exponential decay of ground state correlations, up to an explicit correction. Second, the dynamical localization of random xy spin chains can be reduced to dynamical localization of an effective one-particle Hamiltonian. In particular, the isotropic xy chain in random exterior magnetic field is dynamically localized.

Hamza, Eman; Stolz, Günter

2011-01-01

167

Seniority in quantum many-body systems

The use of the seniority quantum number in many-body systems is reviewed. A brief summary is given of its introduction by Racah in the context of atomic spectroscopy. Several extensions of Racah's original idea are discussed: seniority for identical nucleons in a single-$j$ shell, its extension to the case of many, non-degenerate $j$ shells and to systems with neutrons and protons. To illustrate its usefulness to this day, a recent application of seniority is presented in Bose--Einstein condensates of atoms with spin.

Van Isacker, P

2010-01-01

168

QUANTUM TUNNELLING AND MAGNETIZATION DYNAMICS IN LOW DIMENSIONAL SYSTEMS

Directory of Open Access Journals (Sweden)

Full Text Available Quantum mechanics allows a system to overcome a classically-unsurmountable energy barrier through a mechanism called Quantum Tunnelling (QT). Although pertaining to the quantum domain, QT is the cause of important physical phenomena that can be detected at the macroscopic scale. Some of them have led to breakthrough applications in electronics (tunnel junctions) and imaging (scanning tunnelling microscope).

ANDREA CORNIA

2011-01-01

169

Optimal state estimation for d-dimensional quantum systems

We establish a connection between optimal quantum cloning and optimal state estimation for d-dimensional quantum systems. In this way we derive an upper limit on the fidelity of state estimation for d-dimensional pure quantum states and, furthermore, for generalized inputs supported on the symmetric subspace.

Bruss, D

1999-01-01

170

Conditional density matrix: systems and subsystems in quantum mechanics

International Nuclear Information System (INIS)

A new quantum mechanical notion - Conditional Density Matrix - is discussed and is applied to describe some physical processes. This notion is a natural generalization of von Neumann density matrix for such processes as divisions of quantum systems into subsystems and reunifications of subsystems into new joint systems. Conditional Density Matrix assigns a quantum state to a subsystem of a composite system on condition that another part of the composite system is in some pure state

2003-01-01

171

Description of an open quantum mechanical system

International Nuclear Information System (INIS)

A model for the description of an open quantum mechanical many-particle system is formulated. It starts from the shell model and treats the continuous states by a coupled channels method. The mixing of the discrete shell model states via the continuum of decay channels results in the genuine decaying states of the system. These states are eigenstates of a non-Hermitean Hamilton operator the eigenvalues of which give both the energies and the widths of the states. All correlations between two particles which are caused by the two-particle residual interaction, are taken into account including those via the continuum. In the formalism describing the open quantum mechanical system, the coupling between the system and its environment appears nonlinearly. If the resonance states start to overlap, a redistribution of the spectroscopic values ('trapping effect') takes place. As a result, the complexity of the system is reduced at high level density, structures in space and time are formed. This redistribution describes, on the one hand, the transition from the well-known nuclear properties at low level density to those at high level density and fits, on the other hand, into the concept of selforganization. (orig.)

1994-01-01

172

Quantum theory of non-integrable systems

International Nuclear Information System (INIS)

[en] We consider here the well known Friedrichs model in which an unstable discrete level is coupled to a continuum. Poincare's theorem prevents the existence of solutions of the eigenvalue problem associated to the Hamiltonian which would be analytic in the coupling constant. We show that these Poincare ''divergences'' can be avoided through a natural time ordering of dynamical states which expresses that the emission of radiation occurs after the preparation of the unstable state. This leads to a generalization of quantum mechanics in which the usual Hamiltonian eigenvalue problem is replaced by a complex eigenvalue problem which can be solved exactly. The usual Hilbert space is in this way extended to include generalized eigenstates involving complex distributions. In this way we may integrate a class of Poincare's non-integrable systems. In our method a quantum mechanical state is expressed as a superposition of basis states with a broken time symmetry. This leads to a temporal description for the evolution of the quantum state which is quite appealing. The particle decays as time goes on and produces a field corresponding to outgoing waves. This decay is irreversible in the sense that we can introduce an operator corresponding to a Lyapunov function whose average value taken over an arbitrary state decreases monotonically and takes its minimum value when the unstable particle has decayed. In this way, spontaneous decay becomes indeed an irreversible event. Also, it should be emphasized that our complex spectral representation can be obtained by a perturbation calculation. The general case involves time ordering in the Liouville space of density matrices. The quantum state has then to be described by a density matrix whose time evolution can be expressed in terms of a complete basis formed by density matrices with a broken time symmetry. (orig.)

1991-04-15

173

On the notion of a macroscopic quantum system

We analyse the notion of macroscopic quantum system from the point of view of the statistical structure of quantum theory. We come to conclusion that the presence of interference of probabilities should be used the main characteristic of quantumness (in the opposition to N. Bohr who permanently emphasized the crucial role of quantum action). In the light of recent experiments with statistical ensembles of people who produced interference of probabilities for special pairs of questions (which can be considered as measurements on people) human being should be considered as a macroscopic quantum system. There is also discussed relation with experiments of A. Zeilinger on interference of probabilities for macromoleculas.

Khrenikov, A Yu

2004-01-01

174

Quantum MIMO n-Systems and Conditions for Stability

In this paper we present some conditions for the (strong) stabilizability of an n-D Quantum MIMO system P(X). It contains two parts. The first part is to introduce the n-D Quantum MIMO systems where the coefficients vary in the algebra of Q-meromorphic functions. Then we introduce some conditions for the stabilizability of these systems. The second part is to show that this Quantum system has the n-D system as its quantum limit and the results for the SISO,SIMO,MISO,MIMO are obtained again as special cases.

Mansourbeigi, Seyed M H

2009-01-01

175

Unstable particles as open quantum systems

We present the probability preserving description of the decaying particle within the framework of quantum mechanics of open systems taking into account the superselection rule prohibiting the superposition of the particle and vacuum. In our approach the evolution of the system is given by a family of completely positive trace preserving maps forming one-parameter dynamical semigroup. We give the Kraus representation for the general evolution of such systems which allows one to write the evolution for systems with two or more particles. Moreover, we show that the decay of the particle can be regarded as a Markov process by finding explicitly the master equation in the Lindblad form. We also show that there are remarkable restrictions on the possible strength of decoherence.

Caban, P; Smolinski, K A; Walczak, Z

2005-01-01

176

Unstable particles as open quantum systems

International Nuclear Information System (INIS)

[en] We present the probability-preserving description of the decaying particle within the framework of quantum mechanics of open systems, taking into account the superselection rule prohibiting the superposition of the particle and vacuum. In our approach the evolution of the system is given by a family of completely positive trace-preserving maps forming a one-parameter dynamical semigroup. We give the Kraus representation for the general evolution of such systems, which allows one to write the evolution for systems with two or more particles. Moreover, we show that the decay of the particle can be regarded as a Markov process by finding explicitly the master equation in the Lindblad form. We also show that there are remarkable restrictions on the possible strength of decoherence

2005-01-01

177

Quantum system characterization with limited resources

The construction and operation of large scale quantum information devices presents a grand challenge. A major issue is the effective control of coherent evolution, which requires accurate knowledge of the system dynamics that may vary from device to device. We review strategies for obtaining such knowledge from minimal initial resources and in an efficient manner, and apply these to the problem of characterization of a qubit embedded into a larger state manifold, made tractable by exploiting prior structural knowledge. We also investigate adaptive sampling for estimation of multiple parameters.

Oi, Daniel

2012-01-01

178

Quantum stochastic resonance in symmetric systems

We investigate the low-temperature quantum stochastic resonance (QSR) phenomenon in a two-level system (TLS) which is coupled to an Ohmic heat bath. In contrast to common belief we find that QSR occurs also for symmetric (i.e., unbiased) TLS's if the viscous friction parameter ? exceeds a critical value: We demonstrate that with respect to the spectral power amplification measure QSR always occurs for ?>1 in contrast, the output signal-to-noise ratio exhibits an amplification only for ?>3/2.

Goychuk, Igor; Hänggi, Peter

1999-05-01

179

Black holes and nonrelativistic quantum systems.

We describe black holes in d+3 dimensions, whose thermodynamic properties correspond to those of a scale-invariant nonrelativistic (d+1)-dimensional quantum system with a dynamical exponent z=2. The gravitational model involves a massive Abelian vector field and a scalar field, in addition to the metric. The energy per particle in the dual theory is |micro|d/(d+2) at any temperature (micro is the chemical potential). The ratio of shear viscosity to entropy density is variant Planck's over 2pi/4pi in any dimension d > or =2. PMID:19257179

Kovtun, Pavel; Nickel, Dominik

2009-01-09

180

Black holes and nonrelativistic quantum systems.

UK PubMed Central (United Kingdom)

We describe black holes in d+3 dimensions, whose thermodynamic properties correspond to those of a scale-invariant nonrelativistic (d+1)-dimensional quantum system with a dynamical exponent z=2. The gravitational model involves a massive Abelian vector field and a scalar field, in addition to the metric. The energy per particle in the dual theory is |micro|d/(d+2) at any temperature (micro is the chemical potential). The ratio of shear viscosity to entropy density is variant Planck's over 2pi/4pi in any dimension d > or =2.

Kovtun P; Nickel D

2009-01-01

181

Efficient Diagonalization of Kicked Quantum Systems

We show that the time evolution operator of kicked quantum systems, although a full matrix of size NxN, can be diagonalized with the help of a new method based on a suitable combination of fast Fourier transform and Lanczos algorithm in just N^2 ln(N) operations. It allows the diagonalization of matrizes of sizes up to N\\approx 10^6 going far beyond the possibilities of standard diagonalization techniques which need O(N^3) operations. We have applied this method to the kicked Harper model revealing its intricate spectral properties.

Ketzmerick, R; Geisel, T

1999-01-01

182

Partitioning technique for discrete quantum systems

International Nuclear Information System (INIS)

We develop the partitioning technique for quantum discrete systems. The graph consists of several subgraphs: a central graph and several branch graphs, with each branch graph being rooted by an individual node on the central one. We show that the effective Hamiltonian on the central graph can be constructed by adding additional potentials on the branch-root nodes, which generates the same result as does the the original Hamiltonian on the entire graph. Exactly solvable models are presented to demonstrate the main points of this paper.

2011-01-01

183

Resonance width distribution for open quantum systems

Recent measurements of resonance widths for low-energy neutron scattering off heavy nuclei show large deviations from the standard Porter-Thomas distribution. We propose a new resonance width distribution based on the random matrix theory for an open quantum system. Two methods of derivation lead to a single analytical expression; in the limit of vanishing continuum coupling, we recover the Porter-Thomas distribution. The result depends on the ratio of typical widths $\\Gamma$ to the energy level spacing $D$ via the dimensionless parameter $\\kappa=(\\pi\\Gamma/2D)$. The new distribution suppresses small widths and increases the probabilities of larger widths.

Shchedrin, Gavriil

2011-01-01

184

Energy transport in closed quantum systems.

UK PubMed Central (United Kingdom)

We examine energy transport in an ensemble of closed quantum systems driven by stochastic perturbations. One can show that the probability and energy fluxes can be described in terms of quantum advection modes (QAMs) associated with the off-diagonal elements of the density matrix. These QAMs play the role of Landauer channels in a system with discrete energy spectrum and the eigenfunctions that cannot be described as plane waves. In order to determine the type of correlations that exist between the direction and magnitudes of each QAM and the average direction of energy and probability fluxes we have numerically solved the time-dependent Schrödinger equation describing a single particle trapped in a parabolic potential well which is perturbed by stochastic ripples. The ripples serve as a localized energy source and are offset to one side of the potential well. As the result a nonzero net energy flux flows from one part of the potential well to another across the symmetry center of the potential. We find that some modes exhibit positive correlation with the direction of the energy flow. Other modes, that carry a smaller energy per unit of the probability flux, anticorrelate with the energy flow and thus provide a backflow of the probability. The overall picture of energy transport that emerges from our results is very different from the conventional one based on a system with continuous energy spectrum.

Levin GA; Jones WA; Walczak K; Yerkes KL

2012-03-01

185

Characterizing and quantifying frustration in quantum many-body systems.

We present a general scheme for the study of frustration in quantum systems. We introduce a universal measure of frustration for arbitrary quantum systems and we relate it to a class of entanglement monotones via an exact inequality. If all the (pure) ground states of a given Hamiltonian saturate the inequality, then the system is said to be inequality saturating. We introduce sufficient conditions for a quantum spin system to be inequality saturating and confirm them with extensive numerical tests. These conditions provide a generalization to the quantum domain of the Toulouse criteria for classical frustration-free systems. The models satisfying these conditions can be reasonably identified as geometrically unfrustrated and subject to frustration of purely quantum origin. Our results therefore establish a unified framework for studying the intertwining of geometric and quantum contributions to frustration. PMID:22243147

Giampaolo, S M; Gualdi, G; Monras, A; Illuminati, F

2011-12-22

186

Characterizing and quantifying frustration in quantum many-body systems.

UK PubMed Central (United Kingdom)

We present a general scheme for the study of frustration in quantum systems. We introduce a universal measure of frustration for arbitrary quantum systems and we relate it to a class of entanglement monotones via an exact inequality. If all the (pure) ground states of a given Hamiltonian saturate the inequality, then the system is said to be inequality saturating. We introduce sufficient conditions for a quantum spin system to be inequality saturating and confirm them with extensive numerical tests. These conditions provide a generalization to the quantum domain of the Toulouse criteria for classical frustration-free systems. The models satisfying these conditions can be reasonably identified as geometrically unfrustrated and subject to frustration of purely quantum origin. Our results therefore establish a unified framework for studying the intertwining of geometric and quantum contributions to frustration.

Giampaolo SM; Gualdi G; Monras A; Illuminati F

2011-12-01

187

Schroedinger-cat states and decoherence in quantum electromechanical systems

International Nuclear Information System (INIS)

Quantum-electromechanical systems are nanoscale mechanical resonators whose high-frequency oscillations are detected by an electronic transducer. Despite their macroscopic size and mechanical, ordinary-matter nature, these resonators can exhibit distinct quantum behavior that is of great interest and promise to an experimental exploration of questions in the foundations of quantum mechanics. After a brief introduction to quantum-electromechanical systems, I will sketch the feasibility and features of superposition states of macroscopically distinct positions in such systems. I will also show how these systems give rise to a new and hitherto hardly explored decoherence model and present some first results for this model

2007-01-01

188

Asymptotically open quantum systems; Asymptotisch offene Quantensysteme

Energy Technology Data Exchange (ETDEWEB)

In the present thesis we investigate the structure of time-dependent equations of motion in quantum mechanics.We start from two coupled systems with an autonomous equation of motion. A limit, in which the dynamics of one of the two systems has a decoupled evolution and imposes a non-autonomous evolution for the second system is identified. A result due to K. Hepp that provides a classical limit for dynamics turns out to be part and parcel for this limit and is generalized in our work. The method introduced by J.S. Howland for the solution of the time-dependent Schroedinger equation is interpreted as such a limit. Moreover, we associate our limit with the modern theory of quantization. (orig.)

Westrich, M.

2008-04-15

189

Algebras and universal quantum computations with higher dimensional systems

Here is discussed application of the Weyl pair to construction of universal set of quantum gates for high-dimensional quantum system. An application of Lie algebras (Hamiltonians) for construction of universal gates is revisited first. It is shown next, how for quantum computation with qubits can be used two-dimensional analog of this Cayley-Weyl matrix algebras, i.e. Clifford algebras, and discussed well known applications to product operator formalism in NMR, Jordan-Wigner construction in fermionic quantum computations. It is introduced universal set of quantum gates for higher dimensional system (``qudit''), as some generalization of these models. Finally it is briefly mentioned possible application of such algebraic methods to design of quantum processors (programmable gates arrays) and discussed generalization to quantum computation with continuous variables.

Vlasov, A Yu

2002-01-01

190

Quantum Clocks and the Origin of Time in Complex Systems

The origin and nature of time in complex systems is explored using Quantum or Feynman clocks and the signals produced by them. Networks of these clocks provide the basis for the evolution of complex systems. The arrow of time is defined for complex systems. Applications in quantum cosmology are indicated.

Hitchcock, S M

1999-01-01

191

Sistemas cuánticos individuales/ Individual Quantum Systems

Scientific Electronic Library Online (English)

Full Text Available Abstract in spanish El Premio Nobel de Física 2012 fue otorgado a Serge Haroche y David J.Wineland por métodos experimentales innovadores que permiten la medición y manipulación de sistemas cuánticos individuales. La primera estudia fotones midiéndolos con átomos, y la segunda estudia iones que manipula con fotones. Las aplicaciones tanto potenciales como ya materializadas para el manejo de sistemas cuánticos están en la vía de revolucionar no solamente la tecnología sino la forma en la que comprendemos el mundo microscópico. Abstract in english The Nobel Prize in Physics for 2012 was awarded to Serge Haroche and David J. Wineland "for ground-breaking experimental methods that enable measuring and manipulation of individual quantum systems". The former deals with photons and measures them with atoms and the latter deals with ions and manipulates them with photons. The potential and actual applications of handling quantum systems are on their way to revolutionize not only technology but the way we understand the microscopic world.

Campos, Jorge A.

2013-01-01

192

Energy Transport in Closed Quantum Systems

We examine energy transport in an ensemble of closed quantum systems driven by stochastic perturbations. One can show that the probability and energy fluxes can be described in terms of quantum advection modes (QAM) associated with the off-diagonal elements of the density matrix. These QAM play the role of Landauer channels in a system with discrete energy spectrum and the eigenfunctions that cannot be described as plane waves. In order to determine the type of correlations that exist between the direction and magnitudes of each QAM and the average direction of energy and probability fluxes we have numerically solved the time-dependent Schr\\"{o}dinger equation describing a single particle trapped in a parabolic potential well which is perturbed by stochastic 'ripples'. The ripples serve as a localized energy source and are offset to one side of the potential well. As the result a non-zero net energy flux flows from one part of the potential well to another across the symmetry center of the potential. We find ...

Levin, G A; Walczak, K; Yerkes, K L

2012-01-01

193

Deformed oscillator algebras for two dimensional quantum superintegrable systems

Quantum superintegrable systems in two dimensions are obtained from their classical counterparts, the quantum integrals of motion being obtained from the corresponding classical integrals by a symmetrization procedure. For each quantum superintegrable systema deformed oscillator algebra, characterized by a structure function specific for each system, is constructed, the generators of the algebra being functions of the quantum integrals of motion. The energy eigenvalues corresponding to a state with finite dimensional degeneracy can then be obtained in an economical way from solving a system of two equations satisfied by the structure function, the results being in agreement to the ones obtained from the solution of the relevant Schrodinger equation. The method shows how quantum algebraic techniques can simplify the study of quantum superintegrable systems, especially in two dimensions.

Bonatsos, Dennis; Kokkotas, K D; Bonatsos, Dennis

1994-01-01

194

On microstates counting in many body polymer quantum systems

International Nuclear Information System (INIS)

Polymer quantum systems are mechanical models quantized in a similar way as loop quantum gravity but in which loops/graphs resembling polymers are replaced by discrete sets of points. Such systems have allowed to study in a simpler context some novel aspects of loop quantum gravity. Although thermal aspects play a crucial role in cosmology and black hole physics little attention has been given to the thermostatistics of many body polymer quantum systems. In this work we explore how the features of a one-dimensional effective polymer gas, affect its microstate counting and hence the corresponding thermodynamical quantities.

2011-10-14

195

Controllability of multi-partite quantum systems and selective excitation of quantum dots

We consider the degrees of controllability of multi-partite quantum systems as well as necessary and sufficient criteria for each case. The results are applied to the problem of simultaneous control of an ensemble of quantum dots with a single laser pulse. Finally, we apply optimal control techniques to demonstrate selective excitation of individual dots for a simultaneously controllable ensemble.

Schirmer, S G; Solomon, A I; Schirmer, Sonia G; Pullen, Ivan C H; Solomon, Allan I

2005-01-01

196

UK PubMed Central (United Kingdom)

The behavior of most physical systems is affected by their natural surroundings. A quantum system with an environment is referred to as open, and its study varies according to the classical or quantum description adopted for the environment. We propose an approach to open quantum systems that allows us to follow the cross-over from quantum to classical environments; to achieve this, we devise an exact parametric representation of the principal system, based on generalized coherent states for the environment. The method is applied to the s = 1/2 Heisenberg star with frustration, where the quantum character of the environment varies with the couplings entering the Hamiltonian H. We find that when the star is in an eigenstate of H, the central spin behaves as if it were in an effective magnetic field, pointing in the direction set by the environmental coherent-state angle variables (?, ?), and broadened according to their quantum probability distribution. Such distribution is independent of ?, whereas as a function of ? is seen to get narrower as the quantum character of the environment is reduced, collapsing into a Dirac-? function in the classical limit. In such limit, because ? is left undetermined, the Von Neumann entropy of the central spin remains finite; in fact, it is equal to the entanglement of the original fully quantum model, a result that establishes a relation between this latter quantity and the Berry phase characterizing the dynamics of the central spin in the effective magnetic field.

Calvani D; Cuccoli A; Gidopoulos NI; Verrucchi P

2013-04-01

197

Energy Technology Data Exchange (ETDEWEB)

The parable of Schroedinger`s cat may lead to several up-to date questions: how to treat open systems in quantum theory, how to treat thermodynamically irreversible processes in the quantum mechanics framework, how to explain, following the quantum theory, the existence, phenomenologically evident, of classical observables, what implies the predicted existence by the quantum theory of non localized macroscopic material object ?.

Primas, H. [Eidgenoessische Technische Hochschule, Zurich (Switzerland)

1992-12-31

198

Quantum Heat Engines; Multiple-State 1D Box System

Directory of Open Access Journals (Sweden)

Full Text Available We evaluate quantum Otto, Diesel and Brayton cycles employing multiple-state 1D box system instead of ideal gas filled cylinder. The work and heat are extracted using the change in the expectation of Hamiltonian of the system which leads to the first law of thermodynamics to quantum system. The first law makes available to redefine the force which is in fact not well defined in a quantum mechanical system and then it is applied to define the quantum version of thermodynamic processes, i.e. isobaric, isovolume and adiabatic. As the results, the efficiency of quantum Otto engine depends only on the compression ratio and will be higher than the efficiency of quantum Diesel which can decrease by the widening of expansion under isobaric process. The efficiency of quantum Brayton engine may reach maximum on certain combination between the wide of box under isobaric expansion and compression, under certain conditions. The amount of levels participated in the quantum heat engine system will potentially reduce the performance of the quantum heat cycles consisting isobaric process, but it can be resisted using isobaric process controller.

Eny Latifah; Agus Purwanto

2013-01-01

199

Analog control of open quantum systems under arbitrary decoherence

We derive and investigate a general non-Markovian equation for the time-dependence of a Hamiltonian that maximizes the fidelity of a desired quantum gate on any finite-dimensional quantum system in the presence of arbitrary bath and noise sources. The method is illustrated for a single-qubit gate implemented on a three-level system.

Clausen, Jens; Kurizki, Gershon

2009-01-01

200

Shrinked systems. Quantum physics on new paths

International Nuclear Information System (INIS)

This introducing textbook for students of higher semesters of physics, chemistry, and informatics treats a in latest time dynamically expanding field of physics. This book deals among others with the themes quantum information theory, quantum communications, quantum computing, teleportation, hidden parameters, which-way-marking, quantum measuring process, POVM, quantum channels and mediates by this not only a deepened understanding of quantum theory but also basic science, in order to follow the fast development of the field respectively to enter a special field of research. Commented recommendations for further literature as well as exercise problems help the reader to find quickly a founded approach to the theoretical foundations of future key technologies. The book can be made to a base of courses and seminars. Because the required basic knowledge in mathematics and quantum theory is presented in introductory chapters, the book is also suited for the self-study.

2005-01-01

201

An Open-System Quantum Simulator with Trapped Ions

The control of quantum systems is of fundamental scientific interest and promises powerful applications and technologies. Impressive progress has been achieved in isolating the systems from the environment and coherently controlling their dynamics, as demonstrated by the creation and manipulation of entanglement in various physical systems. However, for open quantum systems, engineering the dynamics of many particles by a controlled coupling to an environment remains largely unexplored. Here we report the first realization of a toolbox for simulating an open quantum system with up to five qubits. Using a quantum computing architecture with trapped ions, we combine multi-qubit gates with optical pumping to implement coherent operations and dissipative processes. We illustrate this engineering by the dissipative preparation of entangled states, the simulation of coherent many-body spin interactions and the quantum non-demolition measurement of multi-qubit observables. By adding controlled dissipation to coheren...

Barreiro, Julio T; Schindler, Philipp; Nigg, Daniel; Monz, Thomas; Chwalla, Michael; Hennrich, Markus; Roos, Christian F; Zoller, Peter; Blatt, Rainer; 10.1038/nature09801

2011-01-01

202

Experimental feedback control of quantum systems using weak measurements

A goal of the emerging field of quantum control is to develop methods for quantum technologies to function robustly in the presence of noise. Central issues are the fundamental limitations on the available information about quantum systems and the disturbance they suffer in the process of measurement. In the context of a simple quantum control scenario--the stabilization of non-orthogonal states of a qubit against dephasing--we experimentally explore the use of weak measurements in feedback control. We find that, despite the intrinsic difficultly of implementing them, weak measurements allow us to control the qubit better in practice than is even theoretically possible without them. Our work shows that these more general quantum measurements can play an important role for feedback control of quantum systems.

Gillett, G G; Lanyon, B P; Almeida, M P; Barbieri, M; Pryde, G J; O'Brien, J L; Resch, K J; Bartlett, S D; White, A G

2009-01-01

203

Quantum chromodynamics in few-nucleon systems

International Nuclear Information System (INIS)

[en] One of the most important implications of quantum chromodynamics (QCD) is that nuclear systems and forces can be described at a fundamental level. The theory provides natural explanations for the basic features of hadronic physics: the meson and baryon spectra, quark statistics, the structure of the weak and electromagnetic currents of hadrons, the scale-invariance of hadronic interactions at short distances, and evidently, color (i.e., quark and gluon) confinement at large distances. Many different and diverse tests have confirmed the basic predictions of QCD; however, since tests of quark and gluon interactions must be done within the confines of hadrons there have been few truly quantitative checks. Nevertheless, it appears likely that QCD is the fundamental theory of hadronic and nuclear interactions in the same sense that QED gives a precise description of electrodynamic interctions. Topics discussed include exclusive processes in QCD, the deuteron in QCD, reduced nuclear amplitudes, and limitations of traditional nuclear physics. 32 references

1983-01-01

204

Dynamics of initially entangled open quantum systems

International Nuclear Information System (INIS)

Linear maps of matrices describing the evolution of density matrices for a quantum system initially entangled with another are identified and found to be not always completely positive. They can even map a positive matrix to a matrix that is not positive, unless we restrict the domain on which the map acts. Nevertheless, their form is similar to that of completely positive maps. Only some minus signs are inserted in the operator-sum representation. Each map is the difference of two completely positive maps. The maps are first obtained as maps of mean values and then as maps of basis matrices. These forms also prove to be useful. An example for two entangled qubits is worked out in detail. The relation to earlier work is discussed.

2004-01-01

205

Anions, quantum particles in planar systems

International Nuclear Information System (INIS)

Our purpose here is to present a general review of the non-relativistic quantum-mechanical description of excitations that do not obey neither the Fermi-Dirac nor the Bose-Einstein statistics; they rather fulfill an intermediate statistics, the we called 'any-statistics'. As we shall see, this is a peculiarity of (1+1) and (1+2) dimensions, due to the fact that, in two space dimensions, the spin is not quantised, once the rotation group is Abelian. The relevance of studying theories in (1+2) dimensions is justified by the evidence that, in condensed matter physics, there are examples of planar systems, for which everything goes as if the third spatial dimension is frozen. (author)

2000-01-01

206

Quantum integrable systems. Quantitative methods in biology

Quantum integrable systems have very strong mathematical properties that allow an exact description of their energetic spectrum. From the Bethe equations, I formulate the Baxter "T-Q" relation, that is the starting point of two complementary approaches based on nonlinear integral equations. The first one is known as thermodynamic Bethe ansatz, the second one as Kl\\"umper-Batchelor-Pearce-Destri- de Vega. I show the steps toward the derivation of the equations for some of the models concerned. I study the infrared and ultraviolet limits and discuss the numerical approach. Higher rank integrals of motion can be obtained, so gaining some control on the eigenvectors. After, I discuss the Hubbard model in relation to the N = 4 supersymmetric gauge theory. The Hubbard model describes hopping electrons on a lattice. In the second part, I present an evolutionary model based on Turing machines. The goal is to describe aspects of the real biological evolution, or Darwinism, by letting evolve populations of algorithms. ...

Feverati, Giovanni

2011-01-01

207

Many electronic systems (e.g., the cuprate superconductors and heavy fermions) exhibit striking features in their dynamical response over a prominent range of experimental parameters. While there are some empirical suggestions of particular increasing length scales that accompany such transitions in some cases, this identification is not universal and in numerous instances no large correlation length is evident. To better understand, as a matter of principle, such behavior in quantum systems, we extend a known mapping (earlier studied in stochastic or supersymmetric quantum mechanics) between finite temperature classical Fokker-Planck systems and related quantum systems at zero temperature to include general nonequilibrium dynamics. Unlike Feynman mappings or stochastic quantization methods in field theories (as well as more recent holographic type dualities), the classical systems that we consider and their quantum duals reside in the same number of space-time dimensions. The upshot of our very broad and rigorous result is that a Wick rotation exactly relates (i) the dynamics in general finite temperature classical dissipative systems to (ii) zero temperature dynamics in the corresponding dual many-body quantum systems. Using this correspondence, we illustrate that, even in the absence of imposed disorder, many continuum quantum fluid systems (and possible lattice counterparts) may exhibit a zero-point “quantum dynamical heterogeneity” wherein the dynamics, at a given instant, is spatially nonuniform. While the static length scales accompanying this phenomenon do not seem to exhibit a clear divergence in standard correlation functions, the length scale of the dynamical heterogeneities can increase dramatically. We further study “quantum jamming” and illustrate how a hard-core bosonic system can undergo a zero temperature quantum critical metal-to-insulator-type transition with an extremely large effective dynamical exponent z>4 that is consistent with length scales that increase far more slowly than the relaxation time as a putative critical transition is approached. Similar results may hold for spin-liquid-type as well as interacting electronic systems. We suggest ways to analyze experimental data in order to adduce such phenomena. Our approach may be used to analyze other quenched quantum systems.

Nussinov, Zohar; Johnson, Patrick; Graf, Matthias J.; Balatsky, Alexander V.

2013-05-01

208

Quantum Knots and Lattices, or a Blueprint for Quantum Systems that Do Rope Tricks

Using the cubic honeycomb (cubic tessellation) of Euclidean 3-space, we define a quantum system whose states, called quantum knots, represent a closed knotted piece of rope, i.e., represent the particular spatial configuration of a knot tied in a rope in 3-space. This quantum system, called a quantum knot system, is physically implementable in the same sense as Shor's quantum factoring algorithm is implementable. To define a quantum knot system, we replace the standard three Reidemeister knot moves with an equivalent set of three moves, called respectively wiggle, wag, and tug, so named because they mimic how a dog might wag its tail. We argue that these moves are in fact more "physics friendly" because, unlike the Reidemeister moves, they respect the differential geometry of 3-space, and moreover they can be transformed into infinitesimal moves. These three moves wiggle, wag, and tug generate a unitary group, called the lattice ambient group, which acts on the state space of the quantum system. The lattice a...

Lomonaco, Samuel J

2009-01-01

209

Geometric phases and quantum phase transitions in open systems

The relationship between quantum phase transition and complex geometric phase for open quantum system governed by the non-Hermitian effective Hamiltonian with the accidental crossing of the eigenvalues is established. In particular, the geometric phase associated with the ground state of the one-dimensional dissipative Ising model in a transverse magnetic field is evaluated, and it is demonstrated that related quantum phase transition is of the first order.

Nesterov, Alexander I

2008-01-01

210

Quantum Cost Efficient Reversible BCD Adder for Nanotechnology Based Systems

Reversible logic allows low power dissipating circuit design and founds its application in cryptography, digital signal processing, quantum and optical information processing. This paper presents a novel quantum cost efficient reversible BCD adder for nanotechnology based systems using PFAG gate. It has been demonstrated that the proposed design offers less hardware complexity and requires minimum number of garbage outputs than the existing counterparts. The remarkable property of the proposed designs is that its quantum realization is given in NMR technology.

Islam, Md Saiful; Begum, Zerina

2011-01-01

211

Classical and quantum simulations of many-body systems

Energy Technology Data Exchange (ETDEWEB)

This thesis is devoted to recent developments in the fields of classical and quantum simulations of many-body systems. We describe new classical algorithms that overcome problems apparent in conventional renormalization group and Monte Carlo methods. These algorithms make possible the detailed study of finite temperature properties of 2-D classical and 1-D quantum systems, the investigation of ground states of 2-D frustrated or fermionic systems and the analysis of time evolutions of 2-D quantum systems. Furthermore, we propose new 'analog' quantum simulators that are able to realize interesting models such as a Tonks-Girardeau gas or a frustrated spin-1/2 XY model on a trigonal lattice. These quantum simulators make use of optical lattices and trapped ions and are technically feasible. In fact, the Tonks-Girardeau gas has been realized experimentally and we provide a detailed comparison between the experimental data and the theoretical predictions. (orig.)

Murg, Valentin

2008-04-07

212

Classical and quantum simulations of many-body systems

International Nuclear Information System (INIS)

This thesis is devoted to recent developments in the fields of classical and quantum simulations of many-body systems. We describe new classical algorithms that overcome problems apparent in conventional renormalization group and Monte Carlo methods. These algorithms make possible the detailed study of finite temperature properties of 2-D classical and 1-D quantum systems, the investigation of ground states of 2-D frustrated or fermionic systems and the analysis of time evolutions of 2-D quantum systems. Furthermore, we propose new ''analog'' quantum simulators that are able to realize interesting models such as a Tonks-Girardeau gas or a frustrated spin-1/2 XY model on a trigonal lattice. These quantum simulators make use of optical lattices and trapped ions and are technically feasible. In fact, the Tonks-Girardeau gas has been realized experimentally and we provide a detailed comparison between the experimental data and the theoretical predictions. (orig.)

2008-01-01

213

The emergence of chaos in an open quantum system

Energy Technology Data Exchange (ETDEWEB)

We take a simple open quantum system and demonstrate that it can exhibit behaviour, similar to that found in its chaotic classical analogue, when it is treated as an individual system, rather than using a statistical ensemble. ((orig.))

Spiller, T.P. (School of Mathematical and Physical Sciences, University of Sussex, Falmer, Brighton, Sussex, BN1 9QH (United Kingdom)); Ralph, J.F. (School of Mathematical and Physical Sciences, University of Sussex, Falmer, Brighton, Sussex, BN1 9QH (United Kingdom))

1994-11-07

214

The Geometric Phase in Quantum Systems

International Nuclear Information System (INIS)

The discovery of the geometric phase is one of the most interesting and intriguing findings of the last few decades. It led to a deeper understanding of the concept of phase in quantum mechanics and motivated a surge of interest in fundamental quantum mechanical issues, disclosing unexpected applications in very diverse fields of physics. Although the key ideas underlying the existence of a purely geometrical phase had already been proposed in 1956 by Pancharatnam, it was Michael Berry who revived this issue 30 years later. The clarity of Berry's seminal paper, in 1984, was extraordinary. Research on the topic flourished at such a pace that it became difficult for non-experts to follow the many different theoretical ideas and experimental proposals which ensued. Diverse concepts in independent areas of mathematics, physics and chemistry were being applied, for what was (and can still be considered) a nascent arena for theory, experiments and technology. Although collections of papers by different authors appeared in the literature, sometimes with ample introductions, surprisingly, to the best of my knowledge, no specific and exhaustive book has ever been written on this subject. The Geometric Phase in Quantum Systems is the first thorough book on geometric phases and fills an important gap in the physical literature. Other books on the subject will undoubtedly follow. But it will take a fairly long time before other authors can cover that same variety of concepts in such a comprehensive manner. The book is enjoyable. The choice of topics presented is well balanced and appropriate. The appendices are well written, understandable and exhaustive - three rare qualities. I also find it praiseworthy that the authors decided to explicitly carry out most of the calculations, avoiding, as much as possible, the use of the joke 'after a straightforward calculation, one finds...' This was one of the sentences I used to dislike most during my undergraduate studies. A student is inexperienced in such matters and needs to look at details. This book is addressed to graduate physics and chemistry students and was written thinking of students. However, I would recommend it also to young and mature physicists, even those who are already 'into' the subject. It is a comprehensive work, jointly written by five researchers. After a simple introduction to the subject, the book gradually provides deeper concepts, more advanced theory and finally an interesting introduction and explanation of recent experiments. For its multidisciplinary features, this work could not have been written by one single author. The collaborative effort is undoubtedly one of its most interesting qualities. I would definitely recommend it to anyone who wants to learn more on the geometric phase, a topic that is both beautiful and intriguing. (book review)

2003-12-12

215

Entangled Systems New Directions in Quantum Physics

An introductory textbook for advanced students of physics, chemistry and computer science, covering an area of physics that has lately witnessed rapid expansion. The topics treated here include quantum information, quantum communication, quantum computing, teleportation and hidden parameters, thus imparting not only a well-founded understanding of quantum theory as such, but also a solid basis of knowledge from which readers can follow the rapid development of the topic or delve deeper into a more specialized branch of research. Commented recommendations for further reading as well as end-of-chapter problems help the reader to quickly access the theoretical basics of future key technologies

Audretsch, Jürgen

2007-01-01

216

Quantum groups, orthogonal polynomials and applications to some dynamical systems

International Nuclear Information System (INIS)

The first part is concerned with the introduction of quantum groups as an extension of Lie groups. In particular, we study the case of unitary enveloping algebras in dimension 2. We then connect the quantum group formalism to the construction of g CGC recurrent relations. In addition, we construct g-deformed Krawtchouck and Meixner orthogonal polynomials and list their respective main characteristics. The second part deals with some dynamical systems from a classical, a quantum and a gp-analogue point of view. We investigate the Coulomb Kepler system by using the canonical namical systems which contain as special cases some interesting systems for nuclear of atomic physics and for quantum chemistry, such as the Hartmann system, the ring-shaped oscillator, the Smarodinsky-Winternitz system, the Aharonov-Bohen system and the dyania of Dirac and Schroedinger. (author)

1993-01-01

217

Inequalities Detecting Quantum Entanglement for $2\\otimes d$ Systems

We present a set of inequalities for detecting quantum entanglement of $2\\otimes d$ quantum states. For $2\\otimes 2$ and $2\\otimes 3$ systems, the inequalities give rise to sufficient and necessary separability conditions for both pure and mixed states. For the case of $d>3$, these inequalities are necessary conditions for separability, which detect all entangled states that are not positive under partial transposition and even some entangled states with positive partial transposition. These inequalities are given by mean values of local observables and present an experimental way of detecting the quantum entanglement of $2\\otimes d$ quantum states and even multi-qubit pure states.

Zhao, Ming-Jing; Fei, Shao-Ming; Wang, Zhi-Xi; 10.1103/PhysRevA.83.052120

2011-01-01

218

The Propagation of Quantum Information Through a Spin System

It has been recently suggested that the dynamics of a quantum spin system may provide a natural mechanism for transporting quantum information. We show that one dimensional rings of qubits with fixed (time-independent) interactions, constant around the ring, allow high fidelity communication of quantum states. We show that the problem of maximising the fidelity of the quantum communication is related to a classical problem in fourier wave analysis. By making use of this observation we find that if both communicating parties have access to limited numbers of qubits in the ring (a fraction that vanishes in the limit of large rings) it is possible to make the communication arbitrarily good.

Osborne, T J; Osborne, Tobias J.; Linden, Noah

2003-01-01

219

Image storage, retrieval, compression and segmentation in a quantum system

A set of quantum states for M colors and another set of quantum states for N coordinates are proposed in this paper to represent M colors and coordinates of the N pixels in an image respectively. We design an algorithm by which an image of N pixels and m different colors is stored in a quantum system just using 2N+m qubits. An algorithm for quantum image compression is proposed. Simulation result on the Lena image shows that compression ratio of lossless is 2.058. Moreover, an image segmentation algorithm based on quantum search quantum search which can find all solutions in the expected times in O(tsqrt{N} ) is proposed, where N is the number of pixels and t is the number of targets to be segmented.

Li, Hai-Sheng; Qingxin, Zhu; Lan, Song; Shen, Chen-Yi; Zhou, Rigui; Mo, Jia

2013-06-01

220

Non-Hermitian Quantum Systems and Time-Optimal Quantum Evolution in Vicinity of Exceptional Point

Recently the quantum brachistochrone problem has been considered for non-Hermitian $\\cal PT$-symmetric quantum system, and it has been shown that the optimal evolution time required to transform a given initial state $|\\psi_i>$ into specific final state $|\\psi_f>$ can be made arbitrary small. Here we study the similar problem for generic non-Hermitian Hamiltonian and find that the same answer can be obtained. We show that at the exceptional point the distance between $|\\psi_i>$ and $|\\psi_f>$ becomes virtually vanishing and quantum tunneling occurring at the exceptional point yields the shortest timed required to evolve from the initial state $|\\psi_i>$ into the final state $|\\psi_f>$. These results may have applications in holonomic quantum computation and quantum information processing.

Nesterov, A I

2007-01-01

221

Security Proof for Quantum Key Distribution Using Qudit Systems

We provide security bounds against coherent attacks for two families of quantum key distribution protocols that use $d$-dimensional quantum systems. In the asymptotic regime, both the secret key rate for fixed noise and the robustness to noise increase with $d$. The finite-key corrections are found to be almost insensitive to $d\\lesssim 20$.

Sheridan, Lana

2010-01-01

222

Adiabatic Response of Quantum Systems Pinching a Gap Closure

A vanishing cause can lead to a large response in quantum systems which undergo cyclic deformations that pinch a point of level crossing. We call such behavior homeopathic. We illustrate this behavior by studying charge circulation in quantum models of necklaces of atoms driven by a running wave of small amplitude.

Avron, J E

1998-01-01

223

Recycling of quantum information: Multiple observations of quantum systems

Given a finite number of copies of an unknown qubit state that have already been measured optimally, can one still extract any information about the original unknown state? We give a positive answer to this question and quantify the information obtainable by a given observer as a function of the number of copies in the ensemble, and of the number of independent observers that, one after the other, have independently measured the same ensemble of qubits before him. The optimality of the protocol is proven and extensions to other states and encodings are also studied. According to the general lore, the state after a measurement has no information about the state before the measurement. Our results manifestly show that this statement has to be taken with a grain of salt, specially in situations where the quantum states encode confidential information.

Rapcan, Peter; Munoz-Tapia, Ramon; Bagan, Emilio; Buzek, Vladimir

2007-01-01

224

International Nuclear Information System (INIS)

In this paper, within a parasupersymmetric and quantum deformed formalism, we introduce a class of bound-state problems which represents the coupling of a three-level atom with a two-dimensional potential system. We consider second-order parasupersymmetric quantum-mechanical models and a nonlinear deformed algebraic formulation for shape-invariant potential systems to study the quantum dynamics of physical observable, such as atomic level occupation, level transition probabilities, entropy and entanglement, in terms of time and deformation intensity. An application is given for a couple of shape-invariant potentials widely used to model quantum confined systems in several fields of physics, assuming a simple exponential form for the nonlinear deformation function. (paper)

2013-02-08

225

On the kinetic theory of quantum systems

International Nuclear Information System (INIS)

[en] The contents of this thesis which deals with transport phenomena of specific gases, plasmas and fluids, can be separated into two distinct parts. In the first part a statistical way is suggested to estimate the neutrino mass. Herefore use is made of the fact that massive neutrinos possess a non-zero volume viscosity in contrast with massless neutrinos. The second part deals with kinetic theory of strongly condensed quantum systems of which examples in nature are: liquid Helium, heavy nuclei, electrons in a metal and the interior of stars. In degenerate systems fermions in general interact strongly so that ordinary kinetic theory is not directly applicable. For such cases Landau-Fermi-liquid theory, in which the strongly interacting particles are replaced by much weaker interacting quasiparticles, proved to be very useful. A method is developed in this theory to calculate transport coefficients. Applications of this method on liquid 3Helium yield surprisingly good agreement with experimental results for thermal conductivities. (Auth.)

1986-01-01

226

Fourier Synthesis Methods for Control of Inhomogeneous Quantum Systems

Finding control laws (pulse sequences) that can compensate for dispersions in parameters which govern the evolution of a quantum system is an important problem in the fields of coherent spectroscopy, imaging, and quantum information processing. The use of composite pulse techniques for such tasks has a long and widely known history. In this paper, we introduce the method of Fourier synthesis control law design for compensating dispersions in quantum system dynamics. We focus on system models arising in NMR spectroscopy and NMR imaging applications.

Pryor, Brent

2007-01-01

227

Multi-particle correlations in quaternionic quantum systems

International Nuclear Information System (INIS)

The authors investigated the outcomes of measurements on correlated, few-body quantum systems described by a quaternionic quantum mechanics that allows for regions of quaternionic curvature. It was found that a multi particles interferometry experiment using a correlated system of four nonrelativistic, spin-half particles has the potential to detect the presence of quaternionic curvature. Two-body systems, however, are shown to give predictions identical to those of standard quantum mechanics when relative angles are used in the construction of the operators corresponding to measurements of particle spin components. 15 refs

1994-01-01

228

Electron transport in the multi-terminal quantum dot system

Directory of Open Access Journals (Sweden)

Full Text Available The time-dependent electron transport through a multi-terminal quantum dot system is studied. External microwave fields with arbitrary amplitudes, phases and frequencies are applied to different parts of the system considered. The dependence of the average current and average differential conductance on different parameters of the external microwave fields is analyzed. Special attention is paid to the photon–electron pump effect observed for some values of the quantum dot system parameters.

Ewa TARANKO; Ryszard TARANKO; Malgorzata WIERTEL

2005-01-01

229

Molecular dynamics investigations on a quantum system in a thermostat

The model quantum system of fermions in a one dimensional harmonic oscillator potential is investigated by a molecular dynamics method at constant temperature. Although in quantum mechanics the equipartition theorem cannot be used like in the Nose-Hoover-thermostat it is possible to couple an additional degree of freedom to the system which acts as a thermometer and drives the system towards the desired temperature via complex time steps.

Schnack, J

1998-01-01

230

Entangled Quantum State Discrimination using Pseudo-Hermitian System

We demonstrate how to discriminate two non-orthogonal, entangled quantum state which are slightly different from each other by using pseudo-Hermitian system. The positive definite metric operator which makes the pseudo-Hermitian systems fully consistent quantum theory is used for such a state discrimination. We further show that non-orthogonal states can evolve through a suitably constructed pseudo-Hermitian Hamiltonian to orthogonal states. Such evolution ceases at exceptional points of the pseudo-Hermitian system.

Ghatak, Ananya

2012-01-01

231

Quantum trajectory approach to the geometric phase: open bipartite systems

International Nuclear Information System (INIS)

Through the quantum trajectory approach, we calculate the geometric phase acquired by a bipartite system subjected to decoherence. The subsystems that compose the bipartite system interact with each other and then are entangled in the evolution. The geometric phase due to the quantum jump for both the bipartite system and its subsystems is calculated and analysed. As an example, we present two coupled spin-1/2 particles to detail the calculations.

1160-01-00

232

Quantum trajectory approach to the geometric phase: open bipartite systems

Energy Technology Data Exchange (ETDEWEB)

Through the quantum trajectory approach, we calculate the geometric phase acquired by a bipartite system subjected to decoherence. The subsystems that compose the bipartite system interact with each other and then are entangled in the evolution. The geometric phase due to the quantum jump for both the bipartite system and its subsystems is calculated and analysed. As an example, we present two coupled spin-1/2 particles to detail the calculations.

Yi, X X; Liu, D P; Wang, W [Department of Physics, Dalian University of Technology, Dalian 116024 (China)

2005-10-15

233

Quantum discord amplification of fermionic systems in an accelerated frame

Quantum discord of fermionic systems in the relativistic regime, that is, beyond the single-mode approximation (SMA) is investigated. It is shown that quantum discord is amplified for the fermionic system in non-inertial frames irrespective of the choice of state, region and level of mixedness. This ensures that the phenomenon of amplification can actually happen in the relativistic regime. It is seen that quantum discord converges at infinite acceleration limit, which means that it becomes independent of qR (Unruh modes) beyond SMA. This implies that most of the tensor product structures already used in the literature to compute quantum field correlations in relativistic quantum information cannot give rise to physical results. The dynamics of quantum discord is investigated under amplitude damping, depolarizing and flipping channels. The vanishing behavior of quantum discord is seen for higher level of decoherence in the infinite acceleration limit. The depolarizing channel dominantly affects the fermionic quantum discord as compared to the amplitude damping channel. It means that the depolarizing channel has most destructive influence on the discord of the fermionic systems. Moreover, the effect of environment on the discord is much stronger than that of the acceleration of non-inertial frames.

Ramzan, M.

2013-10-01

234

H-Infinity Control of Linear Quantum Stochastic Systems

The purpose of this paper is to formulate and solve a H-infinity controller synthesis problem for a class of non-commutative linear stochastic systems which includes many examples of interest in quantum technology. The paper includes results on the class of such systems for which the quantum commutation relations are preserved (such a requirement must be satisfied in a physical quantum system). A quantum version of standard (classical) dissipativity results are presented and from this a quantum version of the Strict Bounded Real Lemma is derived. This enables a quantum version of the two Riccati solution to the H-infinity control problem to be presented. This result leads to controllers which may be realized using purely quantum, purely classical or a mixture of quantum and classical elements. This issue of physical realizability of the controller is examined in detail, and necessary and sufficient conditions are given. Our results are constructive in the sense that we provide explicit formulas for the Hamilt...

James, M R; Petersen, I R

2007-01-01

235

Open quantum spin systems in semiconductor quantum dots and atoms in optical lattices

International Nuclear Information System (INIS)

In this Thesis, we study open quantum spin systems from different perspectives. The first part is motivated by technological challenges of quantum computation. An important building block for quantum computation and quantum communication networks is an interface between material qubits for storage and data processing and travelling photonic qubits for communication. We propose the realisation of a quantum interface between a travelling-wave light field and the nuclear spins in a quantum dot strongly coupled to a cavity. Our scheme is robust against cavity decay as it uses the decay of the cavity to achieve the coupling between nuclear spins and the travelling-wave light fields. A prerequiste for such a quantum interface is a highly polarized ensemble of nuclear spins. High polarization of the nuclear spin ensemble is moreover highly desirable as it protects the potential electron spin qubit from decoherence. Here we present the theoretical description of an experiment in which highly asymmetric dynamic nuclear spin pumping is observed in a single self-assembled InGaAs quantum dot. The second part of this Thesis is devoted to fundamental studies of dissipative spin systems. We study general one-dimensional spin chains under dissipation and propose a scheme to realize a quantum spin system using ultracold atoms in an optical lattice in which both coherent interaction and dissipation can be engineered and controlled. This system enables the study of non-equilibrium and steady state physics of open and driven spin systems. We find, that the steady state expectation values of different spin models exhibit discontinuous behaviour at degeneracy points of the Hamiltonian in the limit of weak dissipation. This effect can be used to dissipatively probe the spectrum of the Hamiltonian. We moreover study spin models under the aspect of state preparation and show that dissipation drives certain spin models into highly entangled state. Finally, we study a spin chain with subwavelength interatomic distances that exhibits long rage interactions. What lies at the heart of all these approaches is the endeavour to include the coupling to the environment into the description of the physical system with the aim of harnessing dissipative processes. While decoherence masks or destroys quantum effects and is considered as the main adversary of any quantum information application, we turn the existence of the dissipative coupling of spin systems to the environment into a fruitful resource.

2012-01-01

236

A robust, scanning quantum system for nanoscale sensing and imaging

Controllable atomic-scale quantum systems hold great potential as sensitive tools for nanoscale imaging and metrology. Possible applications range from nanoscale electric and magnetic field sensing to single photon microscopy, quantum information processing, and bioimaging. At the heart of such schemes is the ability to scan and accurately position a robust sensor within a few nanometers of a sample of interest, while preserving the sensor's quantum coherence and readout fidelity. These combined requirements remain a challenge for all existing approaches that rely on direct grafting of individual solid state quantum systems or single molecules onto scanning-probe tips. Here, we demonstrate the fabrication and room temperature operation of a robust and isolated atomic-scale quantum sensor for scanning probe microscopy. Specifically, we employ a high-purity, single-crystalline diamond nanopillar probe containing a single Nitrogen-Vacancy (NV) color center. We illustrate the versatility and performance of our sc...

Maletinsky, P; Grinolds, M S; Hausmann, B; Lukin, M D; Walsworth, R -L; Loncar, M; Yacoby, A

2011-01-01

237

Photoluminescence of a microcavity quantum dot system in the quantum strong-coupling regime.

UK PubMed Central (United Kingdom)

The Jaynes-Cummings model, describing the interaction between a single two-level system and a photonic mode, has been used to describe a large variety of systems, ranging from cavity quantum electrodynamics, trapped ions, to superconducting qubits coupled to resonators. Recently there has been renewed interest in studying the quantum strong-coupling (QSC) regime, where states with photon number greater than one are excited. This regime has been recently achieved in semiconductor nanostructures, where a quantum dot is trapped in a planar microcavity. Here we study the quantum strong-coupling regime by calculating its photoluminescence (PL) properties under a pulsed excitation. We discuss the changes in the PL as the QSC regime is reached, which transitions between a peak around the cavity resonance to a doublet. We particularly examine the variations of the PL in the time domain, under regimes of short and long pulse times relative to the microcavity decay time.

Ishida N; Byrnes T; Nori F; Yamamoto Y

2013-01-01

238

The Dalton quantum chemistry program system

DEFF Research Database (Denmark)

Dalton is a powerful general-purpose program system for the study of molecular electronic structure at the Hartree–Fock, Kohn–Sham, multiconfigurational self-consistent-field, Møller–Plesset, configuration-interaction, and coupled-cluster levels of theory. Apart from the total energy, a wide variety of molecular properties may be calculated using these electronic-structure models. Molecular gradients and Hessians are available for geometry optimizations, molecular dynamics, and vibrational studies, whereas magnetic resonance and optical activity can be studied in a gauge-origin-invariant manner. Frequency-dependent molecular properties can be calculated using linear, quadratic, and cubic response theory. A large number of singlet and triplet perturbation operators are available for the study of one-, two-, and three-photon processes. Environmental effects may be included using various dielectric-medium and quantum-mechanics/molecular-mechanics models. Large molecules may be studied using linear-scaling and massively parallel algorithms. Dalton is distributed at no cost from http://www.daltonprogram.org for a number of UNIX platforms.

Aidas, Kestutis; Angeli, Celestino

2013-01-01

239

Dynamical Phase Transitions in Quantum Systems

Directory of Open Access Journals (Sweden)

Full Text Available Many years ago Bohr characterized the fundamental differences between the two extreme cases of quantum mechanical many-body problems known at that time: between the compound states in nuclei at extremely high level density and the shell-model states in atoms at low level density. It is shown in the present paper that the compound nucleus states at high level density are the result of a dynamical phase transition due to which they have lost any spectroscopic relation to the individual states of the nucleus. The last ones are shell-model states which are of the same type as the shell-model states in atoms. Mathematically, dynamical phase transitions are caused by singular (exceptional) points at which the trajectories of the eigenvalues of the non-Hermitian Hamilton operator cross. In the neighborhood of these singular points, the phases of the eigenfunctions are not rigid. It is possible therefore that some eigenfunctions of the system align to the scattering wavefunctions of the environment by decoupling (trapping) the remaining ones from the environment. In the Schrödinger equation, nonlinear terms appear in the neighborhood of the singular points.

Ingrid Rotter

2010-01-01

240

The DALTON quantum chemistry program system

DEFF Research Database (Denmark)

Dalton is a powerful general-purpose program system for the study of molecular electronic structure at the Hartree–Fock, Kohn–Sham, multiconfigurational self-consistent-field, Møller–Plesset, configuration-interaction, and coupled-cluster levels of theory. Apart from the total energy, a wide variety of molecular properties may be calculated using these electronic-structure models. Molecular gradients and Hessians are available for geometry optimizations, molecular dynamics, and vibrational studies, while magnetic resonance and optical activity can be studied in a gauge-origininvariant manner. Frequency-dependent molecular properties can be calculated using linear, quadratic, and cubic response theory. A large number of singlet and triplet perturbation operators are available for the study of one-, two-, and three-photon processes. Environmental effects may be included using various dielectric-medium and quantum-mechanics/molecular-mechanics models. Large molecules may be studied using linear-scaling and massively parallel algorithms. Dalton is distributed at no cost from http://www.daltonprogram.org for a number of UNIX platforms.

Aidas, Kestutis; Angeli, Celestino

2013-01-01

241

Decoherence of adiabatically steered quantum systems

We study the effect of Markovian environmental noise on the dynamics of a two-level quantum system which is steered adiabatically by an external driving field. We express the master equation taking consistently into account all the contributions to the lowest non-vanishing order in the coupling to the Markovian environment. We study the master equation numerically and analytically and we find that, in the adiabatic limit, a zero-temperature environment does not affect the ground state evolution. As a physical application, we discuss extensively how the environment affects Cooper pair pumping. The adiabatic ground state pumping appears to be robust against environmental noise. In fact, the relaxation due to the environment is required to avoid the accumulation of small errors from each pumping cycle. We show that neglecting the non-secular terms in the master equation leads to unphysical results, such as charge non-conservation. We discuss also a possible way to control the environmental noise in a realistic p...

Solinas, P; Salmilehto, J; Pekola, J P

2010-01-01

242

The Geometric Phase in Quantum Systems

Energy Technology Data Exchange (ETDEWEB)

The discovery of the geometric phase is one of the most interesting and intriguing findings of the last few decades. It led to a deeper understanding of the concept of phase in quantum mechanics and motivated a surge of interest in fundamental quantum mechanical issues, disclosing unexpected applications in very diverse fields of physics. Although the key ideas underlying the existence of a purely geometrical phase had already been proposed in 1956 by Pancharatnam, it was Michael Berry who revived this issue 30 years later. The clarity of Berry's seminal paper, in 1984, was extraordinary. Research on the topic flourished at such a pace that it became difficult for non-experts to follow the many different theoretical ideas and experimental proposals which ensued. Diverse concepts in independent areas of mathematics, physics and chemistry were being applied, for what was (and can still be considered) a nascent arena for theory, experiments and technology. Although collections of papers by different authors appeared in the literature, sometimes with ample introductions, surprisingly, to the best of my knowledge, no specific and exhaustive book has ever been written on this subject. The Geometric Phase in Quantum Systems is the first thorough book on geometric phases and fills an important gap in the physical literature. Other books on the subject will undoubtedly follow. But it will take a fairly long time before other authors can cover that same variety of concepts in such a comprehensive manner. The book is enjoyable. The choice of topics presented is well balanced and appropriate. The appendices are well written, understandable and exhaustive - three rare qualities. I also find it praiseworthy that the authors decided to explicitly carry out most of the calculations, avoiding, as much as possible, the use of the joke 'after a straightforward calculation, one finds...' This was one of the sentences I used to dislike most during my undergraduate studies. A student is inexperienced in such matters and needs to look at details. This book is addressed to graduate physics and chemistry students and was written thinking of students. However, I would recommend it also to young and mature physicists, even those who are already 'into' the subject. It is a comprehensive work, jointly written by five researchers. After a simple introduction to the subject, the book gradually provides deeper concepts, more advanced theory and finally an interesting introduction and explanation of recent experiments. For its multidisciplinary features, this work could not have been written by one single author. The collaborative effort is undoubtedly one of its most interesting qualities. I would definitely recommend it to anyone who wants to learn more on the geometric phase, a topic that is both beautiful and intriguing. (book review)

Pascazio, S [Dipartimento di Fisica, Universita di Bari (Italy)

2003-12-12

243

Closed-Loop and Robust Control of Quantum Systems

For most practical quantum control systems, it is important and difficult to attain robustness and reliability due to unavoidable uncertainties in the system dynamics or models. Three kinds of typical approaches (e.g., closed-loop learning control, feedback control, and robust control) have been proved to be effective to solve these problems. This work presents a self-contained survey on the closed-loop and robust control of quantum systems, as well as a brief introduction to a selection of basic theories and methods in this research area, to provide interested readers with a general idea for further studies. In the area of closed-loop learning control of quantum systems, we survey and introduce such learning control methods as gradient-based methods, genetic algorithms (GA), and reinforcement learning (RL) methods from a unified point of view of exploring the quantum control landscapes. For the feedback control approach, the paper surveys three control strategies including Lyapunov control, measurement-based control, and coherent-feedback control. Then such topics in the field of quantum robust control as H? control, sliding mode control, quantum risk-sensitive control, and quantum ensemble control are reviewed. The paper concludes with a perspective of future research directions that are likely to attract more attention.

Wang, Lin-Cheng

2013-01-01

244

Quantum feedback in a weakly driven cavity QED system

International Nuclear Information System (INIS)

Quantum feedback in strongly coupled systems can probe a regime where one quantum of excitation is a large fluctuation. We present theoretical and experimental studies of quantum feedback in an optical cavity QED system. The time evolution of the conditional state, following a photodetection, can be modified by changing the drive of the cavity. For the appropriate feedback, the conditional state can be captured in a new steady state and then released. The feedback protocol requires resonance operation, and proper amplitude and delay for the change in the drive. We demonstrate the successful use of feedback in the suppression of the vacuum Rabi oscillations for the length of the feedback pulse and their subsequent return to steady state. The feedback works only because we have an entangled quantum system, rather than an analogous correlated classical system.

1179-01-00

245

Quantum Hall Ferromagnetism in a Two-Dimensional Electron System

Experiments on a nearly spin degenerate two-dimensional electron system reveals unusual hysteretic and relaxational transport in the fractional quantum Hall effect regime. The transition between the spin-polarized (with fill fraction $\

Eom, J; Kang, W; Campman, K L; Gossard, A C; Bichler, M; Wegscheider, W

2000-01-01

246

Estimation of Quantum Correlations in Two-Qubit NMR systems

We study evolution of quantum correlations in ensembles of two-qubit nuclear spin systems via nuclear magnetic resonance techniques. We use discord as a measure of quantum correlations and the Werner state as an explicit example. We first introduce different ways of measuring discord and geometric discord in two-qubit systems, and then we describe (a) quantitative measurement of discord for Werner-like states prepared using an entangling pulse sequence, (b) the efficiency of dynamical decoupling sequences in preserving quantum correlations, and (c) the evolution of discord for a singlet-triplet mixed state during a radio-frequency spin-lock.

Katiyar, Hemant; Mahesh, T S; Patel, Apoorva

2012-01-01

247

Quantum Brayton cycle with coupled systems as working substance.

UK PubMed Central (United Kingdom)

We explore the quantum version of the Brayton cycle with a composite system as the working substance. The actual Brayton cycle consists of two adiabatic and two isobaric processes. Two pressures can be defined in our isobaric process; one corresponds to the external magnetic field (characterized by F(x)) exerted on the system, while the other corresponds to the coupling constant between the subsystems (characterized by F(y)). As a consequence, we can define two types of quantum Brayton cycle for the composite system. We find that the subsystem experiences a quantum Brayton cycle in one quantum Brayton cycle (characterized by F(x)), whereas the subsystem's cycle is quantum Otto cycle in another Brayton cycle (characterized by F(y)). The efficiency for the composite system equals to that for the subsystem in both cases, but the work done by the total system is usually larger than the sum of the work done by the two subsystems. The other interesting finding is that for the cycle characterized by F(y), the subsystem can be a refrigerator, while the total system is a heat engine. The result in this paper can be generalized to a quantum Brayton cycle with a general coupled system as the working substance.

Huang XL; Wang LC; Yi XX

2013-01-01

248

Photonic reagent control of dynamically homologous quantum systems

International Nuclear Information System (INIS)

The general objective of quantum control is the manipulation of atomic scale physical and chemical phenomena through the application of external control fields. These tailored fields, or photonic reagents, exhibit systematic properties analogous to those of ordinary laboratory reagents. This analogous behavior is explored further here by considering the controlled response of a family of homologous quantum systems to a single common photonic reagent. A level set of dynamically homologous quantum systems is defined as the family that produces the same value(s) for a target physical observable(s) when controlled by a common photonic reagent. This paper investigates the scope of homologous quantum system control using the level set exploration technique (L-SET). L-SET enables the identification of continuous families of dynamically homologous quantum systems. Each quantum system is specified by a point in a hypercube whose edges are labeled by Hamiltonian matrix elements. Numerical examples are presented with simple finite level systems to illustrate the L-SET concepts. Both connected and disconnected families of dynamically homologous systems are shown to exist.

2007-03-07

249

Quantum Brayton cycle with coupled systems as working substance.

We explore the quantum version of the Brayton cycle with a composite system as the working substance. The actual Brayton cycle consists of two adiabatic and two isobaric processes. Two pressures can be defined in our isobaric process; one corresponds to the external magnetic field (characterized by F(x)) exerted on the system, while the other corresponds to the coupling constant between the subsystems (characterized by F(y)). As a consequence, we can define two types of quantum Brayton cycle for the composite system. We find that the subsystem experiences a quantum Brayton cycle in one quantum Brayton cycle (characterized by F(x)), whereas the subsystem's cycle is quantum Otto cycle in another Brayton cycle (characterized by F(y)). The efficiency for the composite system equals to that for the subsystem in both cases, but the work done by the total system is usually larger than the sum of the work done by the two subsystems. The other interesting finding is that for the cycle characterized by F(y), the subsystem can be a refrigerator, while the total system is a heat engine. The result in this paper can be generalized to a quantum Brayton cycle with a general coupled system as the working substance. PMID:23410319

Huang, X L; Wang, L C; Yi, X X

2013-01-31

250

Plausibility of quantum coherent states in biological systems

International Nuclear Information System (INIS)

In this paper we briefly discuss the necessity of using quantum mechanics as a fundamental theory applicable to some key functional aspects of biological systems. This is especially relevant to three important parts of a neuron in the human brain, namely the cell membrane, microtubules (MT) and ion channels. We argue that the recently published papers criticizing the use of quantum theory in these systems are not convincing.

2011-07-08

251

Quantum encodings in spin systems and harmonic oscillators

International Nuclear Information System (INIS)

We show that higher-dimensional versions of qubits, or qudits, can be encoded into spin systems and into harmonic oscillators, yielding important advantages for quantum computation. Whereas qubit-based quantum computation is adequate for analyses of quantum vs classical computation, in practice qubits are often realized in higher-dimensional systems by truncating all but two levels, thereby reducing the size of the precious Hilbert space. We develop natural qudit gates for universal quantum computation, and exploit the entire accessible Hilbert space. Mathematically, we give representations of the generalized Pauli group for qudits in coupled spin systems and harmonic oscillators, and include analyses of the qubit and the infinite-dimensional limits.

2002-01-01

252

Manipulating quantum information on the controllable systems or subspaces

In this paper, we explore how to constructively manipulate quantum information on the controllable systems or subspaces. It is revealed that one can make full use of distinguished properties of Pauli operators to design control Hamiltonian based on the geometric parametrization of quantum states. It is demonstrated in this research that Bang-Bang controls, triangle-function controls and square-function control can be utilized to manipulate controllable qubits or encoded qubits on controllable subspace for both open quantum dynamical systems and uncontrollable closed quantum dynamical systems. Furthermore, we propose a new kind of time-energy performance index to trade-off time and energy resource cost, and comprehensively discuss how to design control magnitude to minimize a kind of time-energy performance. A comparison has been made among these three kind of optimal control. It is underlined in this research that the optimal time performance can be always expressed as J^{*} =\\lamda{\\cdot}t^{*}_{f} +E^{*} for...

Zhang, Ming

2010-01-01

253

Galois quantum systems, irreducible polynomials and Riemann surfaces

International Nuclear Information System (INIS)

Finite quantum systems in which the position and momentum take values in the Galois field GF(pl), are studied. Ideas from the subject of field extension are transferred in the context of quantum mechanics. The Frobenius automorphisms in Galois fields lead naturally to the 'Frobenius formalism' in a quantum context. The Hilbert space splits into 'Frobenius subspaces' which are labeled with the irreducible polynomials associated with the ypl-y. The Frobenius maps transform unitarily the states of a Galois quantum system and leave fixed all states in some of its Galois subsystems (where the position and momentum take values in subfields of GF(pl)). An analytic representation of these systems in the l-sheeted complex plane shows deeper links between Galois theory and Riemann surfaces

2006-01-01

254

Risk-sensitive optimal control of quantum systems

International Nuclear Information System (INIS)

The importance of feedback control is being increasingly appreciated in quantum physics and applications. This paper describes the use of optimal control methods in the design of quantum feedback control systems, and in particular the paper formulates and solves a risk-sensitive optimal control problem. The resulting risk-sensitive optimal control is given in terms of an unnormalized conditional state, whose dynamics include the cost function used to specify the performance objective. The risk-sensitive conditional dynamic equation describes the evolution of our knowledge of the quantum system tempered by our purpose for the controlled quantum system. Robustness properties of risk-sensitive controllers are discussed and an example is provided

2004-01-01

255

Scavenging quantum information: Multiple observations of quantum systems

Given an unknown state of a qudit that has already been measured optimally, can one still extract any information about the original unknown state? Clearly, after a maximally informative measurement, the state of the system `collapses' into a post-measurement state from which the {\\em{same}} observer cannot obtain further information about the original state of the system. However, the system still encodes a significant amount of information about the original preparation for a second observer who is unaware of the actions of the first one. We study how a series of independent observers can obtain, or scavenge, information about the unknown state of a system (quantified by the fidelity) when they sequentially measure it. We give closed-form expressions for the estimation fidelity, when one or several qudits are available to carry information about the single-qudit state, and study the `classical' limit when an arbitrarily large number of observers can obtain (nearly) complete information on the system. In add...

Rapcan, Peter; Munoz-Tapia, Ramon; Bagan, Emilio; Buzek, Vladimir

2011-01-01

256

Approaches to open quantum systems: Decoherence, localisation and all that

International Nuclear Information System (INIS)

[en] This thesis is mainly concerned with issues in quantum open systems and the foundations of quantum theory. Chapter I introduces the aim, background and main results which take place in the following chapters. Chapters II and III are used to study and compare the decoherent histories approach, the environment-induced decoherence and the localisation properties of the solutions to the stochastic Schrodinger equation in quantum jump simulation and quantum state diffusion approaches, for a quantum two-level system model. We show, in particular, that there is a close connection between the decoherent histories and the quantum jump simulation, complementing a connection with the quantum state diffusion approach noted earlier by Diosi, Gisin, Halliwell and Percival. In the case of the decoherent histories analysis, the degree of approximate decoherence is discussed in detail. As by-product, by using the von Neumann entropy, we also discuss the predictability and its relation to the upper bounds of degree of decoherence. In Chapter IV, we give an alternative and elementary derivation of the Hu-Paz-Ghang master equation for quantum Brownian motion in a general environment, which involves tracing the evolution equation for the Wigner function. We also discuss the master equation in some special cases. This master equation provides a very useful tool to study the decoherence of a quantum system due to the interaction with its environment. In Chapter V, a derivation of the parameter-based uncertainty relation between position and momentum is given. This uncertainty relation can be regarded as an exact counterpart of the time-energy uncertainty relation. The final chapter is a rather brief summary of the thesis. (author)

1998-01-01

257

Deterministic constant-temperature dynamics for dissipative quantum systems

Energy Technology Data Exchange (ETDEWEB)

A novel method is introduced in order to treat the dissipative dynamics of quantum systems interacting with a bath of classical degrees of freedom. The method is based upon an extension of the Nose-Hoover chain (constant temperature) dynamics to quantum-classical systems. Both adiabatic and nonadiabatic numerical calculations on the relaxation dynamics of the spin-boson model show that the quantum-classical Nose-Hoover chain dynamics represents the thermal noise of the bath in an accurate and simple way. Numerical comparisons, both with the constant-energy calculation and with the quantum-classical Brownian motion treatment of the bath, show that the quantum-classical Nose-Hoover chain dynamics can be used to introduce dissipation in the evolution of a quantum subsystem even with just one degree of freedom for the bath. The algorithm can be computationally advantageous in modelling, within computer simulation, the dynamics of a quantum subsystem interacting with complex molecular environments. (fast track communication)

Sergi, Alessandro [Dipartimento di Fisica, Universita degli Studi di Messina, Contrada Papardo 98166 Messina (Italy)

2007-04-27

258

Deterministic constant-temperature dynamics for dissipative quantum systems

International Nuclear Information System (INIS)

[en] A novel method is introduced in order to treat the dissipative dynamics of quantum systems interacting with a bath of classical degrees of freedom. The method is based upon an extension of the Nose-Hoover chain (constant temperature) dynamics to quantum-classical systems. Both adiabatic and nonadiabatic numerical calculations on the relaxation dynamics of the spin-boson model show that the quantum-classical Nose-Hoover chain dynamics represents the thermal noise of the bath in an accurate and simple way. Numerical comparisons, both with the constant-energy calculation and with the quantum-classical Brownian motion treatment of the bath, show that the quantum-classical Nose-Hoover chain dynamics can be used to introduce dissipation in the evolution of a quantum subsystem even with just one degree of freedom for the bath. The algorithm can be computationally advantageous in modelling, within computer simulation, the dynamics of a quantum subsystem interacting with complex molecular environments. (fast track communication)

2007-04-27

259

Quantum state reconstruction from dynamical systems theory

When an informationally incomplete set of observables is considered there are several solutions to the quantum state reconstruction problem using von Neumann measurements. The set of solutions are known as Pauli partners, which are not easy to find even numerically. We present, in a self-contained paper, a new way to find this solutions using the physical imposition operator. We show that every Pauli partner is an attractive fixed point of this operator, which means that we can find complete sets of Pauli partners very efficiently. As a particular case, we found numerically 24 mutually unbiased bases in dimension N=23 in less than 30 seconds in a standard PC. We hope that the algorithm presented can be adapted to construct MU Constellations, SIC-POVMs, Equiangular Tight Frames and Quantum t-Designs, which could open new possibilities to find numerical solutions to these open problems related with quantum information theory.

Goyeneche, D

2011-01-01

260

Decohering histories and open quantum systems

I briefly review the "decohering histories" or "consistent histories" formulation of quantum theory, due to Griffiths, Omnès, and Gell-Mann and Hartle (and the subject of my graduate work with George Sudarshan). I also sift through the many meanings that have been attached to decohering histories, with an emphasis on the most basic one: Decoherence of appropriate histories is needed to establish that quantum mechanics has the correct classical limit. Then I will describe efforts to find physical mechanisms that do this. Since most work has focused on density matrix versions of decoherence, I'll consider the relation between the two formulations, which historically has not been straightforward. Finally, I'll suggest a line of research that would use recent results by Sudarshan to illuminate this aspect of the classical limit of quantum theory.

Chisolm, Eric D.

2009-11-01

261

Decoherence Control in Open Quantum System via Classical Feedback

In this work we propose a novel strategy using techniques from systems theory to completely eliminate decoherence and also provide conditions under which it can be done so. A novel construction employing an auxiliary system, the bait, which is instrumental to decoupling the system from the environment is presented. Our approach to decoherence control in contrast to other approaches in the literature involves the bilinear input affine model of quantum control system which lends itself to various techniques from classical control theory, but with non-trivial modifications to the quantum regime. The elegance of this approach yields interesting results on open loop decouplability and Decoherence Free Subspaces(DFS). Additionally, the feedback control of decoherence may be related to disturbance decoupling for classical input affine systems, which entails careful application of the methods by avoiding all the quantum mechanical pitfalls. In the process of calculating a suitable feedback the system has to be restru...

Ganesan, N; Ganesan, Narayan; Tarn, Tzyh Jong

2006-01-01

262

Dissipation-assisted quantum computation in atom-cavity systems

The principal obstacle to quantum information processing with many qubits is decoherence. One source of decoherence is spontaneous emission which causes loss of energy and information. Inability to control system parameters with high precision is another possible source of error. Strategies aimed at overcoming one kind of error typically increase sensitivity to others. As a solution we propose quantum computing with dissipation-assisted quantum gates. These can be run relatively fast while achieving fidelities close to one and have a success rate better than 75%.

Beige, A; Knight, P L; Beige, Almut; Cable, Hugo; Knight, Peter L.

2003-01-01

263

Time-resolved electron transport in quantum-dot systems

International Nuclear Information System (INIS)

In this thesis the time-resolved electron transport in quantum dot systems was studied. For this two different formalisms were presented: The nonequilibrium Green functions and the generalized quantum master equations. For both formalisms a propagation method for the numerical calculation of time-resolved expectation values, like the occupation and the electron current, was developed. For the demonstration of the propagation method two different question formulations were considered. On the one hand the stochastically driven resonant-level model was studied. On the other hand the pulse-induced transport through a double quantum dot was considered.

2010-01-01

264

Unknown system boundaries cannot be determined within quantum Darwinism

Observers restricted to interactions with environmental degrees of freedom that nondestructively encode pointer states of a system of interest S cannot determine from such interactions which degrees of freedom of S interact directly or indirectly with the environment E. Without a specification of the S-E boundary, such observers cannot use einselection and quantum Darwinism to calculate the pointer states of S or their environmental encodings. Quantum Darwinism requires S-E boundary specifications assumed or stipulated on the basis of classical-scale observations, and therefore cannot be regarded as providing a predictive, purely quantum-mechanical explanation of the "emergence" of classicality.

Fields, Chris

2010-01-01

265

Scalar Material Reference Systems and Loop Quantum Gravity

In the past, the possibility to employ (scalar) material reference systems in order to describe classical and quantum gravity directly in terms of gauge invariant (Dirac) observables has been emphasised frequently. This idea has been picked up more recently in Loop Quantum Gravity (LQG) with the aim to perform a reduced phase space quantisation of the theory thus possibly avoiding problems with the (Dirac) operator constraint quantisation method for constrained system. In this work, we review the models that have been studied on the classical and/or the quantum level and parametrise the space of theories so far considered. We then describe the quantum theory of a model that, to the best of our knowledge, so far has only been considered classically. This model could arguably called the optimal one in this class of models considered as it displays the simplest possible true Hamiltonian while at the same time reducing all constraints of General Relativity.

Giesel, Kristina

2012-01-01

266

Experimental Quantum Computing to Solve Systems of Linear Equations

Solving linear systems of equations is ubiquitous in all areas of science and engineering. With rapidly growing data sets, such a task can be intractable for classical computers, as the best known classical algorithms require a time proportional to the number of variables N. A recently proposed quantum algorithm shows that quantum computers could solve linear systems in a time scale of order log?(N), giving an exponential speedup over classical computers. Here we realize the simplest instance of this algorithm, solving 2×2 linear equations for various input vectors on a quantum computer. We use four quantum bits and four controlled logic gates to implement every subroutine required, demonstrating the working principle of this algorithm.

Cai, X.-D.; Weedbrook, C.; Su, Z.-E.; Chen, M.-C.; Gu, Mile; Zhu, M.-J.; Li, Li; Liu, Nai-Le; Lu, Chao-Yang; Pan, Jian-Wei

2013-06-01

267

Quantum computation by coupled quantum dot system and controlled NOT operation

A quantum computer based on an asymmetric coupled dot system has been proposed and shown to operate as the controlled NOT gate. The basic idea is (1) the electron is localized in one of the asymmetric coupled dots. (2)The electron transfer takes place from one dot to the other when the energy-levels of the coupled dots are set close. (3)The Coulomb interaction between the coupled dots mutually affects the energy levels of the other coupled dots. The proposed system can be realized by developing the technology of the single electron memory using Si nanocrystals and the direct combination of the quantum circuit and the conventional circuit is possible.

Tanamoto, T

2000-01-01

268

Stochastic representation of quantum interactions and two-lewel systems

Stochastic representation for interaction of quantum systems is formulated which allows to replace some of them by equivalent but purely commutative random sources. The formalism is applied to two-level systems interacting with Gaussian thermal bath. Strong-coupling non-Marcovian effects and besides long-living fluctuations in common susceptibility of two systems subjected to the same bath are considered.

Kuzovlev, Y E

2003-01-01

269

Photon statistics: Nonlinear spectroscopy of single quantum systems

International Nuclear Information System (INIS)

[en] A unified description of multitime correlation functions, nonlinear response functions, and quantum measurements is developed using a common generating function which allows a direct comparison of their information content. A general formal expression for photon counting statistics from single quantum objects is derived in terms of Liouville-space correlation functions of the material system by making a single assumption that a spontaneous emission is described by a master equation

2003-01-01

270

Quantum simulators, continuous-time automata, and translationally invariant systems.

UK PubMed Central (United Kingdom)

The general problem of finding the ground state energy of lattice Hamiltonians is known to be very hard, even for a quantum computer. We show here that this is the case even for translationally invariant systems in 1D. We also show that a quantum computer can be built in a 1D chain with a fixed, translationally invariant Hamitonian consisting of nearest-neighbor interactions only. The result of the computation is obtained after a prescribed time with high probability.

Vollbrecht KG; Cirac JI

2008-01-01

271

Projected wave functions for fractionalized phases of quantum spin systems

Gutzwiller projection allows a construction of an assortment of variational wave functions for strongly correlated systems. For quantum spin S=1/2 models, Gutzwiller-projected wave functions have resonating-valence-bond structure and may represent states with fractional quantum numbers for the excitations. Using insights obtained from field-theoretical descriptions of fractionalization in two dimensions, we construct candidate wave functions of fractionalized states by projecting specific superconducting states. We explicitly demonstrate the presence of topological order in these states.

Ivanov, D A

2002-01-01

272

Scaling law and stability for a noisy quantum system

We show that a scaling law exists for the near resonant dynamics of cold kicked atoms in the presence of a randomly fluctuating pulse amplitude. Analysis of a quasi-classical phase-space representation of the quantum system with noise allows a new scaling law to be deduced. The scaling law and associated stability are confirmed by comparison with quantum simulations and experimental data.

Sadgrove, Mark; Parkins, Scott; Leonhardt, Rainer

2008-01-01

273

Decoherence-free quantum dynamics in circuit QED system

We study decoherence in a circuit QED system consisting of a charge qubit and two superconducting transmission line resonators (TLRs). We show that in the dispersive regime of the circuit QED system one TLR can be used as an auxiliary subsystem to realize decoherence-free quantum dynamics of the bipartite target system consisting of the charge qubit and the other TLR conditioned on the auxiliary TLR initially being a proper number state. Our study gives new insight into control and manipulation of decoherence in quantum systems.

Liao, Ping; Liao, Jie-Qiao; Kuang, Le-Man

2009-01-01

274

Symplectic group and invariants of quantum systems

Energy Technology Data Exchange (ETDEWEB)

The purpose of the paper is to analyze tendencies developed during the last decade in quantum theory in connection with the intense application of group-theoretic methods (methods of the theory of representations of Lie groups) and variational methods in formulating and solving physical problems.

Man' ko, V.I.

1987-03-20

275

Wait-Free Synchronization in Quantum-Based Multiprogrammed Systems

UK PubMed Central (United Kingdom)

)James H. Anderson, Rohit Jain, and David OttDepartment of Computer ScienceUniversity of North Carolina at Chapel HillAbstract. We consider wait-free synchronization in multiprogrammeduniprocessor and multiprocessor systems in which the processes bound toeach processor are scheduled for execution using a scheduling quantum.We show that, in such systems, any object with consensus number P inHerlihy's wait-free hierarchy is universal for any number of processes executingon P processors, provided the scheduling quantum is of a certainsize. We give an asymptotically tight characterization of how large thescheduling quantum must be for this result to hold.1 IntroductionThis paper is concerned with wait-free synchronization in multiprogrammed systems.In such systems, several processes may be bound to the same processor. Inrelated previous work, Ramamurthy, Moir, and Anderson considered wait-freesynchronization in multiprogrammed systems in which processes on the ...

276

Wait-Free Synchronization in Quantum-Based Multiprogrammed Systems

UK PubMed Central (United Kingdom)

)James H. Anderson, Rohit Jain, and David OttDepartment of Computer ScienceUniversity of North Carolina at Chapel HillAbstract. We consider wait-free synchronization in multiprogrammeduniprocessor and multiprocessor systems in which the processes bound toeach processor are scheduled for execution using a scheduling quantum.We show that, in such systems, any object with consensus number P inHerlihy's wait-free hierarchy is universal for any number of processes executingon P processors, provided the scheduling quantum is of a certainsize. We give an asymptotically tight characterization of how large thescheduling quantum must be for this result to hold.1 IntroductionThis paper is concerned with wait-free synchronization in multiprogrammed systems.In such systems, several processes may be bound to the same processor. Inrelated previous work, Ramamurthy, Moir, and Anderson considered wait-freesynchronization in multiprogrammed systems in which processes on the sameproce...

277

Effects of symmetry breaking in finite quantum systems

The review considers the peculiarities of symmetry breaking and symmetry transformations and the related physical effects in finite quantum systems. Some types of symmetry in finite systems can be broken only asymptotically. However, with a sufficiently large number of particles, crossover transitions become sharp, so that symmetry breaking happens similarly to that in macroscopic systems. This concerns, in particular, global gauge symmetry breaking, related to Bose-Einstein condensation and superconductivity, or isotropy breaking, related to the generation of quantum vortices, and the stratification in multicomponent mixtures. A special type of symmetry transformation, characteristic only for finite systems, is the change of shape symmetry. These phenomena are illustrated by the examples of several typical mesoscopic systems, such as trapped atoms, quantum dots, atomic nuclei, and metallic grains. The specific features of the review are: (i) the emphasis on the peculiarities of the symmetry breaking in finit...

Birman, J L; Yukalov, V I

2013-01-01

278

Quantum demolition filtering and optimal control of unstable systems.

A brief account of the quantum information dynamics and dynamical programming methods for optimal control of quantum unstable systems is given to both open loop and feedback control schemes corresponding respectively to deterministic and stochastic semi-Markov dynamics of stable or unstable systems. For the quantum feedback control scheme, we exploit the separation theorem of filtering and control aspects as in the usual case of quantum stable systems with non-demolition observation. This allows us to start with the Belavkin quantum filtering equation generalized to demolition observations and derive the generalized Hamilton-Jacobi-Bellman equation using standard arguments of classical control theory. This is equivalent to a Hamilton-Jacobi equation with an extra linear dissipative term if the control is restricted to Hamiltonian terms in the filtering equation. An unstable controlled qubit is considered as an example throughout the development of the formalism. Finally, we discuss optimum observation strategies to obtain a pure quantum qubit state from a mixed one. PMID:23091216

Belavkin, V P

2012-11-28

279

Quantum demolition filtering and optimal control of unstable systems.

UK PubMed Central (United Kingdom)

A brief account of the quantum information dynamics and dynamical programming methods for optimal control of quantum unstable systems is given to both open loop and feedback control schemes corresponding respectively to deterministic and stochastic semi-Markov dynamics of stable or unstable systems. For the quantum feedback control scheme, we exploit the separation theorem of filtering and control aspects as in the usual case of quantum stable systems with non-demolition observation. This allows us to start with the Belavkin quantum filtering equation generalized to demolition observations and derive the generalized Hamilton-Jacobi-Bellman equation using standard arguments of classical control theory. This is equivalent to a Hamilton-Jacobi equation with an extra linear dissipative term if the control is restricted to Hamiltonian terms in the filtering equation. An unstable controlled qubit is considered as an example throughout the development of the formalism. Finally, we discuss optimum observation strategies to obtain a pure quantum qubit state from a mixed one.

Belavkin VP

2012-11-01

280

A quantum information perspective of fermionic quantum many-body systems

International Nuclear Information System (INIS)

In this Thesis fermionic quantum many-body system are theoretically investigated from a quantum information perspective. Quantum correlations in fermionic many-body systems, though central to many of the most fascinating effects of condensed matter physics, are poorly understood from a theoretical perspective. Even the notion of ''paired'' fermions which is widely used in the theory of superconductivity and has a clear physical meaning there, is not a concept of a systematic and mathematical theory so far. Applying concepts and tools from entanglement theory, we close this gap, developing a pairing theory allowing to unambiguously characterize paired states. We develop methods for the detection and quantification of pairing according to our definition which are applicable to current experimental setups. Pairing is shown to be a quantum correlation distinct from any notion of entanglement proposed for fermionic systems, giving further understanding of the structure of highly correlated quantum states. In addition, we show the resource character of paired states for precision metrology, proving that BCS-states allow phase measurements at the Heisenberg limit. Next, the power of fermionic systems is considered in the context of quantum simulations, where we study the possibility to simulate Hamiltonian time evolutions on a cubic lattice under the constraint of translational invariance. Given a set of translationally invariant local Hamiltonians and short range interactions we determine time evolutions which can and those which can not be simulated. Bosonic and finite-dimensional quantum systems (''spins'') are included in our investigations. Furthermore, we develop new techniques for the classical simulation of fermionic many-body systems. First, we introduce a new family of states, the fermionic Projected Entangled Pair States (fPEPS) on lattices in arbitrary spatial dimension. These are the natural generalization of the PEPS known for spin systems, and they approximate efficiently ground and thermal states of systems with short-range interaction. We give an explicit mapping between fPEPS and PEPS, allowing to extend previous simulation methods to fermions. In addition, we show that fPEPS naturally arise as exact ground states of certain fermionic Hamiltonians, and give an example that exhibits criticality while fulfilling the area law. Finally, we derive methods for the determination of ground and thermal states, as well as the time evolution, of interacting fermionic systems using generalized Hartree-Fock theory (gHFT). With the computational complexity scaling polynomially with the number of particles, this method can deal with large systems. As a benchmark we apply our methods to the translationally invariant Hubbard model with attractive interaction and find excellent agreement with known results. (orig.)

2009-01-01

281

Numerical approaches to complex quantum, semiclassical and classical systems

International Nuclear Information System (INIS)

[en] In this work we analyse the capabilities of several numerical techniques for the description of different physical systems. Thereby, the considered systems range from quantum over semiclassical to classical and from few- to many-particle systems. In chapter 1 we investigate the behaviour of a single quantum particle in the presence of an external disordered background (static potentials). Starting from the quantum percolation problem, we address the fundamental question of a disorder induced (Anderson-) transition from extended to localised single-particle eigenstates. Distinguishing isolating from conducting states by applying a local distribution approach for the local density of states (LDOS), we detect the quantum percolation threshold in two- and three-dimensions. Extending the quantum percolation model to a quantum random resistor model, we comment on the possible relevance of our results to the influence of disorder on the conductivity in graphene sheets. For the calculation of the LDOS as well as for the Chebyshev expansion of the time evolution operator, the kernel polynomial method (KPM) is the key numerical technique. In chapter 2 we examine how a single quantum particle is influenced by retarded bosonic fields that are inherent to the system. Within the Holstein model, these bosonic degrees of freedom (phonons) give rise to an infinite dimensional Hilbert space, posing a true many-particle problem. Constituting a minimal model for polaron formation, the Holstein model allows us to study the optical absorption and activated transport in polaronic systems. Using a two-dimensional variant of the KPM, we calculate for the first time quasi-exactly the optical absorption and dc-conductivity as a function of temperature. In chapter 3 we come back to the time evolution of a quantum particle in an external, static potential and investigate the capability of semiclassical approximations to it. We address basic quantum effects as tunneling, interference and anharmonicity. To this end we consider the linearised semiclassical propagator method, the Wigner-Moyal approach and the recently proposed quantum tomography. Finally, in chapter 4 we calculate the dynamics of a classical many-particle system under the influence of external fields. Considering a low-temperature rf-plasma, we investigate the interplay of the plasma dynamics and the motion of dust particles, immersed into the plasma for diagnostic reasons. (orig.)

2008-01-01

282

Numerical approaches to complex quantum, semiclassical and classical systems

Energy Technology Data Exchange (ETDEWEB)

In this work we analyse the capabilities of several numerical techniques for the description of different physical systems. Thereby, the considered systems range from quantum over semiclassical to classical and from few- to many-particle systems. In chapter 1 we investigate the behaviour of a single quantum particle in the presence of an external disordered background (static potentials). Starting from the quantum percolation problem, we address the fundamental question of a disorder induced (Anderson-) transition from extended to localised single-particle eigenstates. Distinguishing isolating from conducting states by applying a local distribution approach for the local density of states (LDOS), we detect the quantum percolation threshold in two- and three-dimensions. Extending the quantum percolation model to a quantum random resistor model, we comment on the possible relevance of our results to the influence of disorder on the conductivity in graphene sheets. For the calculation of the LDOS as well as for the Chebyshev expansion of the time evolution operator, the kernel polynomial method (KPM) is the key numerical technique. In chapter 2 we examine how a single quantum particle is influenced by retarded bosonic fields that are inherent to the system. Within the Holstein model, these bosonic degrees of freedom (phonons) give rise to an infinite dimensional Hilbert space, posing a true many-particle problem. Constituting a minimal model for polaron formation, the Holstein model allows us to study the optical absorption and activated transport in polaronic systems. Using a two-dimensional variant of the KPM, we calculate for the first time quasi-exactly the optical absorption and dc-conductivity as a function of temperature. In chapter 3 we come back to the time evolution of a quantum particle in an external, static potential and investigate the capability of semiclassical approximations to it. We address basic quantum effects as tunneling, interference and anharmonicity. To this end we consider the linearised semiclassical propagator method, the Wigner-Moyal approach and the recently proposed quantum tomography. Finally, in chapter 4 we calculate the dynamics of a classical many-particle system under the influence of external fields. Considering a low-temperature rf-plasma, we investigate the interplay of the plasma dynamics and the motion of dust particles, immersed into the plasma for diagnostic reasons. (orig.)

Schubert, Gerald

2008-11-03

283

Quantum dynamics of bio-molecular systems in noisy environments

We discuss three different aspects of the quantum dynamics of bio-molecular systems and more generally complex networks in the presence of strongly coupled environments. Firstly, we make a case for the systematic study of fundamental structural elements underlying the quantum dynamics of these systems, identify such elements and explore the resulting interplay of quantum dynamics and environmental decoherence. Secondly, we critically examine some existing approaches to the numerical description of system-environment interaction in the non-perturbative regime and present a promising new method that can overcome some limitations of existing methods. Thirdly, we present an approach towards deciding and quantifying the non-classicality of the action of the environment and the observed system-dynamics. We stress the relevance of these tools for strengthening the interplay between theoretical and experimental research in this field.

Plenio, M B

2012-01-01

284

Hidden symmetries enhance quantum transport in Light Harvesting systems

For more than 50 years we have known that photosynthetic systems harvest solar energy with almost unit {\\it quantum efficiency}. However, recent experimental evidence of {\\it quantum coherence} during the excitonic energy transport in photosynthetic organisms challenges our understanding of this fundamental biological function. Currently, and despite numerous efforts, the causal connection between coherence and efficiency is still a matter of debate. We show, through the study of extensive simulations of quantum coherent transport on networks, that three dimensional structures characterized by centro-symmetric Hamiltonians are statistically more efficient than random arrangements. Moreover, we demonstrate that the experimental data available for the electronic Hamiltonians of the Fenna-Mathew-Olson (FMO) complex of sulfur bacteria and of the crypophyte PC645 complex of marine algae are consistent with this strong correlation of centro-symmetry with quantum efficiency. These results show that what appears to b...

Zech, Tobias; Wellens, Thomas; Buchleitner, Andreas

2012-01-01

285

Quantum computing with collective ensembles of multi-level systems

We propose a new physical approach for encoding and processing of quantum information in ensembles of multi-level quantum systems, where the different bits are not carried by individual particles but associated with the collective population of different internal levels. One- and two-bit gates are implemented by collective internal state transitions taking place in the presence of an excitation blockade mechanism which restricts the population of each internal state to the values zero and unity. 10-20 bit quantum computers can be built via this scheme in single trapped clouds of ground state atoms subject to the Rydberg excitation blockade mechanism, and the linear dependence between register size and the number of internal quantum states in atoms offers realistic means to reach larger registers.

Brion, E; Saffman, M

2007-01-01

286

Quantum computing with collective ensembles of multilevel systems.

UK PubMed Central (United Kingdom)

We propose a new physical approach for encoding and processing of quantum information in ensembles of multilevel quantum systems, where the different bits are not carried by individual particles but associated with the collective population of different internal levels. One- and two-bit gates are implemented by collective internal state transitions taking place in the presence of an excitation blockade mechanism, which restricts the population of each internal state to the values zero and unity. Quantum computers with 10-20 bits can be built via this scheme in single trapped clouds of ground state atoms subject to the Rydberg excitation blockade mechanism, and the linear dependence between register size and the number of internal quantum states in atoms offers realistic means to reach larger registers.

Brion E; Mølmer K; Saffman M

2007-12-01

287

Unifying relation for quantum systems driven out of equilibrium

We extend a classical relation derived by Crooks to quantum systems driven out of equilibrium and show that it provides a unified way of deriving both known and new results for these systems. For a fluid driven to a steady state with a shear flow, we use it to prove the fluctuation theorem for shear stress to obtain the Green-Kubo formula for shear viscosity in terms of the symmetrized correlation function of the shear stress operator. We also show that a generalized entropy for a quantum system in a steady heat conduction state satisfies extensions of the Clausius and the Gibbs relations.

Matsuoka, Hiroshi

2011-01-01

288

Entangled quantum state discrimination using a pseudo-Hermitian system

International Nuclear Information System (INIS)

[en] We demonstrate how to discriminate two non-orthogonal, entangled quantum states which are slightly different from each other by using a pseudo-Hermitian system. The positive definite metric operator which makes the pseudo-Hermitian system fully consistent with quantum theory is used for such a state discrimination. We further show that non-orthogonal states can evolve through a suitably constructed pseudo-Hermitian Hamiltonian to orthogonal states. Such an evolution ceases at exceptional points of the pseudo-Hermitian system. (paper)

2012-09-07

289

Preparing thermal states of quantum systems by dimension reduction.

We present an algorithm that prepares thermal Gibbs states of one dimensional quantum systems on a quantum computer without any memory overhead, and in a time significantly shorter than other known alternatives. Specifically, the time complexity is dominated by the quantity N(?h?/T), where N is the size of the system, ?h? is a bound on the operator norm of the local terms of the Hamiltonian (coupling energy), and T is the temperature. Given other results on the complexity of thermalization, this overall scaling is likely optimal. For higher dimensions, our algorithm lowers the known scaling of the time complexity with the dimension of the system by one. PMID:21231028

Bilgin, Ersen; Boixo, Sergio

2010-10-22

290

Preparing thermal states of quantum systems by dimension reduction.

UK PubMed Central (United Kingdom)

We present an algorithm that prepares thermal Gibbs states of one dimensional quantum systems on a quantum computer without any memory overhead, and in a time significantly shorter than other known alternatives. Specifically, the time complexity is dominated by the quantity N(?h?/T), where N is the size of the system, ?h? is a bound on the operator norm of the local terms of the Hamiltonian (coupling energy), and T is the temperature. Given other results on the complexity of thermalization, this overall scaling is likely optimal. For higher dimensions, our algorithm lowers the known scaling of the time complexity with the dimension of the system by one.

Bilgin E; Boixo S

2010-10-01

291

N-Photon Wavepackets Interacting with an Arbitrary Quantum System

We present a theoretical framework that describes a wavepacket of light prepared in a state of definite photon number interacting with an arbitrary quantum system (e.g. a quantum harmonic oscillator or a multi-level atom). Within this framework we derive master equations for the system as well as for output field quantities such as quadratures and photon flux. These results are then generalized to wavepackets with arbitrary spectral distribution functions. Finally, we obtain master equations and output field quantities for systems interacting with wavepackets in multiple spatial and/or polarization modes.

Baragiola, Ben Q; Branczyk, Agata M; Combes, Joshua

2012-01-01

292

The power of quantum systems on a line

We study the computational strength of quantum particles (each of finite dimensionality) arranged on a line. First, we prove that it is possible to perform universal adiabatic quantum computation using a one-dimensional quantum system (with 9 states per particle). This might have practical implications for experimentalists interested in constructing an adiabatic quantum computer. Building on the same construction, but with some additional technical effort and 12 states per particle, we show that the problem of approximating the ground state energy of a system composed of a line of quantum particles is QMA-complete; QMA is a quantum analogue of NP. This is in striking contrast to the fact that the analogous classical problem, namely, one dimensional MAX-2-SAT with nearest neighbor constraints, is in P. The proof of the QMA-completeness result requires an additional idea beyond the usual techniques in the area: Not all illegal configurations can be ruled out by local checks, so instead we rule out such illegal ...

Aharonov, Dorit; Kempe, Julia

2007-01-01

293

Kepler-16 Circumbinary System Validates Quantum Celestial Mechanics

Directory of Open Access Journals (Sweden)

Full Text Available We report the application of quantum celestial mechanics (QCM) to the Kepler-16 circumbinary system which has a single planet orbiting binary stars with the important system parameters known to within one percent. Other gravitationally bound systems such as the Solar System of planets and the Jovian satellite systems have large uncertainties in their total angular momentum. Therefore, Kepler-16 allows us for the ?rst time to determine whether the QCM predicted angular momentum per mass quantization isvalid.

Potter F.; Preston H. G.

2012-01-01

294

Bohmian Mechanics In A Macroscopic Quantum System

In the so called `causal' interpretation of quantum mechanics, an electron is considered as a particle and such particle is influenced not only by a classical but also by a so called quantum potential. This idea was developed by Professor Bohm in an important paper. In this paper we use some of the basics of this interpretation in a financial option pricing environment. The causal interpretation allows for trajectories. Path breaking work by Professors Bohm and Hiley and Khrennikov and Choustova have made that the causal interpretation is a step closer to potential applications in social science. In this paper we consider the wave function as a wave of information. We consider the gradient of the phase of this wave function and show how the option price could be influenced by this gradient.

Haven, Emmanuel

2006-01-01

295

The 20th century saw the first revolution of quantum mechanics, setting the rules for our understanding of light, matter, and their interaction. The 21st century is focused on using these quantum mechanical laws to develop technologies which allows us to solve challenging practical problems. One of the directions is the use quantum devices which promise to surpass the best computers and best known classical algorithms for solving certain tasks. Crucial to the design of realistic devices and technologies is to account for the open nature of quantum systems and to cope with their interactions with the environment. In the first part of this dissertation, we show how to tackle classical optimization problems of interest in the physical sciences within one of these quantum computing paradigms, known as quantum annealing (QA). We present the largest implementation of QA on a biophysical problem (six different experiments with up to 81 superconducting quantum bits). Although the cases presented here can be solved on a classical computer, we present the first implementation of lattice protein folding on a quantum device under the Miyazawa-Jernigan model. This is the first step towards studying optimization problems in biophysics and statistical mechanics using quantum devices. In the second part of this dissertation, we focus on the problem of excitonic energy transfer. We provide an intuitive platform for engineering exciton transfer dynamics and we show that careful consideration of the properties of the environment leads to opportunities to engineer the transfer of an exciton. Since excitons in nanostructures are proposed for use in quantum information processing and artificial photosynthetic designs, our approach paves the way for engineering a wide range of desired exciton dynamics. Finally, we develop the theory for a two-dimensional electronic spectroscopic technique based on fluorescence (2DFS) and challenge previous theoretical results claiming its equivalence to the two-dimensional photon echo (2DPE) technique which is based on polarization. Experimental realization of this technique confirms our theoretical predictions. The new technique is more sensitive than 2DPE as a tool for conformational determination of excitonically coupled chromophores and offers the possibility of applying two-dimensional electronic spectroscopy to single-molecules.

Perdomo, Alejandro

296

Optimizing entangling quantum gates for physical systems

Optimal control theory is a versatile tool that presents a route to significantly improving figures of merit for quantum information tasks. We combine it here with the geometric theory for local equivalence classes of two-qubit operations to derive an optimization algorithm that determines the best entangling two-qubit gate for a given physical setting. We demonstrate the power of this approach for trapped polar molecules and neutral atoms.

Müller, M M; Murphy, M; Yuan, H; Vala, J; Whaley, K B; Calarco, T; Koch, C P

2011-01-01

297

Electronic transport and noise in quantum dot systems

International Nuclear Information System (INIS)

In this thesis we describe the current and shot noise properties of quantum dot systems. Their transport characteristics reveal information about interesting quantum mechanical effects such as the energy quantization and electronic correlations due to Coulomb interactions of electrons. Based on a diagrammatic real time approach we developed a numerical method to describe the current and shot noise. The method includes all relevant quantities such as the electron spin, the Coulomb interaction as well as thedelocalized nature of the electronic wavefunctions in coupled quantum dots. Our approach is based on a perturbative expansion in terms of the coupling constant to the leads and thus allows to describe sequential tunneling as well as co-tunneling transport in local as well as non-local multilevel systems. For a system of a double quantum dot we analyzed in detail the influence of asymmetries on the electronic transport properties and found strong correlations. In contrast, larger systems such as three and more coupled quantum dots display a strong noise enhancement even in fully symmetric situations due to their complex delocalized wavefunctions. Within the Coulomb blockade transport is governed by co-tunneling processes. In particular we investigated the regime of co-tunneling assisted sequential tunneling and described characteristic features in the differential conductance as well as the noise properties. (orig.)

2007-01-01

298

Quantum railroads and directed localization at the juncture of quantum Hall systems

The integer quantum Hall effect (QHE) and one-dimensional Anderson localization (AL) are limiting special cases of a more general phenomenon, directed localization (DL), predicted to occur in disordered one-dimensional wave guides called "quantum railroads" (QRR). Here we explain the surprising results of recent measurements by Kang et al. [Nature 403, 59 (2000)] of electron transfer between edges of two-dimensional electron systems and identify experimental evidence of QRR's in the general, but until now entirely theoretical, DL regime that unifies the QHE and AL. We propose direct experimental tests of our theory.

Nonoyama, S; Nonoyama, Shinji; Kirczenow, George

2002-01-01

299

Quantum System Identification: Hamiltonian Estimation using Spectral and Bayesian Analysis

Identifying the Hamiltonian of a quantum system from experimental data is considered. General limits on the identifiability of model parameters with limited experimental resources are investigated, and a specific Bayesian estimation procedure is proposed and evaluated for a model system where a-priori information about the Hamiltonian's structure is available.

Schirmer, S G

2009-01-01

300

Effective Strategies for Identifying Model Parameters for Open Quantum Systems

The problem of identifiability of model parameters for open quantum systems is considered by investigating two-level dephasing systems. We discuss under which conditions full information about the Hamiltonian and dephasing parameters can be obtained. Using simulated experiments several different strategies for extracting model parameters from limited and noisy data are compared.

Gong, Er-ling; Schirmer, S G; Sun, Zhi-Qiang; Zhang, Ming

2010-01-01

301

Hidden symmetry of the quantum Calogero-Moser system.

DEFF Research Database (Denmark)

The hidden symmetry of the quantum Calogero-Moser system with an inverse-square potential is algebraically demonstrated making use of Dunkl's operators. We find the underlying algebra explaining the super-integrability phenomenon for this system. Applications to related multi-variable Bessel functions are also discussed.

Kuzentsov, Vadim b

1996-01-01

302

Cooling of quantum systems through optimal control and dissipation

Based on an exact non-Markovian open systems quantum dynamics, we demonstrate how to reduce the entropy of an open system through a cooperative effect of driving and dissipation. We illustrate the controlled dynamics in phase space in terms of Wigner functions and discuss the applicability of approximate approaches using master equations.

Schmidt, Rebecca; Rohrer, Selina; Ankerhold, Joachim; Stockburger, Jürgen T.

2012-11-01

303

Electrical control of spontaneous emission and strong coupling for a single quantum dot

DEFF Research Database (Denmark)

We report the design, fabrication and optical investigation of electrically tunable single quantum dots—photonic crystal defect nanocavities operating in both the weak and strong coupling regimes of the light–matter interaction. Unlike previous studies where the dot–cavity spectral detuning was varied by changing the lattice temperature, or by the adsorption of inert gases at low temperatures, we demonstrate that the quantum-confined Stark effect can be employed to quickly and reversibly switch the dot–cavity coupling simply by varying a gate voltage. Our results show that exciton transitions from individual dots can be tuned by4 meV relative to the nanocavity mode before the emission quenches due to carrier tunneling escape. This range is much larger than the typical linewidth of the high-Q cavity modes (100?eV) allowing us to explore and contrast regimes where the dots couple to the cavity or decay by spontaneous emission into the two-dimensional photonic bandgap. In the weak-coupling regime, we show that the dot spontaneous emission rate can be tuned using a gate voltage, with Purcell factors>7. New information is obtained on the nature of the dot–cavity coupling in the weak coupling regime, and electrical control of zerodimensional polaritons is demonstrated for the highest-Q cavities (Q > 12 000). Vacuum Rabi splittings up to 120?eV are observed, larger than the linewidths of either the decoupled exciton ( 6 40?eV) or cavity mode. These observations represent a voltage switchable optical nonlinearity at the single photon level, paving the way towards on-chip dot-based nano-photonic devices that can be integrated with passive optical components.

Laucht, A.; Hofbauer, F.

2009-01-01

304

Electrical control of spontaneous emission and strong coupling for a single quantum dot

International Nuclear Information System (INIS)

We report the design, fabrication and optical investigation of electrically tunable single quantum dots-photonic crystal defect nanocavities operating in both the weak and strong coupling regimes of the light-matter interaction. Unlike previous studies where the dot-cavity spectral detuning was varied by changing the lattice temperature, or by the adsorption of inert gases at low temperatures, we demonstrate that the quantum-confined Stark effect can be employed to quickly and reversibly switch the dot-cavity coupling simply by varying a gate voltage. Our results show that exciton transitions from individual dots can be tuned by ?4 meV relative to the nanocavity mode before the emission quenches due to carrier tunneling escape. This range is much larger than the typical linewidth of the high-Q cavity modes (?100 ?eV) allowing us to explore and contrast regimes where the dots couple to the cavity or decay by spontaneous emission into the two-dimensional photonic bandgap. In the weak-coupling regime, we show that the dot spontaneous emission rate can be tuned using a gate voltage, with Purcell factors ?7. New information is obtained on the nature of the dot-cavity coupling in the weak coupling regime, and electrical control of zero-dimensional polaritons is demonstrated for the highest-Q cavities (Q?12 000). Vacuum Rabi splittings up to ?120 ?eV are observed, larger than the linewidths of either the decoupled exciton (??40 ?eV) or cavity mode. These observations represent a voltage switchable optical nonlinearity at the single photon level, paving the way towards on-chip dot-based nano-photonic devices that can be integrated with passive optical components.

2009-01-01

305

Adiabatic passage and ensemble control of quantum systems

This paper considers population transfer between eigenstates of a finite quantum ladder controlled by a classical electric field. Using an appropriate change of variables, we show that this setting can be set in the framework of adiabatic passage, which is known to facilitate ensemble control of quantum systems. Building on this insight, we present a mathematical proof of robustness for a control protocol -- chirped pulse -- practiced by experimentalists to drive an ensemble of quantum systems from the ground state to the most excited state. We then propose new adiabatic control protocols using a single chirped and amplitude shaped pulse, to robustly perform any permutation of eigenstate populations, on an ensemble of systems with badly known coupling strengths. Such adiabatic control protocols are illustrated by simulations achieving all 24 permutations for a 4-level ladder.

Leghtas, Zaki; Rouchon, Pierre

2010-01-01

306

Description and control of decoherence in quantum bit systems

International Nuclear Information System (INIS)

The description and control of decoherence of quantum bit systems have become a field of increasing interest during the last decade. We discuss different techniques to estimate and model decoherence sources of solid state quantum bit realizations. At first, we derive a microscopic, perturbation theoretical approach for Lindblad master equations of a spin-Boson model at low temperatures. A different sort of decoherence is investigate by means of the bistable fluctuator model. For this particular but nevertheless for solid state qubits relevant noise source, we present a suitably designed dynamical decoupling method (so-called quantum bang-bang). This works as a high-pass filter, suppressing low frequency parts of the noise most effectively and thus being a promising method to compensate the ubiquituous 1/f noise. Furthermore, we investigate the behaviour of a two coupled spin system exposed to collective and localized bath. For this dressed-spin system we receive by means of scaling-analysis in first order a quantum phase diagram. On that we can identify the various quantum dynamical and entanglement phases.

2005-01-01

307

Quantum spin ladder systems associated with SU(2|2)

Energy Technology Data Exchange (ETDEWEB)

Two integrable quantum spin ladder systems will be introduced associated with the fundamental SU 92/20 solution of the Yang-baxter equation. The first model is a generalized quantum Ising system with Ising rung interactions. In the second model the addition of extra interactions allows us to impose Heisenberg rung interactions without violating integrability. The existence of a Bethe ansatz solution for both models allows us to investigate the elementary excitations for antiferromagnetic rung couplings. We find that the first model does not show a gap whilst in the second case there is a gap for all positive values of the rung coupling. (author)

Foerster, A. [Rio Grande do Sul Univ., Porto Alegre, RS (Brazil). Inst. de Fisica]. E-mail: angela@if.ufrgs.br; Hibberd, K.E. [Centro Brasileiro de Pesquisas Fisicas (CBPF), Rio de Janeiro, RJ (Brazil)]. E-mail: keh@cbpf.br; Links, J.R. [Queensland Univ., St. Lucia, QLD (Australia). Centre for Mathematical Physics, Dept. of Mathematics]. E-mail: jrl@maths.uq.edu.au; Roditi, I. [State Univ. of New York, Stony Brook (United States). C.N. Yang Inst. for Theoretical Physics]. E-mail: roditi@cbpf.br

2000-12-01

308

Correlations of neutral kaons - an open quantum system approach

International Nuclear Information System (INIS)

[en] Full text: I will present the quantum mechanical model of decaying particles based on the open quantum system approach. I will discuss the decay of a single neutral kaon. In our model the time evolution of the density matrix describing such a kaon is given by the appropriate Lindblad equation. The corresponding Lindblad operators are constructed explicitly in two cases - when we assume CP-invariance and without this assumption. The complete positivity of the time evolution is proved by the explicit construction of the corresponding Krauss operators. We use this results to calculate the correlation function in neutral kaon system. (author)

2005-01-01

309

Is the Joint Entropy Behavior Dependent on Investigated Quantum Systems?

We have extensively investigated the time dependent entropy (or Leipnik's entropy) of harmonic and damped harmonic oscillators by using time dependent wave function obtained by the Feynman path integral method. In the literature, the joint entropy of the quantum mechanical system are shown monotonically increase with time but this result is not valid for simple harmonic and damped harmonic oscillator. In this study, while our results for simple harmonic oscillator are in agrement with literature, the joint entropy of damped harmonic oscillator shows negative value and remarkable discontinuity with time. Therefore, it is shown that the joint entropy behavior depends on investigated quantum systems.

Ozcan, O; Sever, R

2007-01-01

310

Bayesian parameter inference from continuously monitored quantum systems

DEFF Research Database (Denmark)

We review the introduction of likelihood functions and Fisher information in classical estimation theory, and we show how they can be defined in a very similar manner within quantum measurement theory. We show that the stochastic master equations describing the dynamics of a quantum system subject to a definite set of measurements provides likelihood functions for unknown parameters in the system dynamics, and we show that the estimation error, given by the Fisher information, can be identified by stochastic master equation simulations. For large parameter spaces we describe and illustrate the efficient use of Markov chain Monte Carlo sampling of the likelihood function.

Gammelmark, SØren; MØlmer, Klaus

2013-01-01

311

Arbitrarily Accurate Dynamical Control in Open Quantum Systems

We show that open-loop dynamical control techniques may be used to synthesize unitary transformations in open quantum systems in such a way that decoherence is perturbatively compensated for to a desired (in principle arbitrarily high) level of accuracy, which depends only on the strength of the relevant errors, and the achievable rate of control modulation. Our constructive and fully analytical solution employs concatenated dynamically corrected gates, and is applicable independently of detailed knowledge of the system-environment interactions and environment dynamics. Explicit implications for boosting quantum gate fidelities are addressed.

Khodjasteh, Kaveh; Viola, Lorenza

2009-01-01

312

Dynamic stabilization of a quantum many-body spin system.

UK PubMed Central (United Kingdom)

We demonstrate dynamic stabilization of a strongly interacting quantum spin system realized in a spin-1 atomic Bose-Einstein condensate. The spinor Bose-Einstein condensate is initialized to an unstable fixed point of the spin-nematic phase space, where subsequent free evolution gives rise to squeezing and quantum spin mixing. To stabilize the system, periodic microwave pulses are applied that rotate the spin-nematic many-body fluctuations and limit their growth. The stability diagram for the range of pulse periods and phase shifts that stabilize the dynamics is measured and compares well with a stability analysis.

Hoang TM; Gerving CS; Land BJ; Anquez M; Hamley CD; Chapman MS

2013-08-01

313

Thermodynamic length for far-from-equilibrium quantum systems.

UK PubMed Central (United Kingdom)

We consider a closed quantum system initially at thermal equilibrium and driven by arbitrary external parameters. We derive a lower bound on the entropy production which we express in terms of the Bures angle between the nonequilibrium and the corresponding equilibrium state of the system. The Bures angle is an angle between mixed quantum states and defines a thermodynamic length valid arbitrarily far from equilibrium. As an illustration, we treat the case of a time-dependent harmonic oscillator for which we obtain analytic expressions for generic driving protocols.

Deffner S; Lutz E

2013-02-01

314

Sign Rules for Anisotropic Quantum Spin Systems

We present new and exact ``sign rules'' for various spin-s anisotropic spin-lattice models. It is shown that, after a simple transformation which utilizes these sign rules, the ground-state wave function of the transformed Hamiltonian is positive-definite. Using these results exact statements for various expectation values of off-diagonal operators are presented, and transitions in the behavior of these expectation values are observed at particular values of the anisotropy. Furthermore, the effects of sign rules in variational calculations and quantum Monte Carlo calculations are considered. They are illustrated by a simple variational treatment of a one-dimensional anisotropic spin model.

Bishop, R F; Parkinson, J B

1999-01-01

315

Classical representation of a quantum system at equilibrium

International Nuclear Information System (INIS)

Complete text of publication follows. A quantum system at equilibrium is represented by a corresponding classical system, chosen to reproduce the thermodynamic and structural properties. The objective is to develop a means for exploiting strong coupling classical methods (e.g., MD, integral equations, DFT) to describe quantum systems. The classical system has an effective temperature, local chemical potential, and pair interaction that are defined by requiring equivalence of the grand potential and its functional derivatives with respect to the external and pair potentials for the classical and quantum systems. Practical inversion of this mapping for the classical properties is effected via the hypernetted chain approximation, leading to representations as functionals of the quantum pair correlation function (similar in spirit to the approach of Dharma-wardana and Perrot). The parameters of the classical system are determined such that ideal gas, weak coupling RPA, and strong coupling pair limits are preserved. The potential advantages of this approach are discussed. Research supported by NSF/DOE Partnership in Basic Plasma Science Award DE-FG02-07ER54946, and by US DOE Grant DE-SC0002139.

2011-01-01

316

Classical Representation of a Quantum System at Equilibrium

A quantum system at equilibrium is represented by a corresponding classical system, chosen to reproduce the thermodynamic and structural properties. The objective is to develop a means for exploiting strong coupling classical methods (e.g., MD, integral equations, DFT) to describe quantum systems. The classical system has an effective temperature, local chemical potential, and pair interaction that are defined by requiring equivalence of the grand potential and its functional derivatives with respect to the external and pair potentials for the classical and quantum systems. Practical inversion of this mapping for the classical properties is effected via the hypernetted chain approximation, leading to representations as functionals of the quantum pair correlation function (similar in spirit to the approach of Dharma-wardana and Perrot [1]). The parameters of the classical system are determined such that ideal gas, weak coupling RPA, and strong coupling pair limits are preserved. The potential advantages of this approach are discussed. Research supported by US DOE Grant DE-SC0002139. [4pt] [1] M. W. C. Dharma-wardana and F. Perrot, Phys. Rev. Lett. 84, 959 (2000).

Dutta, Sandipan; Dufty, James

2011-11-01

317

Dynamics of Large Quantum Systems: Equilibration, Thermalization and Interactions

The question of how/whether large quantum systems equilibrate and/or thermalize when prepared in an out-of-equilibrium state has been of enormous interest given recent experimental progress. We address this question in fermionic [1,2] and bosonic [3] systems, by following the dynamics of the full density matrix. We particularly study the case of two large-twin systems connected by a weak link (a quantum impurity), and we show that the total system equilibrates and thermalizes when the weak link is susceptible to incoherent and inelastic processes. We thus provide an experimentally feasible prescription for equilibrating and thermalizing large finite quantum systems. Our calculations are based on extending methods originally developed to treat subsystem dynamics (such as impurity), namely, the quantum Langevin equation method, the well known fermionic trace formula, and an iterative path integral approach. We also explore the role of interactions. While the fermionic system [1,2] shares many common features with the bosonic analog [3], we will describe certain crucial differences that arise as a result of different statistics.[4pt] [1] M. Kulkarni, K. L. Tiwari, D. Segal, arXiv:1206.2408[0pt] [2] M. Kulkarni, K. L. Tiwari, D. Segal, arXiv:1208.5725[0pt] [3] M. Kulkarni and D. Segal (in preparation)

Segal, Dvira; Kulkarni, Manas; Tiwari, Kunal

2013-03-01

318

International Nuclear Information System (INIS)

We establish an intriguing connection between quantum phase transitions and bifurcations in the reduced fidelity between two different reduced density matrices for quantum lattice many-body systems with symmetry-breaking order. Our finding is based on the observation that in the conventional Landau–Ginzburg–Wilson paradigm a quantum system undergoing a phase transition is characterized in terms of spontaneous symmetry breaking that is captured by a local-order parameter, which in turn results in an essential change of the reduced density matrix in the symmetry-broken phase. Two quantum systems on an infinite lattice in one spatial dimension, i.e. a quantum Ising model in a transverse magnetic field and a quantum spin-1/2 XYX model in an external magnetic field, are considered in the context of the tensor network algorithm based on the matrix product state representation. (paper)

2011-12-09

319

Energy Technology Data Exchange (ETDEWEB)

We establish an intriguing connection between quantum phase transitions and bifurcations in the reduced fidelity between two different reduced density matrices for quantum lattice many-body systems with symmetry-breaking order. Our finding is based on the observation that in the conventional Landau-Ginzburg-Wilson paradigm a quantum system undergoing a phase transition is characterized in terms of spontaneous symmetry breaking that is captured by a local-order parameter, which in turn results in an essential change of the reduced density matrix in the symmetry-broken phase. Two quantum systems on an infinite lattice in one spatial dimension, i.e. a quantum Ising model in a transverse magnetic field and a quantum spin-1/2 XYX model in an external magnetic field, are considered in the context of the tensor network algorithm based on the matrix product state representation. (paper)

Liu Jinhua; Shi Qianqian; Zhao Jianhui; Zhou Huanqiang, E-mail: huanqiang.zhou@gmail.com [Centre for Modern Physics and Department of Physics, Chongqing University, Chongqing 400044 (China)

2011-12-09

320

Efficient entanglement concentration for quantum dot and optical microcavities systems

A recent paper (Chuan Wang in Phys Rev A 86:012323, 2012) discussed an entanglement concentration protocol (ECP) for partially entangled electrons using a quantum dot and microcavity coupled system. In his paper, each two-electron spin system in a partially entangled state can be concentrated with the assistance of an ancillary quantum dot and a single photon. In this paper, we will present an efficient ECP for such entangled electrons with the help of only one single photon. Compared with the protocol of Wang, the most significant advantage is that during the whole ECP, the single photon only needs to pass through one microcavity which will increase the total success probability if the cavity is imperfect. The whole protocol can be repeated to get a higher success probability. With the feasible technology, this protocol may be useful in current long-distance quantum communications.

Sheng, Yu-Bo; Zhou, Lan; Wang, Lei; Zhao, Sheng-Mei

2013-05-01

321

Nuclear spintronics: quantum Hall and nano-systems

The electron spin transport in condensed matter, Spintronics, is a subject of rapidly growing interest both scientifically and from the point of view of applications to modern and future electronics. In many cases the electron spin transport cannot be described adequately without accounting for the hyperfine interaction between electron and nuclear spins. Under extreme conditions of high magnetic fileds, ultra-low temperatures, ultra high isotopical cleanness etc., the nuclear spins in these sytems are very promizing candidates for the qubits: the basic elements of emerging quantum memory, logics and hopefully quantum computers. Here we review the progress in the Nuclear Spintronics i.e. in physics and applications of hyperfine interactions in such exotic systems, as superconducting, quantum Hall, mesoscopic and nano-systems.

Vagner, I D

2004-01-01

322

Dielectric Response of Periodic Systems from Quantum Monte Carlo Calculations

We present a novel approach that allows to calculate the dielectric response of periodic systems in the quantum Monte Carlo formalism. We employ a many-body generalization for the electric enthalpy functional, where the coupling with the field is expressed via the Berry-phase formulation for the macroscopic polarization. A self-consistent local Hamiltonian then determines the ground-state wavefunction, allowing for accurate diffusion quantum Monte Carlo calculations where the polarization's fixed point is estimated from the average on an iterative sequence, sampled via forward-walking. This approach has been validated for the case of an isolated hydrogen atom, and then applied to a periodic system, to calculate the dielectric susceptibility of molecular-hydrogen chains. The results found are in excellent agreement with the best estimates obtained from the extrapolation of quantum-chemistry calculations.

Umari, P; Galli, G; Marzari, N; Galli, Giulia; Marzari, Nicola

2005-01-01

323

Quantum States and Phases in Driven Open Quantum Systems with Cold Atoms

An open quantum system, whose time evolution is governed by a master equation, can be driven into a given pure quantum state by an appropriate design of the system-reservoir coupling. This points out a route towards preparing many body states and non-equilibrium quantum phases by quantum reservoir engineering. Here we discuss in detail the example of a \\emph{driven dissipative Bose Einstein Condensate} of bosons and of paired fermions, where atoms in an optical lattice are coupled to a bath of Bogoliubov excitations via the atomic current representing \\emph{local dissipation}. In the absence of interactions the lattice gas is driven into a pure state with long range order. Weak interactions lead to a weakly mixed state, which in 3D can be understood as a depletion of the condensate, and in 1D and 2D exhibits properties reminiscent of a Luttinger liquid or a Kosterlitz-Thouless critical phase at finite temperature, with the role of the ``finite temperature'' played by the interactions.

Diehl, S; Kantian, A; Kraus, B; Büchler, H P; Zoller, P

2008-01-01

324

Topics in quantum information and the theory of open quantum systems

This thesis examines seven topics in the areas of deterministic open-quantum-system dynamics, quantum measurements, and quantum error correction (QEC). The first topic concerns weak measurements and their universality as a means of generating quantum operations. It is shown that every generalized measurement can be implemented as a sequence of weak (infinitesimal) measurements. The second topic is an application of this result to the theory of entanglement. Necessary and sufficient differential conditions for entanglement monotones are derived and are used to find a new entanglement monotone for three-qubit states. The third topic is a study of the performance of different master equations for the description of non-Markovian dynamics. The system studied is a qubit coupled to a spin bath via the Ising interaction. The fourth topic investigates continuous QEC in the presence of non-Markovian noise. It is shown that due to the existence of a Zeno regime in non-Markovian dynamics, the performance of continuous Q...

Oreshkov, Ognyan

2008-01-01

325

Electron spin and charge switching in a coupled quantum dot quantum ring system

Few-electron systems confined in a quantum dot laterally coupled to a surrounding quantum ring in the presence of an external magnetic field are studied by exact diagonalization. The distribution of electrons between the dot and the ring is influenced by the relative strength of the dot and ring confinement, the gate voltage and the magnetic field which induces transitions of electrons between the two parts of the system. These transitions are accompanied by changes in the periodicity of the Aharonov-Bohm oscillations of the ground-state angular momentum. The singlet-triplet splitting for a two electron system with one electron confined in the dot and the other in the ring exhibits piecewise linear dependence on the external field due to the Aharonov-Bohm effect for the ring-confined electron, in contrast to smooth oscillatory dependence of the exchange energy for laterally coupled dots in the side-by-side geometry.

Szafran, B; Bednarek, S

2004-01-01

326

Quantum systems with finite Hilbert space: Galois fields in quantum mechanics

International Nuclear Information System (INIS)

A 'Galois quantum system' in which the position and momentum take values in the Galois field GF(pl) is considered. It is comprised of l-component systems which are coupled in a particular way and is described by a certain class of Hamiltonians. Displacements in the GF(pl) x GF(pl) phase space and the corresponding Heisenberg-Weyl group are studied. Symplectic transformations are shown to form the Sp(2, GF(pl)) group. Wigner and Weyl functions are defined and their properties are studied. Frobenius symmetries, which are based on Frobenius automorphisms in the theory of Galois fields, are a unique feature of these systems (for l ? 2). If they commute with the Hamiltonian, there are constants of motion which are discussed. An analytic representation in the l-sheeted complex plane provides an elegant formalism that embodies the properties of Frobenius transformations. The difference between a Galois quantum system and other finite quantum systems where the position and momentum take values in the ring [Zpl] is discussed. (topical review)

2007-08-17

327

Quantum systems with finite Hilbert space: Galois fields in quantum mechanics

Energy Technology Data Exchange (ETDEWEB)

A 'Galois quantum system' in which the position and momentum take values in the Galois field GF(p{sup l}) is considered. It is comprised of l-component systems which are coupled in a particular way and is described by a certain class of Hamiltonians. Displacements in the GF(p{sup l}) x GF(p{sup l}) phase space and the corresponding Heisenberg-Weyl group are studied. Symplectic transformations are shown to form the Sp(2, GF(p{sup l})) group. Wigner and Weyl functions are defined and their properties are studied. Frobenius symmetries, which are based on Frobenius automorphisms in the theory of Galois fields, are a unique feature of these systems (for l {>=} 2). If they commute with the Hamiltonian, there are constants of motion which are discussed. An analytic representation in the l-sheeted complex plane provides an elegant formalism that embodies the properties of Frobenius transformations. The difference between a Galois quantum system and other finite quantum systems where the position and momentum take values in the ring [Z{sub p}{sup l}] is discussed. (topical review)

Vourdas, A [Department of Computing, University of Bradford, Bradford BD7 1DP (United Kingdom)

2007-08-17

328

Localization in the non-analytic quantum kicked systems

Numerical investigations on non-analytic quantum kicked systems are presented. A new type of localization - power-law localization is found to be universal in the nonanalytic systems. With increasing the perturbation strength, a transition from perturbative localization to pseudo-integrable system, to dynamical localization and to complete extension is clearly demonstrated. The dependence of the localization length on perturbation is given in different parameter regimes.

Liu, J; Cheng, C G

2002-01-01

329

Understanding Quantum Entanglement of Bipartite System Based on Bell Inequality

We present an alternative definition of quantum entanglement for bipartite system based on Bell inequality and operators' noncommutativity. A state is said to be entangled, if the maximum of CHSH expectation value $F_{\\max}$ is obtain by noncommutative measures on each particle of the bipartite system; otherwise, the state is a disentangled state. A uniform measure quantifying the degree of entanglement for any state of the bipartite system is also proposed.

Fu Li Bin; Zhao, X G; Chen Shi Gang; Fu, Li-Bin; Chen, Jing-Ling; Zhao, Xian-Geng; Chen, Shi-Gang

2001-01-01

330

Thermodynamic phase diagram of the quantum hall skyrmion system

We numerically study the interacting quantum Hall Skyrmion system based on the Chern-Simons action. By noticing that the action is invariant under global spin rotations in the spin space with respect to the magnetic field direction, we obtain the low-energy effective action for a many-Skyrmion system. Performing extensive molecular dynamics simulations, we establish the thermodynamic phase diagram for a many-Skyrmion system. PMID:11017419

Moon; Mullen

2000-01-31

331

Thermodynamic phase diagram of the quantum hall skyrmion system

UK PubMed Central (United Kingdom)

We numerically study the interacting quantum Hall Skyrmion system based on the Chern-Simons action. By noticing that the action is invariant under global spin rotations in the spin space with respect to the magnetic field direction, we obtain the low-energy effective action for a many-Skyrmion system. Performing extensive molecular dynamics simulations, we establish the thermodynamic phase diagram for a many-Skyrmion system.

Moon K; Mullen K

2000-01-01

332

Open quantum system of two coupled harmonic oscillators

International Nuclear Information System (INIS)

[en] On the basis of the Lindblad theory for open quantum systems master equations for a system consisting of two harmonic oscillators are derived. The time dependence of expectation values, Wigner function and Weyl operator are obtained and discussed. The chosen system can be applied for the description of the charge and mass asymmetry degrees of freedom in deep inelastic collisions in nuclear physics

1986-01-01

333

Open quantum system of two coupled harmonic oscillators

International Nuclear Information System (INIS)

[en] On the basis of the Lindblad theory for open quantum systems are derived master equations for a system consisting of two harmonic oscillators. The time-dependence of expectation values, Wigner-function and Weyl operator are obtained and discussed. The chosen system can be applied for the description of the charge and mass asymmetry degrees of freedom in deep inelastic collisions in nuclear physics

1986-01-01

334

Quantum systems with positions and momenta on a Galois field

International Nuclear Information System (INIS)

Quantum systems with positions and momenta in the Galois field GF(pe), are considered. The Heisenberg-Weyl group of displacements and the Sp(2,GF(pe)) group of symplectic transformations, are studied. Frobenius symmetries, are a unique feature of these systems and lead to constants of motion. The engineering of such systems from l spins with j = (p - 1)/2, which are coupled in a particular way, is discussed

2008-03-01

335

Quantum systems with positions and momenta on a Galois field

Energy Technology Data Exchange (ETDEWEB)

Quantum systems with positions and momenta in the Galois field GF(p{sup e}), are considered. The Heisenberg-Weyl group of displacements and the Sp(2,GF(p{sup e})) group of symplectic transformations, are studied. Frobenius symmetries, are a unique feature of these systems and lead to constants of motion. The engineering of such systems from l spins with j = (p - 1)/2, which are coupled in a particular way, is discussed.

Vourdas, A [Department of Computing, University of Bradford, Bradford BD7 1DP (United Kingdom)], E-mail: A.Vourdas@Bradford.ac.uk

2008-03-01

336

Microscopic Properties of Quantum Annealing -- Application to Fully Frustrated Ising Systems

In this paper we show quantum fluctuation effect of fully frustrated Ising spin systems. Quantum annealing has been expected to be an efficient method to find ground state of optimization problems. However it is not clear when to use the quantum annealing. In order to clarify when the quantum annealing works well, we have to study microscopic properties of quantum annealing. In fully frustrated Ising spin systems, there are macroscopically degenerated ground states. When we apply quantum annealing to fully frustrated systems, we cannot obtain each ground state with the same probability. This nature is consistent with "order by disorder" which is well-known mechanism in frustrated systems.

Tanaka, Shu

2011-01-01

337

Nonlinear Quantum Wave Equation of Radiation Electron and Dissipative Systems

As well known, an electron will produce radiative reaction force when the electron emits electromagnetism radiation. The electron radiative effect had not been considered in Schr\\"{o}dinger wave equation. In this paper, we give the nonlinear quantum wave equations for the radiative electron and some dissipative systems.

Wu, Xiang-Yao; Zhang, Bai-Jun; Wu, Yi-Heng

2010-01-01

338

Dynamical entropy, quantum K-systems and clustering

International Nuclear Information System (INIS)

[en] The two possibilities to define a quantum K-system, either using algebraic relations or using properties of the dynamical entropy, are compared. It is shown that under the additional assumption of strong asymptotic abelianess the algebraic relations imply the properties of the dynamical entropy. 14 refs. (Author)

1989-01-01

339

The Thermodynamic Limit of Quantum Coulomb Systems. Part II. Applications

In a previous paper, we have developed a general theory of thermodynamic limits. We apply it here to three different Coulomb quantum systems, for which we prove the convergence of the free energy per unit volume. The first system is the crystal for which the nuclei are classical particles arranged periodically in space and only the electrons are quantum particles. We recover and generalize a previous result of Fefferman. In the second example, both the nuclei and the electrons are quantum particles, submitted to a periodic magnetic field. We thereby extend a seminal result of Lieb and Lebowitz. Finally, in our last example we take again classical nuclei but optimize their position. To our knowledge such a system was never treated before. The verification of the assumptions introduced in the previous paper uses several tools which have been introduced before in the study of large quantum systems. In particular, an electrostatic inequality of Graf and Schenker is one main ingredient of our new approach.

Hainzl, Christian; Solovej, Jan Philip

2008-01-01

340

A note on the gravity screening in quantum systems

We discuss how, in the theoretical scenario presented in [1], the gravity screening and the gravity impulse which seem to be produced under certain conditions by high temperature superconductors are expected to be an entropic response to the flow of part of the system into a deeper quantum regime.

Gregori, Andrea

2011-01-01

341

Nonlinear von Neumann equations for quantum dissipative systems

International Nuclear Information System (INIS)

For pure states nonlinear Schroedinger equations, the so-called Schroedinger-Langevin equations are well-known to model quantum dissipative systems of the Langevin type. For mixtures it is shown that these wave equations do not extend to master equations, but to corresponding nonlinear von Neumann equations. Solutions for the damped harmonic oscillator are discussed. (Author)

342

Landau quantum systems an approach based on symmetry

We show that the Landau quantum systems (or integer quantum Hall effect systems) in a plane, sphere or a hyperboloid, can be explained in a complete meaningful way from group-theoretical considerations concerning the symmetry group of the corresponding configuration space. The crucial point in our development is the role played by locality and its appropriate mathematical framework, the fiber bundles. In this way the Landau levels can be understood as the local equivalence classes of the symmetry group. We develop a unified treatment that supplies the correct geometric way to recover the planar case as a limit of the spherical or the hyperbolic quantum systems when the curvature goes to zero. This is an interesting case where a contraction procedure gives rise to nontrivial cohomology starting from a trivial one. We show how to reduce the quantum hyperbolic Landau problem to a Morse system using horocyclic coordinates. An algebraic analysis of the eigenvalue equation allow us to build ladder operators which c...

Negro, J; Rodríguez-Marco, A

2001-01-01

343

Real-Space Entanglement Spectrum of Quantum Hall Systems

We study the real-space entanglement spectrum for fractional quantum Hall systems, which maintains locality along the spatial cut, and provide evidence that it possesses a scaling property. We also consider the closely-related particle entanglement spectrum, and carry out the Schmidt decomposition of the Laughlin state analytically at large size.

Dubail, J; Rezayi, E H

2012-01-01

344

Spontaneous Symmetry Breaking in Quantum Systems. A review for Scholarpedia

The mechanism of spontaneous symmetry breaking in quantum systems is briefly reviewed, rectifying part of the standard wisdom on logical and mathematical grounds. The crucial role of the localization properties of the time evolution for the conclusion of the Goldstone theorem is emphasized.

Strocchi, F

2012-01-01

345

Quantum limitations for spin measurements on systems of arbitrary spin

International Nuclear Information System (INIS)

The existence of additive conserved quantities implies, as well known, an unavoidable error in the measurement of those observables which do not commute with the conserved quantities. We introduce a formal procedure to derive a lower bound for the error in the case of a measurement of a spin component for a quantum system of arbitrary spin. (author).

1981-01-01

346

Performance of Photon-Pair Quantum Key Distribution Systems

We analyze the quantitative improvement in performance provided by a novel quantum key distribution (QKD) system that employs a correlated photon source (CPS) and a photon-number resolving detector (PNR). Our calculations suggest that given current technology, the CPR implementation offers an improvement of several orders of magnitude in secure bit rate over previously described implementations.

Walton, Z D; Atatüre, M; Saleh, B E A; Teich, M C

2001-01-01

347

On transport in quantum Hall systems with constrictions

Motivated by recent experimental findings, we study transport in a simple phenomenological model of a quantum Hall edge system with a gate-voltage controlled constriction lowering the local filling factor. The current backscattered from the constriction is seen to arise from the matching of the properties of the edge-current excitations in the constriction ($\

Lal, S

2006-01-01

348

Trojan-horse attacks on quantum-key-distribution systems

International Nuclear Information System (INIS)

General Trojan-horse attacks on quantum-key-distribution systems, i.e., attacks on Alice or Bob's system via the quantum channel, are analyzed. We illustrate the power of such attacks with today's technology and conclude that all systems must implement active counter measures. In particular, all systems must include an auxiliary detector that monitors any incoming light. We show that such counter measures can be efficient, provided that enough additional privacy amplification is applied to the data. We present a practical way to reduce the maximal information gain that an adversary can gain using Trojan-horse attacks. This does reduce the security analysis of the two-way plug-and-play implementation to those of the standard one-way systems.

2006-01-01

349

Unpredictability of wave function's evolution in nonintegrable quantum systems

It is shown that evolution of wave functions in nonintegrable quantum systems is unpredictable for a long time T because of rapid growth of number of elementary computational operations $\\mathcal O(T)\\sim T^\\alpha$. On the other hand, the evolution of wave functions in integrable systems can be predicted by the fast algorithms $\\mathcal O(T)\\sim (log_2 T)^\\beta$ for logarithmically short time and thus there is an algorithmic "compressibility" of their dynamics. The difference between integrable and nonintegrable systems in our approach looks identically for classical and quantum systems. Therefore the minimal number of bit operations $\\mathcal O(T)$ needed to predict a state of system for time interval T can be used as universal sign of chaos.

Ivanov, I B

2002-01-01

350

Quantum teleportation and entanglement swapping for spin systems

Energy Technology Data Exchange (ETDEWEB)

We analyse quantum teleportation (QT) and entanglement swapping (ES) for spin systems. If the permitted operations are restricted to the Ising interaction, plus local rotations and spin measurements, high-fidelity teleportation is achievable for quantum states that are close to the maximally weighted spin state. ES is achieved, and is maximized for a combination of entangled states and Bell measurements that is different from the QT case. If more general local unitary transformations are considered, then it is possible to achieve perfect teleportation and ES. (author)

Berry, Dominic W. [Department of Physics and Centre for Advanced Computing-Algorithms and Cryptography, Macquarie University, Sydney, NSW (Australia)]. E-mail: berry@ics.mq.edu.au; Sanders, Barry C. [Department of Physics and Centre for Advanced Computing-Algorithms and Cryptography, Macquarie University, Sydney, NSW (Australia); Quantum Entanglement Project, Edward L. Ginzton Laboratory, Stanford University, CA (United States)

2002-02-01

351

Quantum teleportation and entanglement swapping for spin systems

International Nuclear Information System (INIS)

We analyse quantum teleportation (QT) and entanglement swapping (ES) for spin systems. If the permitted operations are restricted to the Ising interaction, plus local rotations and spin measurements, high-fidelity teleportation is achievable for quantum states that are close to the maximally weighted spin state. ES is achieved, and is maximized for a combination of entangled states and Bell measurements that is different from the QT case. If more general local unitary transformations are considered, then it is possible to achieve perfect teleportation and ES. (author)

2002-01-01

352

Formal Analysis of Quantum Systems using Process Calculus

Quantum communication and cryptographic protocols are well on the way to becoming an important practical technology. Although a large amount of successful research has been done on proving their correctness, most of this work does not make use of familiar techniques from formal methods, such as formal logics for specification, formal modelling languages, separation of levels of abstraction, and compositional analysis. We argue that these techniques will be necessary for the analysis of large-scale systems that combine quantum and classical components, and summarize the results of initial investigation using behavioural equivalence in process calculus. This paper is a summary of Simon Gay's invited talk at ICE'11.

Davidson, Timothy A S; Nagarajan, Rajagopal; 10.4204/EPTCS.59.9

2011-01-01

353

Quantum Dynamical Entropies and Complexity in Dynamical Systems

We analyze the behaviour of two quantum dynamical entropies in connection with the classical limit. Using strongly chaotic classical dynamical systems as models (Arnold Cat Maps and Sawtooth Maps), we also propose a discretization procedure that resembles quantization; even in this case, studies of quantum dynamical entropy production are carried out and the connection with the continuous limit is explored. In both case (quantization and discretization) the entropy production converge to the Kolmogorov-Sinai invariant on time-scales that are logarithmic in the quantization (discretization) parameter.

Cappellini, V

2004-01-01

354

Frustration, entanglement, and factorization in quantum spin systems

We investigate the separability properties of quantum ground states in frustrated spin systems. We prove that the existence of fully factorized ground states is compatible with increasing degrees of frustration up to a critical threshold above which only entangled ground states are permitted. The separability threshold identifies a frustration-driven transition between classical-like and entanglement-dominated regimes. We determine the critical degree of frustration and the form of the exact factorized ground-state solutions in various classes of non exactly solvable frustrated quantum spin models with finite-range as well as infinite-range interactions.

Giampaolo, Salvatore M; Illuminati, Fabrizio

2009-01-01

355

Feshbach PQ Partitioning for Quantum Open Systems: Stochastic Approach

Dynamics of a state of interest coupled to a non-Markovian environment is studied for the first time by concatenating the quantum state diffusion (QSD) equation and the Feshbach PQ partitioning technique. An exact one-dimensional stochastic master equation is derived as a general tool for controlling an arbitrary component of the system. The exact one-dimensional stochastic master equation can be efficiently solved beyond the widely adapted second-order master equation. The generality and applicability of this hybrid approach is justified and exemplified by several coherence control problems concerning quantum state protection against leakage and decoherence.

Jing, Jun; You, J Q; Yu, Ting

2011-01-01

356

Quantum phase transitions in Bose-Fermi systems

Quantum phase transitions in a system of N bosons with angular momentum L=0,2 (s,d) and a single fermion with angular momentum j are investigated both classically and quantum mechanically. It is shown that the presence of the odd fermion strongly influences the location and nature of the phase transition, especially the critical value of the control parameter at which the phase transition occurs. Experimental evidence for the U(5)-SU(3) (spherical to axially-deformed) transition in odd-even nuclei is presented.

Petrellis, D; Iachello, F

2011-01-01

357

Classical and quantum anomalous diffusion in a system of 2$\\delta$-kicked Quantum Rotors

We study the dynamics of cold atoms subjected to {\\em pairs} of closely time-spaced $\\delta$-kicks from standing waves of light. The classical phase space of this system is partitioned into momentum cells separated by trapping regions. In a certain range of parameters it is shown that the classical motion is well described by a process of anomalous diffusion. We investigate in detail the impact of the underlying classical anomalous diffusion on the quantum dynamics with special emphasis on the phenomenon of dynamical localization. Based on the study of the quantum density of probability, its second moment and the return probability we identify a region of weak dynamical localization where the quantum diffusion is still anomalous but the diffusion rate is slower than in the classical case. Moreover we examine how other relevant time scales such as the quantum-classical breaking time or the one related to the beginning of full dynamical localization are modified by the classical anomalous diffusion. Finally we ...

Wang, J; Garcia-Garcia, Antonio M.; Wang, Jiao

2007-01-01

358

We present some deterministic schemes to construct universal quantum gates, that is, controlled- not, three-qubit Toffoli, and Fredkin gates, between flying photon qubits and stationary electron-spin qubits assisted by quantum dots inside double-sided optical microcavities. The control qubit of our gates is encoded on the polarization of the moving single photon and the target qubits are encoded on the confined electron spins in quantum dots inside optical microcavities. Our schemes for these universal quantum gates on a hybrid system have some advantages. First, all the gates are accomplished with a success probability of 100% in principle. Second, our schemes require no additional qubits. Third, the control qubits of the gates are easily manipulated and the target qubits are perfect for storage and processing. Fourth, the gates do not require that the transmission for the uncoupled cavity is balanceable with the reflectance for the coupled cavity, in order to get a high fidelity. Fifth, the devices for the three universal gates work in both the weak coupling and the strong coupling regimes, and they are feasible in experiment.

Wei, Hai-Rui; Deng, Fu-Guo

2013-02-01

359

Quantum kinetic theory of phonon-assisted carrier transitions in nitride-based quantum-dot systems

A microscopic theory for the interaction of carriers with LO phonons is used to study the ultrafast carrier dynamics in nitride-based semiconductor quantum dots. It is shown that the efficiency of scattering processes is directly linked to quasi-particle renormalizations. The electronic states of the interacting system are strongly modified by the combined influence of quantum confinement and polar coupling. Inherent electrostatic fields, typical for InGaN/GaN quantum dots, do not limit the fast scattering channels.

Seebeck, J; Gärtner, P; Jahnke, F

2005-01-01

360

Unifying variational methods for simulating quantum many-body systems.

We introduce a unified formulation of variational methods for simulating ground state properties of quantum many-body systems. The key feature is a novel variational method over quantum circuits via infinitesimal unitary transformations, inspired by flow equation methods. Variational classes are represented as efficiently contractible unitary networks, including the matrix-product states of density matrix renormalization, multiscale entanglement renormalization (MERA) states, weighted graph states, and quantum cellular automata. In particular, this provides a tool for varying over classes of states, such as MERA, for which so far no efficient way of variation has been known. The scheme is flexible when it comes to hybridizing methods or formulating new ones. We demonstrate the functioning by numerical implementations of MERA, matrix-product states, and a new variational set on benchmarks. PMID:18517924

Dawson, C M; Eisert, J; Osborne, T J

2008-03-31

361

Unifying Variational Methods for Simulating Quantum Many-Body Systems

International Nuclear Information System (INIS)

We introduce a unified formulation of variational methods for simulating ground state properties of quantum many-body systems. The key feature is a novel variational method over quantum circuits via infinitesimal unitary transformations, inspired by flow equation methods. Variational classes are represented as efficiently contractible unitary networks, including the matrix-product states of density matrix renormalization, multiscale entanglement renormalization (MERA) states, weighted graph states, and quantum cellular automata. In particular, this provides a tool for varying over classes of states, such as MERA, for which so far no efficient way of variation has been known. The scheme is flexible when it comes to hybridizing methods or formulating new ones. We demonstrate the functioning by numerical implementations of MERA, matrix-product states, and a new variational set on benchmarks.

2008-04-04

362

Ground-state geometric quantum computing in superconducting systems

International Nuclear Information System (INIS)

We present a theoretical proposal for the implementation of geometric quantum computing based on a Hamiltonian which has a doubly degenerate ground state. Thus the system which is steered adiabatically, remains in the ground-state. The proposed physical implementation relies on a superconducting circuit composed of three SQUIDs and two superconducting islands with the charge states encoding the logical states. We obtain a universal set of single-qubit gates and implement a nontrivial two-qubit gate exploiting the mutual inductance between two neighboring circuits, allowing us to realize a fully geometric ground-state quantum computing. The introduced paradigm for the implementation of geometric quantum computing is expected to be robust against environmental effects.

2010-01-01

363

Limits of Gaudin Systems: Classical and Quantum Cases

Directory of Open Access Journals (Sweden)

Full Text Available We consider the XXX homogeneous Gaudin system with N sites, both in classical and the quantum case. In particular we show that a suitable limiting procedure for letting the poles of its Lax matrix collide can be used to define new families of Liouville integrals (in the classical case) and new ''Gaudin'' algebras (in the quantum case). We will especially treat the case of total collisions, that gives rise to (a generalization of) the so called Bending flows of Kapovich and Millson. Some aspects of multi-Poisson geometry will be addressed (in the classical case). We will make use of properties of ''Manin matrices'' to provide explicit generators of the Gaudin Algebras in the quantum case.

Alexander Chervov; Gregorio Falqui; Leonid Rybnikov

2009-01-01

364

Persistent current magnification in a double quantum-ring system

The electronic transport in a system of two quantum rings side-coupled to a quantum wire is studied via a single-band tunneling tight-binding Hamiltonian. We derived analytical expressions for the conductance, density of states and the persistent current when the rings are threaded by magnetic fluxes. We found a clear manifestation of the presence of bound states in each one of those physical quantities when either the flux difference or the sum of the fluxes are zero or integer multiples of a quantum of flux. These bound states play an important role in the magnification of the persistent current in the rings. We also found that the persistent current keeps a large amplitude even for strong ring-wire coupling.

Orellana, P A

2005-01-01

365

Separability and ground state factorization in quantum spin systems

We investigate the existence and the properties of fully separable (fully factorized) ground states in quantum spin systems. Exploiting techniques of quantum information and entanglement theory we extend a recently introduced method and construct a general, self-contained theory of ground state factorization in frustration-free quantum spin models defined on lattices in any spatial dimension and for interactions of arbitrary range. We show that, quite generally, non exactly solvable models in external field admit exact, fully factorized ground state solutions. Unentangled ground states occur at finite values of the Hamiltonian parameters satisfying well defined balancing conditions between external fields and interaction strengths. These conditions are analytically determined together with the type of magnetic orderings compatible with factorization and the corresponding values of the fundamental observables such as energy and magnetization. The method is applied to a series of examples of increasing complexi...

Giampaolo, S M; Illuminati, F

2009-01-01

366

Experimental probes of emergent symmetries in the quantum Hall system

Experiments studying renormalization group flows in the quantum Hall system provide significant evidence for the existence of an emergent holomorphic modular symmetry Gamma(0)(2). We briefly review this evidence and show that, for the lowest temperatures, the experimental determination of the position of the quantum critical points agrees to the parts per mille level with the prediction from Gamma(0)(2). We present evidence that experiments giving results that deviate substantially from the symmetry predictions are not cold enough to be in the quantum critical domain. We show how the modular symmetry extended by a non-holomorphic particle hole duality leads to an extensive web of dualities related to those in plateau insulator transitions, and we derive a formula relating dual pairs (B, B(d)) of magnetic field strengths across any transition. The experimental data obtained for the transition studied so far is in excellent agreement with the duality relations following from this emergent symmetry, and rule out...

Lutken, C A

2011-01-01

367

Quantum-to-classical crossover of quasi-bound states in open quantum systems

In the semiclassical limit of open ballistic quantum systems, we demonstrate the emergence of instantaneous decay modes guided by classical escape faster than the Ehrenfest time. The decay time of the associated quasi-bound states is smaller than the classical time of flight. The remaining long-lived quasi-bound states obey random-matrix statistics, renormalized in compliance with the recently proposed fractional Weyl law for open systems [W. T. Lu, S. Sridhar, and M. Zworski, Phys. Rev. Lett. 91, 154101 (2003)]. We validate our theory numerically for a model system, the open kicked rotator.

Schomerus, H; Schomerus, Henning; Tworzydlo, Jakub

2004-01-01

368

Non-Locality Distillation is Impossible for Isotropic Quantum Systems

Non-locality is a powerful resource for various communication and information theoretic tasks, e.g., to establish a secret key between two parties, or to reduce the communication complexity of distributed computing. Typically, the more non-local a system is, the more useful it is as a resource for such tasks. We address the issue of non-locality distillation, i.e., whether it is possible to create a strongly non-local system by local operations on several weakly non-local ones. More specifically, we consider a setting where non-local systems can be realized via measurements on underlying shared quantum states. The hardest instances for non-locality distillation are the isotropic quantum systems: if a certain isotropic system can be distilled, then all systems of the same non-locality can be distilled as well. The main result of this paper is that non-locality cannot be distilled from such isotropic quantum systems. Our results are based on the theory of cross norms defined over the tensor product of certain B...

Dukaric, Dejan D

2011-01-01

369

Superintegrability and higher order constants for quantum systems

We refine a method for finding a canonical form for symmetry operators of arbitrary order for the Schroedinger eigenvalue equation on any 2D Riemannian manifold, real or complex, that admits a separation of variables in some orthogonal coordinate system. As examples we treat two potentials with parameter k (one of which is the Tremblay, Turbiner, and Winternitz system) that have been shown to be classically superintegrable for all rational numbers k. We apply the canonical operator method to give a constructive proof that each of these systems is also quantum superintegrable for all rational k. We also develop the classical analog of the quantum canonical form for a symmetry. It is clear that our methods will generalize to other Hamiltonian systems.

Kalnins, E G; Miller, W

2010-01-01

370

On the quantum dynamics of non-commutative systems

Scientific Electronic Library Online (English)

Full Text Available Abstract in english This is a review paper concerned with the global consistency of the quantum dynamics of non-commutative systems. Our point of departure is the theory of constrained systems, since it provides a unified description of the classical and quantum dynamics for the models under investigation. We then elaborate on recently reported results concerned with the sufficient conditions for the existence of the Born series and unitarity and turn, afterwards, into analyzing the function (more) al quantization of non-commutative systems. The compatibility between the operator and the functional approaches is established in full generality. The intricacies arising in connection with the explicit computation of path integrals, for the systems under scrutiny, is illustrated by presenting the detailed calculation of the Feynman kernel for the non-commutative two dimensional harmonic oscillator.

Bemfica, F. S.; Girotti, H. O.

2008-06-01

371

Invariant theoretic approach to uncertainty relations for quantum systems

We present a general framework and procedure to derive uncertainty relations for observables of quantum systems in a covariant manner. All such relations are consequences of the positive semidefiniteness of the density matrix of a general quantum state. Particular emphasis is given to the action of unitary symmetry operations of the system on the chosen observables, and the covariance of the uncertainty relations under these operations. The general method is applied to the case of an $n$-mode system to recover the $Sp(2n,\\,R)$-covariant multi mode generalization of the single mode Schr\\"{o}dinger-Robertson Uncertainty Principle; and to the set of all polynomials in canonical variables for a single mode system. In the latter situation, the case of the fourth order moments is analyzed in detail, exploiting covariance under the homogeneous Lorentz group $SO(2,\\,1)$ of which the symplectic group $Sp(2,\\,R)$ is the double cover.

Ivan, J Solomon; Mukunda, N; Simon, R

2012-01-01

372

Macroscopic quantum in bosonic systems. Local theories

International Nuclear Information System (INIS)

[en] A general method to deduce topological macroscopic quantum waves (MQWs) solutions in non relativistic local field theories is presented. It is shown that every theory lambda bar phi bar sup(n) (with lambda>0 and n>2) exhibits subsonic MQWs. Explicit solutions are given for the bar phi bar2 and bar phi bar6 models. The fact that these topological waves are a common feature of a wide class of bosonic theories is an important support for our conjecture sup((1,2)) that they exist in liquid 4He[pt] Apresenta-se um metodo geral para deduzir solucoes de ondas quanticas macroscopicas MQWs topologicas em teorias de campos locais nao relativisticos. Mostra-se que toda teoria lambda barra phi barra sup(n) (com lambda >0 e n>2) exibe MQWs subsonicas. Dao-se solucoes explicitas para os modelos barra phi barra2 e barra phi barra6. O fato de que essas ondas topologicas sao uma caracteristica comum de uma grande classe de teorias bosonicas, e um importante suporte para nossa conjectura de que ela existe em 4He liquido

1979-01-01

373

Theory of ground state factorization in quantum cooperative systems.

We introduce a general analytic approach to the study of factorization points and factorized ground states in quantum cooperative systems. The method allows us to determine rigorously the existence, location, and exact form of separable ground states in a large variety of, generally nonexactly solvable, spin models belonging to different universality classes. The theory applies to translationally invariant systems, irrespective of spatial dimensionality, and for spin-spin interactions of arbitrary range. PMID:18518481

Giampaolo, Salvatore M; Adesso, Gerardo; Illuminati, Fabrizio

2008-05-13

374

Analysis of Lyapunov Method for Control of Quantum Systems

We present a detailed analysis of the convergence properties of Lyapunov control for finite-dimensional quantum systems based on the application of the LaSalle invariance principle and stability analysis from dynamical systems and control theory. For a certain class of ideal Hamiltonians, convergence results are derived both pure-state and mixed-state control, and the effectiveness of the method for more realistic Hamiltonians is discussed.

Wang, Xiaoting

2008-01-01

375

Novel finite temperature conductivity in quantum Hall systems

We study quantum Hall systems (mainly the integer case) at finite temperatures and show that there is a novel temperature dependence even for a pure system, thanks to the `anomalous' nature of generators of translation. The deviation of Hall conductivity from its zero temperature value is controlled by a parameter T_0 =\\pi \\rho /m^\\ast N which is sample specific and hence the universality of quantization is lost at finite temperatures.

Mandal, S S; Ravishankar, V; Mandal, Sudhansu S

1995-01-01

376

Wave functions for open quantum systems and stochastic Schrodinger equations

It is shown that evolution of an open quantum system can be exactly described in terms of wave function which obeys Schrodinger equation with randomly varying parameters whose statistics is universally determined by separate dynamics of the system's environment. Corresponding stochastic evolution of the wave function is unitary on average, and this property implies optical theorem for inelastic scattering as demonstrated by the example of one-dimensional conducting channel with thermally fluctuating potential perturbation.

Kuzovlev, Y E

2005-01-01

377

Magnetic light scattering in low-dimensional quantum spin systems

An overview of one- and two-dimensional quantum spin systems based on transition-metal oxides and halides of current interest is given, such as spin-Peierls, spin-dimer, geometrically frustrated and ladder systems. The most significant and outstanding contributions of magnetic light scattering to the understanding of these materials are discussed and compared to results of other spectroscopies and thermodynamic measurements.

Lemmens, P; Gros, C

2003-01-01

378

Theory of ground state factorization in quantum cooperative systems.

UK PubMed Central (United Kingdom)

We introduce a general analytic approach to the study of factorization points and factorized ground states in quantum cooperative systems. The method allows us to determine rigorously the existence, location, and exact form of separable ground states in a large variety of, generally nonexactly solvable, spin models belonging to different universality classes. The theory applies to translationally invariant systems, irrespective of spatial dimensionality, and for spin-spin interactions of arbitrary range.

Giampaolo SM; Adesso G; Illuminati F

2008-05-01

379

Theory of ground state factorization in quantum cooperative systems

We introduce a general analytic approach to the study of factorization points and factorized ground states in quantum cooperative systems. The method allows to determine rigorously existence, location, and exact form of separable ground states in a large variety of, generally non-exactly solvable, spin models belonging to different universality classes. The theory applies to translationally invariant systems, irrespective of spatial dimensionality, and for spin-spin interactions of arbitrary range.

Giampaolo, S M; Illuminati, F

2008-01-01

380

In this lively look at quantum science, a physicist takes you on an entertaining and enlightening journey through the basics of subatomic physics. Along the way, he examines the paradox of quantum mechanics--beautifully mathematical in theory but confoundingly unpredictable in the real world. Marvel at the Dual Slit experiment as a tiny atom passes through two separate openings at the same time. Ponder the peculiar communication of quantum particles, which can remain in touch no matter how far apart. Join the genius jewel thief as he carries out a quantum measurement on a diamond without ever touching the object in question. Baffle yourself with the bizzareness of quantum tunneling, the equivalent of traveling partway up a hill, only to disappear then reappear traveling down the opposite side. With its clean, colorful layout and conversational tone, this text will hook you into the conundrum that is quantum mechanics.

Al-Khalili, Jim

2003-01-01

381

Unambiguous comparison of the states of multiple quantum systems

International Nuclear Information System (INIS)

We consider N quantum systems initially prepared in pure states and address the problem of unambiguously comparing them. One may ask whether or not all N systems are in the same state. Alternatively, one may ask whether or not the states of all N systems are different. We investigate the possibility of unambiguously obtaining this kind of information. It is found that some unambiguous comparison tasks are possible only when certain linear independence conditions are satisfied. We also obtain measurement strategies for certain comparison tasks which are optimal under a broad range of circumstances, in particular when the states are completely unknown. Such strategies, which we call universal comparison strategies, are found to have intriguing connections with the problem of quantifying the distinguishability of a set of quantum states and also with unresolved conjectures in linear algebra. We finally investigate a potential generalization of unambiguous state comparison, which we term unambiguous overlap filtering.

2004-07-23

382

Quantum correlations in nanostructured two-impurity Kondo systems

We study the ground-state entanglement properties of nanostructured Kondo systems consisting of a pair of impurity spins coupled to a background of confined electrons. The competition between the RKKY-like coupling and the Kondo effect determines the development of quantum correlations between the different parts of the system. A key element is the electronic filling due to confinement. An even electronic filling leads to results similar to those found previously for extended systems, where the properties of the reduced impurity-spin subsystem are uniquely determined by the spin correlation function defining a one-dimensional phase space. An odd filling, instead, breaks spin-rotation symmetry unfolding a two-dimensional phase space showing rich entanglement characteristics as, e.g., the requirement of a larger amount of entanglement for the development of non-local correlations between impurity spins. We check these results by numerical simulations of elliptic quantum corrals with magnetic impurities at the f...

Nizama, Marco; Hallberg, Karen

2012-01-01

383

Decoherence Rates in Large Scale Quantum Computers and Macroscopic Systems

Markovian regime decoherence effects in quantum computers are studied in terms of the fidelity for the situation where the number of qubits N becomes large. A general expression giving the decoherence time scale in terms of Markovian relaxation elements and expectation values of products of system fluctuation operators is obtained, which could also be applied to study decoherence in other macroscopic systems such as Bose condensates and superconductors. A standard circuit model quantum computer involving three-state lambda system ionic qubits is considered, with qubits localised around well-separated positions via trapping potentials. The centre of mass vibrations of the qubits act as a reservoir. Coherent one and two qubit gating processes are controlled by time dependent localised classical electromagnetic fields that address specific qubits, the two qubit gating processes being facilitated by a cavity mode ancilla, which permits state interchange between qubits. With a suitable choice of parameters, it is ...

Dalton, B J

2004-01-01

384

Optimal control for generating quantum gates in open dissipative systems

International Nuclear Information System (INIS)

Optimal control methods for implementing quantum modules with least amount of relaxative loss are devised to give best approximations to unitary gates under relaxation. The potential gain by optimal control fully exploiting known relaxation parameters against time-optimal control (the alternative for unknown relaxation parameters) is explored and exemplified in numerical and in algebraic terms: for instance, relaxation-based optimal control is the method of choice to govern quantum systems within subspaces of weak relaxation whenever the drift Hamiltonian would otherwise drive the system through fast decaying modes. In a standard model system generalizing ideal decoherence-free subspaces to more realistic scenarios, opengrape-derived controls realize a CNOT with fidelities beyond 95% instead of at most 15% for a standard Trotter expansion. As additional benefit their control fields are orders of magnitude lower in power than bang-bang decouplings.

2011-08-14

385

Dynamical Structure Factors in Quantum Many-Body Systems from Quantum Monte Carlo Calculations

An ab-initio method for determining the dynamical structure function of an interacting many--body quantum system has been devised by combining a generalized integral transform method with Quantum Monte Carlo methods. As a first application, the coherent and, separately, the incoherent excitation spectrum of bulk atomic 4He has been computed, both in the low and intermediate momentum range. The peculiar form of the kernel in the integral transform of the dynamical structure function allows to predict, without using any model, both position and width of the collective excitations in the maxon--roton region, as well as the second collective peak. A prediction of the dispersion of the single--particle modes described by the incoherent part is also presented.

Roggero, Alessandro; Orlandini, Giuseppina

2012-01-01

386

Magneto-optical cavity quantum electrodynamics effects in quantum dot - micropillar systems

International Nuclear Information System (INIS)

We report on magneto-optical studies of strongly coupled quantum dot - micropillar cavity systems. Large In0.3Ga0.7As quantum dots (QDs) in the active layer of the micropillar facilitate the observation of strong coupling. In addition, they exhibit a particular large diamagnetic response which is exploited to demonstrate magneto-optical resonance tuning in the strong coupling regime. The magnetic field employed in Faraday configuration induces a transition from strong coupling towards the critical coupling regime which is explained in terms of a magnetic field dependent oscillator strength of the In0.3Ga0.7As QDs. We further study the coherent interaction between spin resolved states of the QDs and microcavity photon modes. A detailed oscillator model is used to extract the associated coupling parameters of the individual spin and cavity modes and reveals an effective coupling between photon modes that is mediated by the exciton spin states.

2011-12-28

387

Light-cone-like spreading of correlations in a quantum many-body system

How fast can correlations spread in a quantum many-body system? Based on the seminal work by Lieb and Robinson, it has recently been shown that several interacting many-body systems exhibit an effective light cone that bounds the propagation speed of correlations. The existence of such a "speed of light" has profound implications for condensed matter physics and quantum information, but has never been observed experimentally. Here we report on the time-resolved detection of propagating correlations in an interacting quantum many-body system. By quenching a one-dimensional quantum gas in an optical lattice, we reveal how quasiparticle pairs transport correlations with a finite velocity across the system, resulting in an effective light cone for the quantum dynamics. Our results open important perspectives for understanding relaxation of closed quantum systems far from equilibrium as well as for engineering efficient quantum channels necessary for fast quantum computations.

Cheneau, Marc; Poletti, Dario; Endres, Manuel; Schauß, Peter; Fukuhara, Takeshi; Gross, Christian; Bloch, Immanuel; Kollath, Corinna; Kuhr, Stefan

2011-01-01

388

Stabilizing open quantum systems by Markovian reservoir engineering

International Nuclear Information System (INIS)

We study open quantum systems whose evolution is governed by a master equation of Kossakowski-Gorini-Sudarshan-Lindblad type and give a characterization of the convex set of steady states of such systems based on the generalized Bloch representation. It is shown that an isolated steady state of the Bloch equation cannot be a center, i.e., that the existence of a unique steady state implies attractivity and global asymptotic stability. Necessary and sufficient conditions for the existence of a unique steady state are derived and applied to different physical models, including two- and four-level atoms, (truncated) harmonic oscillators, and composite and decomposable systems. It is shown how these criteria could be exploited in principle for quantum reservoir engineeing via coherent control and direct feedback to stabilize the system to a desired steady state. We also discuss the question of limit points of the dynamics. Despite the nonexistence of isolated centers, open quantum systems can have nontrivial invariant sets. These invariant sets are center manifolds that arise when the Bloch superoperator has purely imaginary eigenvalues and are closely related to decoherence-free subspaces.

2010-01-01

389

Analysis of Lyapunov Control for Hamiltonian Quantum Systems

We present detailed analysis of the convergence properties and effectiveness of Lyapunov control design for bilinear Hamiltonian quantum systems based on the application of LaSalle's invariance principle and stability analysis from dynamical systems and control theory. For a certain class of Hamiltonians, strong convergence results can be obtained for both pure and mixed state systems. The control Hamiltonians for realistic physical systems, however, generally do not fall in this class. It is shown that the effectiveness of Lyapunov control design in this case is significantly diminished.

Wang, Xiaoting

2008-01-01

390

Dynamical symmetries for superintegrable quantum systems

International Nuclear Information System (INIS)

[en] We study the dynamical symmetries of a class of two-dimensional superintegrable systems on a 2-sphere, obtained by a procedure based on the Marsden-Weinstein reduction, by considering its shape-invariant intertwining operators. These are obtained by generalizing the techniques of factorization of one-dimensional systems. We firstly obtain a pair of noncommuting Lie algebras su(2) that originate the algebra so(4). By considering three spherical coordinate systems, we get the algebra u(3) that can be enlarged by 'reflexions' to so(6). The bounded eigenstates of the Hamiltonian hierarchies are associated to the irreducible unitary representations of these dynamical algebras

2007-01-01

391

Dynamical symmetries for superintegrable quantum systems

We study the dynamical symmetries of a class of two-dimensional superintegrable systems on a 2-sphere, obtained by a procedure based on the Marsden-Weinstein reduction, by considering its shape-invariant intertwining operators. These are obtained by generalizing the techniques of factorization of one-dimensional systems. We firstly obtain a pair of noncommuting Lie algebras $su(2)$ that originate the algebra $so(4)$. By considering three spherical coordinate systems we get the algebra $u(3)$ that can be enlarged by `reflexions' to $so(6)$. The bounded eigenstates of the Hamiltonian hierarchies are associated to the irreducible unitary representations of these dynamical algebras.

Calzada, J A; Del'Olmo, M A

2006-01-01

392

Quantum Optics with Nanomechanical and Solid State Systems

International Nuclear Information System (INIS)

This thesis presents theoretical studies in an interfacing field of quantum optics, nanomechanics and mesoscopic solid state physics and proposes new methods for the generation of particular quantum states and quantum state transfer for selected hybrid systems. The first part of this thesis focuses on the quantum limit of a macroscopic object, a nanomechanical resonator. This is studied for two different physical systems. The first one is a nanomechanical beam incorporated in a superconducting circuit, in particular a loop-shaped Cooper pair box (CPB) - circuit. We present a scheme for ground state cooling of the flexural mode of the nanomechanical beam. Via the Lorentz force coupling of the beam motion to circulating CPB-circuit currents, energy is transferred to the CPB qubit which acts as a dissipative two-level system. The cooling process is driven by a detuned gate-voltage drive acting on the CPB. We analyze the cooling force spectrum and present analytical expressions for the cooling rate and final occupation number for a wide parameter regime. In particular, we find that cooling is optimized in a strong drive regime, and we present the necessary conditions for ground-state cooling. In a second system, we investigate the creation of squeezed states of a mechanical oscillator (a vibrating membrane or a movable mirror) in an optomechanical setup. An optical cavity is driven by squeezed light and couples via radiation pressure to the mechanical oscillator, effectively providing a squeezed heat-bath for the mechanical oscillator. Under the conditions of laser cooling to the ground state, we find an efficient transfer of squeezing with roughly 60% of light squeezing conveyed to the mechanical oscillator (on a dB scale). We determine the requirements on the carrier frequency and the bandwidth of squeezed light. Beyond the conditions for ground state cooling, we predict mechanical squashing to be observable in current systems. The second part of the thesis is concerned with the state transfer of an arbitrary quantum state. It is shown that by optimizing the time-dependent interaction between a harmonic oscillator and a transmission line, the oscillator quantum state can be transferred to another oscillator via the transmission line, with a fidelity that is independent of the initial state of both oscillators. For a transfer time T the fidelity approaches 1 exponentially in [gamma] T where [gamma] is a characteristic damping rate. Hence, a good fidelity is achieved even for a transfer time of a few damping times. Possible implementations of the theory are discussed. (author)

2009-01-01

393

Quantum-classical correspondence principles for locally nonequilibrium driven systems.

Many of the core concepts and (especially field-theoretic) tools of statistical mechanics have developed within the context of thermodynamic equilibrium, where state variables are all taken to be charges, meaning that their values are inherently preserved under reversal of the direction of time. A principle concern of nonequilibrium statistical mechanics is to understand the emergence and stability of currents, quantities whose values change sign under time reversal. Whereas the correspondence between classical charge-valued state variables and their underlying statistical or quantum ensembles is quite well understood, the study of currents away from equilibrium has been more fragmentary, with classical descriptions relying on the asymmetric auxiliary-field formalism of Martin, Siggia, and Rose (and often restricted to the Markovian assumption of Doi and Peliti), while quantum descriptions employ a symmetric two-field formalism introduced by Schwinger and further clarified by Keldysh. In this paper we demonstrate that for quantum ensembles in which superposition is not violated by very strong conditions of decoherence, there is a large natural generalization of the principles and tools of equilibrium, which not only admits but requires the introduction of current-valued state variables. For these systems, not only do Martin-Siggia-Rose (MSR) and Schwinger-Keldysh (SK) field methods both exist, in some cases they provide inequivalent classical and quantum descriptions of identical ensembles. With these systems for examples, we can both study the correspondence between classical and quantum descriptions of currents, and also clarify the nature of the mapping between the structurally homologous but interpretationally different MSR and SK formalisms. PMID:18351989

Smith, Eric

2008-02-11

394

Quantum effects in classical systems having complex energy

International Nuclear Information System (INIS)

On the basis of extensive numerical studies it is argued that there are strong analogies between the probabilistic behavior of quantum systems defined by Hermitian Hamiltonians and the deterministic behavior of classical mechanical systems extended into the complex domain. Three models are examined: the quartic double-well potential V(x) = x4 - 5x2, the cubic potential V(x)=1/2 x2 - gx3, and the periodic potential V(x) = -cos x. For the quartic potential a wave packet that is initially localized in one side of the double-well can tunnel to the other side. Complex solutions to the classical equations of motion exhibit a remarkably analogous behavior. Furthermore, classical solutions come in two varieties, which resemble the even-parity and odd-parity quantum-mechanical bound states. For the cubic potential, a quantum wave packet that is initially in the quadratic portion of the potential near the origin will tunnel through the barrier and give rise to a probability current that flows out to infinity. The complex solutions to the corresponding classical equations of motion exhibit strongly analogous behavior. For the periodic potential a quantum particle whose energy lies between -1 and 1 can tunnel repeatedly between adjacent classically allowed regions and thus execute a localized random walk as it hops from region to region. Moreover, if the energy of the quantum particle lies in a conduction band, then the particle delocalizes and drifts freely through the periodic potential. A classical particle having complex energy executes a qualitatively analogous local random walk, and there exists a narrow energy band for which the classical particle becomes delocalized and moves freely through the potential. (fast track communication)

2008-09-05

395

A formula for the Bloch vector of some Lindblad quantum systems

Energy Technology Data Exchange (ETDEWEB)

Using the Bloch representation of an N-dimensional quantum system and immediate results from quantum stochastic calculus, we establish a closed formula for the Bloch vector, hence also for the density operator, of a quantum system following a Lindblad evolution with selfadjoint Lindblad operators.

Salgado, D.; Sanchez-Gomez, J.L

2004-03-29

396

Low energy properties of non-perturbative quantum systems: a space reduction approach

We propose and test a renormalization procedure which acts in Hilbert space. We test its efficiency on strongly correlated quantum spin systems by working out and analyzing the low-energy spectral properties of frustrated quantum spin systems in different parts of the phase diagram and in the neighbourhood of quantum critical points.

Khalil, Tarek

2007-01-01

397

Low-energy properties of non-perturbative quantum systems: A space reduction approach

International Nuclear Information System (INIS)

[en] We propose and test a renormalization procedure which acts in Hilbert space. We test its efficiency on strongly correlated quantum spin systems by working out and analyzing the low-energy spectral properties of frustrated quantum spin systems in different parts of the phase diagram and in the neighbourhood of quantum critical points

2008-03-24

398

A pseudospectral method for optimal control of open quantum systems.

In this paper, we present a unified computational method based on pseudospectral approximations for the design of optimal pulse sequences in open quantum systems. The proposed method transforms the problem of optimal pulse design, which is formulated as a continuous-time optimal control problem, to a finite-dimensional constrained nonlinear programming problem. This resulting optimization problem can then be solved using existing numerical optimization suites. We apply the Legendre pseudospectral method to a series of optimal control problems on open quantum systems that arise in nuclear magnetic resonance spectroscopy in liquids. These problems have been well studied in previous literature and analytical optimal controls have been found. We find an excellent agreement between the maximum transfer efficiency produced by our computational method and the analytical expressions. Moreover, our method permits us to extend the analysis and address practical concerns, including smoothing discontinuous controls as well as deriving minimum-energy and time-optimal controls. The method is not restricted to the systems studied in this article and is applicable to optimal manipulation of both closed and open quantum systems. PMID:19894930

Li, Jr-Shin; Ruths, Justin; Stefanatos, Dionisis

2009-10-28

399

A pseudospectral method for optimal control of open quantum systems.

UK PubMed Central (United Kingdom)

In this paper, we present a unified computational method based on pseudospectral approximations for the design of optimal pulse sequences in open quantum systems. The proposed method transforms the problem of optimal pulse design, which is formulated as a continuous-time optimal control problem, to a finite-dimensional constrained nonlinear programming problem. This resulting optimization problem can then be solved using existing numerical optimization suites. We apply the Legendre pseudospectral method to a series of optimal control problems on open quantum systems that arise in nuclear magnetic resonance spectroscopy in liquids. These problems have been well studied in previous literature and analytical optimal controls have been found. We find an excellent agreement between the maximum transfer efficiency produced by our computational method and the analytical expressions. Moreover, our method permits us to extend the analysis and address practical concerns, including smoothing discontinuous controls as well as deriving minimum-energy and time-optimal controls. The method is not restricted to the systems studied in this article and is applicable to optimal manipulation of both closed and open quantum systems.

Li JS; Ruths J; Stefanatos D

2009-10-01

400

Experimental probes of emergent symmetries in the quantum Hall system

International Nuclear Information System (INIS)

Experiments studying renormalization group flows in the quantum Hall system provide significant evidence for the existence of an emergent holomorphic modular symmetry ?0(2). We briefly review this evidence and show that, for the lowest temperatures, the experimental determination of the position of the quantum critical points agrees to the parts per mille level with the prediction from ?0(2). We present evidence that experiments giving results that deviate substantially from the symmetry predictions are not cold enough to be in the quantum critical domain. We show how the modular symmetry extended by a non-holomorphic particle-hole duality leads to an extensive web of dualities related to those in plateau-insulator transitions, and we derive a formula relating dual pairs (B,Bd) of magnetic field strengths across any transition. The experimental data obtained for the transition studied so far is in excellent agreement with the duality relations following from this emergent symmetry, and rule out the duality rule derived from the 'law of corresponding states'. Comparing these generalized duality predictions with future experiments on other transitions should provide stringent tests of modular duality deep in the non-linear domain far from the quantum critical points.

2011-09-11

401

Anti-Zeno Effect for Quantum Transport in Disordered Systems

We demonstrate that repeated measurements in disordered systems can induce quantum anti-Zeno effect under certain condition to enhance quantum transport. The enhancement of energy transfer is really exhibited with a simple model under repeated measurements. The optimal measurement interval for the anti-Zeno effect and the maximal efficiency of energy transfer are specified in terms of the relevant physical parameters. Since the environment acts as frequent measurements on the system, the decoherence-induced energy transfer, which has been discussed recently for photosynthetic complexes, may be interpreted in terms of the anti-Zeno effect. We further find an interesting phenomenon, where local decoherence or repeated measurements may even promote entanglement generation between the non-local sites.

Fujii, Keisuke

2010-01-01

402

The unstable system and irreversible motion in quantum theory

International Nuclear Information System (INIS)

It has been shown by Flesia and Piron that the scattering theory of Lax and Phillips, developed primarily for the treatment of hyperbolic systems (e.g., classical scattering of electromagnetic waves on a finite target), can be applied to the quantum theory. In order to construct the correspondence, the continuously infinite family of (isomorphic) Hilbert spaces associated with each value of the time t are taken to be a direct integral representation (with a choice of measure) of a larger Hilbert space which includes the variable t in its measure space. We show that this theory, for which the evolution contracted to the subspace orthogonal to the incoming and outgoing subspaces is a contractive semigroup, provides a natural description of the irreversible motion of an unstable quantum system. (orig.)

1993-01-01

403

Geometrical effects on energy transfer in disordered open quantum systems

We explore various design principles for efficient excitation energy transport in complex quantum systems. We investigate energy transfer efficiency in randomly disordered geometries consisting of up to 20 chromophores to explore spatial and spectral properties of small natural/artificial Light-Harvesting Complexes (LHC). We find significant statistical correlations among highly efficient random structures with respect to ground state properties, excitonic energy gaps, multichromophoric spatial connectivity, and path strengths. These correlations can even exist beyond the optimal regime of environment-assisted quantum transport. For random configurations embedded in spatial dimensions of 30 A and 50 A, we observe that the transport efficiency saturates to its maximum value if the systems contain 7 and 14 chromophores respectively. Remarkably, these optimum values coincide with the number of chlorophylls in (Fenna-Matthews-Olson) FMO protein complex and LHC II monomers, respectively, suggesting a potential nat...

Mohseni, M; Lloyd, S; Omar, Y; Rabitz, H

2013-01-01

404

A Pseudospectral Method for Optimal Control of Open Quantum Systems

In this paper, we present a unified computational method based on pseudospectral approximations for the design of optimal pulse sequences in open quantum systems. The proposed method transforms the problem of optimal pulse design, which is formulated as a continuous time optimal control problem, to a finite dimensional constrained nonlinear programming problem. This resulting optimization problem can then be solved using existing numerical optimization suites. We apply the Legendre pseudospectral method to a series of optimal control problems on open quantum systems that arise in Nuclear Magnetic Resonance (NMR) spectroscopy in liquids. These problems have been well studied in previous literature and analytical optimal controls have been found. We find an excellent agreement between the maximum transfer efficiency produced by our computational method and the analytical expressions. Moreover, our method permits us to extend the analysis and address practical concerns, including smoothing discontinuous controls...

Li, Jr-Shin; Stefanatos, Dionisis

2009-01-01

405

Ground state of the parallel double quantum dot system.

UK PubMed Central (United Kingdom)

We resolve the controversy regarding the ground state of the parallel double quantum dot system near half filling. The numerical renormalization group predicts an underscreened Kondo state with residual spin-1/2 magnetic moment, ln2 residual impurity entropy, and unitary conductance, while the Bethe ansatz solution predicts a fully screened impurity, regular Fermi-liquid ground state, and zero conductance. We calculate the impurity entropy of the system as a function of the temperature using the hybridization-expansion continuous-time quantum Monte Carlo technique, which is a numerically exact stochastic method, and find excellent agreement with the numerical renormalization group results. We show that the origin of the unconventional behavior in this model is the odd-symmetry "dark state" on the dots.

Zitko R; Mravlje J; Haule K

2012-02-01

406

Ground state of the parallel double quantum dot system.

We resolve the controversy regarding the ground state of the parallel double quantum dot system near half filling. The numerical renormalization group predicts an underscreened Kondo state with residual spin-1/2 magnetic moment, ln2 residual impurity entropy, and unitary conductance, while the Bethe ansatz solution predicts a fully screened impurity, regular Fermi-liquid ground state, and zero conductance. We calculate the impurity entropy of the system as a function of the temperature using the hybridization-expansion continuous-time quantum Monte Carlo technique, which is a numerically exact stochastic method, and find excellent agreement with the numerical renormalization group results. We show that the origin of the unconventional behavior in this model is the odd-symmetry "dark state" on the dots. PMID:22401099

Zitko, Rok; Mravlje, Jernej; Haule, Kristjan

2012-02-08

407

Connection between conserved quantities and degeneracies in quantum systems

In the framework of quantum theory, we present one theorem and three corollaries regarding the direct connection between constants of motion of a physical system and degeneracies of its energy eigenvalues. It is shown that this connection emerges when there exist quantum operators which commute with the Hamiltonian, but not with each other. Further it is shown that if the commutator of these operators is a nonvanishing constant number then (a) all the eigenvalues of the system are degenerate, and (b) the degree of degeneracy is infinite. A number of examples are discussed including the parity degeneracy of the hydrogen atom and the infinite degeneracy of the Landau levels of a charged particle in a constant magnetic field.

Fallieros, S.; Hadjimichael, E.

1995-11-01

408

Connection between conserved quantities and degeneracies in quantum systems

International Nuclear Information System (INIS)

In the framework of quantum theory, we present one theorem and three corollaries regarding the direct connection between constants of motion of a physical system and degeneracies of its energy eigenvalues. It is shown that this connection emerges when there exist quantum operators which commute with the Hamiltonian, but not with each other. Further it is shown that if the commutator of these operators is a nonvanishing constant number then (a) all the eigenvalues of the system are degenerate, and (b) the degree of degeneracy is infinite. A number of examples are discussed including the parity degeneracy of the hydrogen atom and the infinite degeneracy of the Landau levels of a charged particle in a constant magnetic field. copyright 1995 American Association of Physics Teachers

1995-01-01

409

Connection between conserved quantities and degeneracies in quantum systems

Energy Technology Data Exchange (ETDEWEB)

In the framework of quantum theory, we present one theorem and three corollaries regarding the direct connection between constants of motion of a physical system and degeneracies of its energy eigenvalues. It is shown that this connection emerges when there exist quantum operators which commute with the Hamiltonian, but not with each other. Further it is shown that if the commutator of these operators is a nonvanishing constant number then (a) all the eigenvalues of the system are degenerate, and (b) the degree of degeneracy is infinite. A number of examples are discussed including the parity degeneracy of the hydrogen atom and the infinite degeneracy of the Landau levels of a charged particle in a constant magnetic field. {copyright} {ital 1995} {ital American} {ital Association} {ital of} {ital Physics} {ital Teachers}.

Fallieros, S. [Department of Physics, Brown University, Providence, Rhode Island 02912 (United States); Hadjimichael, E. [Department of Physics, Fairfield University, Fairfield, Connecticut 06430 (United States)

1995-11-01

410

Geometrical effects on energy transfer in disordered open quantum systems.

UK PubMed Central (United Kingdom)

We explore various design principles for efficient excitation energy transport in complex quantum systems. We investigate energy transfer efficiency in randomly disordered geometries consisting of up to 20 chromophores to explore spatial and spectral properties of small natural/artificial Light-Harvesting Complexes (LHC). We find significant statistical correlations among highly efficient random structures with respect to ground state properties, excitonic energy gaps, multichromophoric spatial connectivity, and path strengths. These correlations can even exist beyond the optimal regime of environment-assisted quantum transport. For random configurations embedded in spatial dimensions of 30 A? or 50 A?, we observe that the transport efficiency saturates to its maximum value if the systems contain around 7 or 14 chromophores, respectively. Remarkably, these optimum values coincide with the number of chlorophylls in the Fenna-Matthews-Olson protein complex and LHC II monomers, respectively, suggesting a potential natural optimization with respect to chromophoric density.

Mohseni M; Shabani A; Lloyd S; Omar Y; Rabitz H

2013-05-01

411

Quantum pumping and dissipation in closed systems

Current can be pumped through a closed system by changing parameters (or fields) in time. Linear response theory (the Kubo formula) allows one to analyze both the charge transport and the associated dissipation effect. We make a distinction between adiabatic and non-adiabatic regimes, and explain the subtle limit of an infinite system. As an example we discuss the following question: What is the amount of charge which is pushed by a moving scatterer? In the low-frequency (DC) limit we can write dQ=-GdX, where dX is the displacement of the scatterer. Thus the issue is to calculate the generalized conductance G.

Cohen, Doron

2005-10-01

412

Quantum pumping and dissipation in closed systems

Current can be pumped through a closed system by changing parameters (or fields) in time. Linear response theory (the Kubo formula) allows to analyze both the charge transport and the associated dissipation effect. We make a distinction between adiabatic and non-adiabatic regimes, and explain the subtle limit of an infinite system. As an example we discuss the following question: What is the amount of charge which is pushed by a moving scatterer? In the low frequency (DC) limit we can write dQ=-GdX, where dX is the displacement of the scatterer. Thus the issue is to calculate the generalized conductance $G$.

Cohen, D

2005-01-01

413

There exist several phenomena (systems) breaking the classical probability laws. Such systems are contextual dependent adaptive systems. In this paper, we present a new mathematical formula to compute the probability in those systems by using the concepts of the adaptive dynamics and quantum information theory -- quantum channels and the lifting. The basic examples of the contextual dependent phenomena can be found in quantum physics. And recently similar examples were found in biological and psychological sciences. Our novel approach is motivated by traditional quantum probability, but it is general enough to describe aforementioned phenomena outside of quantum physics.

Asano, Masanari; Khrennikov, Andrei; Ohya, Masanori; Yamato, Ichiro

2011-01-01

414

Hybrid quantum-well system for wavelength-channel selection.

UK PubMed Central (United Kingdom)

We consider a hybrid quantum-well structure consisting of regions whose properties alternate between active Raman gain and electromagnetically induced transparency. We present both analytical and numerical results that indicate a large light beam defection using spatially inhomogeneous pump and control lasers. We show well-isolated on-chip wavelength selection or channeling capabilities without light field attenuation or distortion, demonstrating the advantages of the system for possible important applications in integrated circuits for optical telecommunications.

Zhu C; Deng L; Hagley EW

2013-07-01

415

Mechanical and chemical spinodal instabilities in finite quantum systems

Energy Technology Data Exchange (ETDEWEB)

Self consistent quantum approaches are used to study the instabilities of finite nuclear systems. The frequencies of multipole density fluctuations are determined as a function of dilution and temperature, for several isotopes. The spinodal region of the phase diagrams is determined and it appears reduced by finite size effects. The role of surface and volume instabilities is discussed. Important chemical effects are associated with mechanical disruption and may lead to isospin fractionation. (authors)

Colonna, M. [Catania Univ., LNS (Italy); Chomaz, Ph. [Grand Accelerateur National d' Ions Lourds (GANIL), 14 - Caen (France); Ayik, S. [Tennessee Technological University, Cookeville TN (United States)

2001-07-01

416

Quantum Super-Integrable Systems as Exactly Solvable Models

Directory of Open Access Journals (Sweden)

Full Text Available We consider some examples of quantum super-integrable systems and the associated nonlinear extensions of Lie algebras. The intimate relationship between super-integrability and exact solvability is illustrated. Eigenfunctions are constructed through the action of the commuting operators. Finite dimensional representations of the quadratic algebras are thus constructed in a way analogous to that of the highest weight representations of Lie algebras.

Allan P. Fordy

2007-01-01

417

Transforming quantum operations: quantum supermaps

We introduce the concept of {\\em quantum supermap}, describing the most general transformation that maps an input quantum operation into an output quantum operation. Since quantum operations include as special cases quantum states, effects, and measurements, quantum supermaps describe all possible transformations between elementary quantum objects (quantum systems as well as quantum devices). After giving the axiomatic definition of supermap, we prove a realization theorem, which shows that any supermap can be physically implemented as a simple quantum circuit. Applications to quantum programming, cloning, discrimination, estimation, information-disturbance trade-off, and tomography of channels are outlined.

Chiribella, G; Perinotti, P

2008-01-01

418

Trojan Horse attacks on Quantum Key Distribution systems

General Trojan horse attacks on quantum key distribution systems are analyzed. We illustrate the power of such attacks with today's technology and conclude that all system must implement active counter-measures. In particular all systems must include an auxiliary detector that monitors any incoming light. We show that such counter-measures can be efficient, provided enough additional privacy amplification is applied to the data. We present a practical way to reduce the maximal information gain that an adversary can gain using Trojan horse attacks.

Gisin, Nicolas; Kraus, B; Zbinden, H; Ribordy, G

2005-01-01

419

Chaos and statistical relaxation in quantum systems of interacting particles

We propose a method to study the transition to chaos in isolated quantum systems of interacting particles. It is based on the concept of delocalization of eigenstates in the energy shell, controlled by the Gaussian form of the strength function. We show that although the fluctuations of energy levels in integrable and non-integrable systems are principally different, global properties of the eigenstates may be quite similar, provided the interaction between particles exceeds some critical value. In this case the quench dynamics can be described analytically, demonstrating the universal statistical relaxation of the systems irrespectively of whether they are integrable or not.

Santos, L F; Izrailev, F M

2011-01-01

420

Average quantum dynamics of closed systems over stochastic Hamiltonians

We develop a master equation formalism to describe the evolution of the average density matrix of a closed quantum system driven by a stochastic Hamiltonian. The average over random processes generally results in decoherence effects in closed system dynamics, in addition to the usual unitary evolution. We then show that, for an important class of problems in which the Hamiltonian is proportional to a Gaussian random process, the 2nd-order master equation yields exact dynamics. The general formalism is applied to study the examples of a two-level system, two atoms in a stochastic magnetic field and the heating of a trapped ion.

Yu, Li

2011-01-01

421

Engineering of quantum systems with variables in GF(pell)

A system comprised of ell subsystems, each of which is described with variables in ?p, is considered. Position and momentum states in this ell-partite system can be labelled with elements in GF(pell). It is shown that the whole quantum formalism can be expressed in terms of Galois arithmetic, if and only if the Hamiltonian belongs to a particular subset of the set of all Hamiltonians of this ell-partite system (we say that these Hamiltonians are compatible with GF(plell)). Several examples of such Hamiltonians are presented.

Vourdas, A.

2011-03-01

422

Engineering of quantum systems with variables in GF(pl)

International Nuclear Information System (INIS)

A system comprised of l subsystems, each of which is described with variables in Zp, is considered. Position and momentum states in this l-partite system can be labelled with elements in GF(pl). It is shown that the whole quantum formalism can be expressed in terms of Galois arithmetic, if and only if the Hamiltonian belongs to a particular subset of the set of all Hamiltonians of this l-partite system (we say that these Hamiltonians are compatible with GF(pll)). Several examples of such Hamiltonians are presented.

2011-03-01

423

Classical and quantum integrability in 3D systems

International Nuclear Information System (INIS)

[en] In this paper, we discuss three situations in which complete integrability of a three-dimensional classical system and its quantum version can be achieved under some conditions. The former is a system with axial symmetry. In the second, we discuss a three-dimensional system without spatial symmetry which admits separation of variables if we use ellipsoidal coordinates. In both cases, and as a condition for integrability, certain conditions arise in the integrals of motion. Finally, we study integrability in the three-dimensional sphere and a particular case associated with the Kepler problem in S3

2008-08-01

424

Simulating quantum systems on classical computers with matrix product states

Energy Technology Data Exchange (ETDEWEB)

In this thesis, the numerical simulation of strongly-interacting many-body quantum-mechanical systems using matrix product states (MPS) is considered. Matrix-Product-States are a novel representation of arbitrary quantum many-body states. Using quantum information theory, it is possible to show that Matrix-Product-States provide a polynomial-sized representation of one-dimensional quantum systems, thus allowing an efficient simulation of one-dimensional quantum system on classical computers. Matrix-Product-States form the conceptual framework of the density-matrix renormalization group (DMRG). After a general introduction in the first chapter of this thesis, the second chapter deals with Matrix-Product-States, focusing on the development of fast and stable algorithms. To obtain algorithms to efficiently calculate ground states, the density-matrix renormalization group is reformulated using the Matrix-Product-States framework. Further, time-dependent problems are considered. Two different algorithms are presented, one based on a Trotter decomposition of the time-evolution operator, the other one on Krylov subspaces. Finally, the evaluation of dynamical spectral functions is discussed, and a correction vector-based method is presented. In the following chapters, the methods presented in the second chapter, are applied to a number of different physical problems. The third chapter deals with the existence of chiral phases in isotropic one-dimensional quantum spin systems. A preceding analytical study based on a mean-field approach indicated the possible existence of those phases in an isotropic Heisenberg model with a frustrating zig-zag interaction and a magnetic field. In this thesis, the existence of the chiral phases is shown numerically by using Matrix-Product-States-based algorithms. In the fourth chapter, we propose an experiment using ultracold atomic gases in optical lattices, which allows a well controlled observation of the spin-charge separation (of different hyperfine states of the ultracold atoms) with current state of the art experimental techniques. Ultracold atoms in optical lattices are well described by (Bose)-Hubbard models. In order to support this proposal, we present numerical results for realistic system parameters. For strongly-correlated systems, for instance cuprates-based high-temperature superconductors, quantum fluctuations play an essential role. The dynamical mean-field theory (DMFT) fully takes local quantum fluctuations into account but neglects any kind of spatial fluctuations. The many-body problem on the lattice is mapped onto an impurity problem, which needs to be solved self-consistently. In the last chapter of this thesis, Matrix-Product-States-based algorithms are used to solve the impurity problem of the dynamical mean-field theory. We present results for a Hubbard model on a one-dimensional lattice and on a Bethe lattice obtained by the dynamical mean-field and compare them with exact results. (orig.)

Kleine, Adrian

2010-11-08

425

Simulating quantum systems on classical computers with matrix product states

International Nuclear Information System (INIS)

[en] In this thesis, the numerical simulation of strongly-interacting many-body quantum-mechanical systems using matrix product states (MPS) is considered. Matrix-Product-States are a novel representation of arbitrary quantum many-body states. Using quantum information theory, it is possible to show that Matrix-Product-States provide a polynomial-sized representation of one-dimensional quantum systems, thus allowing an efficient simulation of one-dimensional quantum system on classical computers. Matrix-Product-States form the conceptual framework of the density-matrix renormalization group (DMRG). After a general introduction in the first chapter of this thesis, the second chapter deals with Matrix-Product-States, focusing on the development of fast and stable algorithms. To obtain algorithms to efficiently calculate ground states, the density-matrix renormalization group is reformulated using the Matrix-Product-States framework. Further, time-dependent problems are considered. Two different algorithms are presented, one based on a Trotter decomposition of the time-evolution operator, the other one on Krylov subspaces. Finally, the evaluation of dynamical spectral functions is discussed, and a correction vector-based method is presented. In the following chapters, the methods presented in the second chapter, are applied to a number of different physical problems. The third chapter deals with the existence of chiral phases in isotropic one-dimensional quantum spin systems. A preceding analytical study based on a mean-field approach indicated the possible existence of those phases in an isotropic Heisenberg model with a frustrating zig-zag interaction and a magnetic field. In this thesis, the existence of the chiral phases is shown numerically by using Matrix-Product-States-based algorithms. In the fourth chapter, we propose an experiment using ultracold atomic gases in optical lattices, which allows a well controlled observation of the spin-charge separation (of different hyperfine states of the ultracold atoms) with current state of the art experimental techniques. Ultracold atoms in optical lattices are well described by (Bose)-Hubbard models. In order to support this proposal, we present numerical results for realistic system parameters. For strongly-correlated systems, for instance cuprates-based high-temperature superconductors, quantum fluctuations play an essential role. The dynamical mean-field theory (DMFT) fully takes local quantum fluctuations into account but neglects any kind of spatial fluctuations. The many-body problem on the lattice is mapped onto an impurity problem, which needs to be solved self-consistently. In the last chapter of this thesis, Matrix-Product-States-based algorithms are used to solve the impurity problem of the dynamical mean-field theory. We present results for a Hubbard model on a one-dimensional lattice and on a Bethe lattice obtained by the dynamical mean-field and compare them with exact results. (orig.)

2010-01-01

426

Optical nanostructures have proven to be meritorious for tailoring the emission properties of quantum emitters. However, unavoidable fabrication imperfections may represent a nuisance. Quite remarkably, disorder offers new opportunities since light can be efficiently confined by random multiple scattering leading to Anderson localization. Here we investigate the effect of such disorder-induced cavities on the emission dynamics of single quantum dots embedded in disordered photonic-crystal waveguides. We present time-resolved measurements of both the total emission from Anderson-localized cavities and from single emitters that are coupled to the cavities. We observe both strongly inhibited and enhanced decay rates relative to the rate of spontaneous emission in a homogeneous medium. From a statistical analysis, we report an average Purcell factor of 2 in without any control on the quantum dot - cavity detuning. By spectrally tuning individual quantum dots into resonance with Anderson-localized modes, a maximum...

Javadi, Alisa; Sapienza, Luca; Thyrrestrup, Henri; Lodahl, Peter

2013-01-01

427

In the quantum Hall regime, electronic correlations in double-layer two-dimensional electron systems are strong because the kinetic energy is quenched by Landau quantization. In this article we point out that these correlations are reflected in the way the partitioning of charge between the two-layers responds to a bias potential. We report on illustrative calculations based on an unrestricted Hartree-Fock approximation which allows for spontaneous inter-layer phase coherence. The possibility of studying inter-layer correlations by capacitive coupling to separately contacted two-dimensional layers is discussed in detail.

Jungwirth, T

1996-01-01

428

Task-dependent control of open quantum systems

We develop a general optimization strategy for performing a chosen unitary or non-unitary task on an open quantum system. The goal is to design a controlled time-dependent system Hamiltonian by variationally minimizing or maximizing a chosen function of the system state, which quantifies the task success (score), such as fidelity, purity, or entanglement. If the time-dependence of the system Hamiltonian is fast enough to be comparable or shorter than the response-time of the bath, then the resulting non-Markovian dynamics is shown to optimize the chosen task score to second order in the coupling to the bath. This strategy can protect a desired unitary system evolution from bath-induced decoherence, but ca also take advantage of the system-bath coupling so as to realize a desired non-unitary effect on the system.

Clausen, Jens; Kurizki, Gershon

2011-01-01

429

Perturbations of supersymmetric systems in quantum mechanics

International Nuclear Information System (INIS)

The methods of supersymmetry are extended to the factorization method. The degeneracy of levels in factorizable systems is broken under perturbations. With the methods of supersymmetry it is possible to state laws on the order of these perturbed energy levels. One proof of a confirmation of these laws has a more algebraic touch and works for first order perturbation theory. The proof of the laws beyond perturbation theory is hard, more of an analytic spirit and exploits convexity properties of the potentials. The convexity properties serve also for an intuitive argument. An important application is the law on the ordering of energy levels in atoms. 8 refs. (Author).

1990-01-01

430

Correlations and Equilibration in Relativistic Quantum Systems

In this article we study the time evolution of an interacting field theoretical system, i.e. \\phi^4-field theory in 2+1 space-time dimensions, on the basis of the Kadanoff-Baym equations for a spatially homogeneous system including the self-consistent tadpole and sunset self-energies. We find that equilibration is achieved only by inclusion of the sunset self-energy. Simultaneously, the time evolution of the scalar particle spectral function is studied for various initial states. We also compare associated solutions of the corresponding Boltzmann equation to the full Kadanoff-Baym theory. This comparison shows that a consistent inclusion of the spectral function has a significant impact on the equilibration rates only if the width of the spectral function becomes larger than 1/3 of the particle mass. Furthermore, based on these findings, the conventional transport of particles in the on-shell quasiparticle limit is extended to particles of finite life time by means of a dynamical spectral function A(X,\\vec{p}...

Cassing, W

2003-01-01

431

Quantum Fourier Transform and Phase Estimation in Qudit System

International Nuclear Information System (INIS)

The quantum Fourier transform and quantum phase estimation are the key components for many quantum algorithms, such as order-finding, factoring, and etc. In this article, the general procedure of quantum Fourier transform and phase estimation are investigated for high dimensional case. They can be seen as subroutines in a main program run in a qudit quantum computer, and the quantum circuits are given. (general)

2011-05-15

432

Numerical approaches to time evolution of complex quantum systems

International Nuclear Information System (INIS)

We examine several numerical techniques for the calculation of the dynamics of quantum systems. In particular, we single out an iterative method which is based on expanding the time evolution operator into a finite series of Chebyshev polynomials. The Chebyshev approach benefits from two advantages over the standard time-integration Crank-Nicholson scheme: speedup and efficiency. Potential competitors are semiclassical methods such as the Wigner-Moyal or quantum tomographic approaches. We outline the basic concepts of these techniques and benchmark their performance against the Chebyshev approach by monitoring the time evolution of a Gaussian wave packet in restricted one-dimensional (1D) geometries. Thereby the focus is on tunnelling processes and the motion in anharmonic potentials. Finally we apply the prominent Chebyshev technique to two highly non-trivial problems of current interest: (i) the injection of a particle in a disordered 2D graphene nanoribbon and (ii) the spatiotemporal evolution of polaron states in finite quantum systems. Here, depending on the disorder/electron-phonon coupling strength and the device dimensions, we observe transmission or localisation of the matter wave.

2009-06-01

433

Vector lattice description of a simple quantum system. Pt. 1

Energy Technology Data Exchange (ETDEWEB)

The cannonical 8-dimensional discrete vector lattice (A, EPSILON, E) with a positive cone EPSILON and an order unit E is considered as the space of random variables over the Boolean algebra 2/sup 8/. The ordered triple of basic variables is determined using the properties of a free Boolean algebra with three free generating elements. It is shown that all triples of basic variables in (A, EPSILON, E) are obtained from one another through the symmetries of the cone as well as that each orthonormal basis generated by basic variables and their products corresponds to a basis of eight atoms through an orthogonal and involuntary transformation. The orthogonal representation of the three-dimensional rotation group in A is considered and the correspondence between the triple of basic variables and the right-handed Cartesian space coordinate systems is referred to as the transformation law for the basic variables. It is shown that such an orthogonal representation of O(3,R) acts on both the lattice cone and the basic variables. It is claimed that the corresponding physical system is the quantum one known as spin-1/2. The classical (and therefore commutative) description is not given. The non-commutativity of the quantum observables is replaced by a property which distinguishes the model both from classical and quantum mechanics. The model is an extention of standard formalism and is reduced to the operator description.

Petrov, A. (Bylgarska Akademiya na Naukite, Sofia. Inst. za Yadrena Izsledvaniya i Yadrena Energetika)

1981-01-01

434

Enhancements to cavity quantum electrodynamics system

We show the planned upgrade of a cavity QED experimental apparatus. The system consists of an optical cavity and an ensemble of ultracold $^{85}$Rb atoms coupled to its mode. We propose enhancements to both. First, we document the building process for a new cavity, with a planned finesse of $\\sim$20000. We address problems of maintaining mirror integrity during mounting and improving vibration isolation. Second, we propose improvements to the cold atom source in order to achieve better optical pumping and control over the flux of atoms. We consider a 2-D optical molasses for atomic beam deflection, and show computer simulation results for evaluating the design. We also examine the possibility of all-optical atomic beam focusing, but find that it requires unreasonable experimental parameters.

Cimmarusti, A D; Norris, D G; Orozco, L A

2011-01-01

435

Quantum chemical modelling of some catalytic transition metal systems

Energy Technology Data Exchange (ETDEWEB)

The present thesis investigates chemical reactions on metal surfaces. Some catalytic transition metal systems are analysed by