Quantum control and process tomography of a semiconductor quantum dot hybrid qubit.

Science.gov (United States)

Kim, Dohun; Shi, Zhan; Simmons, C B; Ward, D R; Prance, J R; Koh, Teck Seng; Gamble, John King; Savage, D E; Lagally, M G; Friesen, Mark; Coppersmith, S N; Eriksson, Mark A

2014-07-03

The similarities between gated quantum dots and the transistors in modern microelectronics--in fabrication methods, physical structure and voltage scales for manipulation--have led to great interest in the development of quantum bits (qubits) in semiconductor quantum dots. Although quantum dot spin qubits have demonstrated long coherence times, their manipulation is often slower than desired for important future applications, such as factoring. Furthermore, scalability and manufacturability are enhanced when qubits are as simple as possible. Previous work has increased the speed of spin qubit rotations by making use of integrated micromagnets, dynamic pumping of nuclear spins or the addition of a third quantum dot. Here we demonstrate a qubit that is a hybrid of spin and charge. It is simple, requiring neither nuclear-state preparation nor micromagnets. Unlike previous double-dot qubits, the hybrid qubit enables fast rotations about two axes of the Bloch sphere. We demonstrate full control on the Bloch sphere with π-rotation times of less than 100 picoseconds in two orthogonal directions, which is more than an order of magnitude faster than any other double-dot qubit. The speed arises from the qubit's charge-like characteristics, and its spin-like features result in resistance to decoherence over a wide range of gate voltages. We achieve full process tomography in our electrically controlled semiconductor quantum dot qubit, extracting high fidelities of 85 per cent for X rotations (transitions between qubit states) and 94 per cent for Z rotations (phase accumulation between qubit states).

Geometric picture of quantum discord for two-qubit quantum states

International Nuclear Information System (INIS)

Shi Mingjun; Jiang Fengjian; Sun Chunxiao; Du Jiangfeng

2011-01-01

Among various definitions of quantum correlations, quantum discord has attracted considerable attention. To find an analytical expression for quantum discord is an intractable task. Exact results are known only for very special states, namely two-qubit X-shaped states. We present in this paper a geometric viewpoint, from which two-qubit quantum discord can be described clearly. The known results on X state discord are restated in the directly perceivable geometric language. As a consequence, the dynamics of classical correlations and quantum discord for an X state in the presence of decoherence is endowed with geometric interpretation. More importantly, we extend the geometric method to the case of more general states, for which numerical as well as analytical results on quantum discord have not yet been obtained. Based on the support of numerical computations, some conjectures are proposed to help us establish the geometric picture. We find that the geometric picture for these states has an intimate relationship with that for X states. Thereby, in some cases, analytical expressions for classical correlations and quantum discord can be obtained.

Non-local correlations via Wigner-Yanase skew information in two SC-qubit having mutual interaction under phase decoherence

Science.gov (United States)

Mohamed, Abdel-Baset A.

2017-10-01

An analytical solution of the master equation that describes a superconducting cavity containing two coupled superconducting charge qubits is obtained. Quantum-mechanical correlations based on Wigner-Yanase skew information, as local quantum uncertainty and uncertainty-induced quantum non-locality, are compared to the concurrence under the effects of the phase decoherence. Local quantum uncertainty exhibits sudden changes during its time evolution and revival process. Sudden death and sudden birth occur only for entanglement, depending on the initial state of the two coupled charge qubits, while the correlations of skew information does not vanish. The quantum correlations of skew information are found to be sensitive to the dephasing rate, the photons number in the cavity, the interaction strength between the two qubits, and the qubit distribution angle of the initial state. With a proper initial state, the stationary correlation of the skew information has a non-zero stationary value for a long time interval under the phase decoherence, that it may be useful in quantum information and computation processes.

Multi-target-qubit unconventional geometric phase gate in a multi-cavity system.

Science.gov (United States)

Liu, Tong; Cao, Xiao-Zhi; Su, Qi-Ping; Xiong, Shao-Jie; Yang, Chui-Ping

2016-02-22

Cavity-based large scale quantum information processing (QIP) may involve multiple cavities and require performing various quantum logic operations on qubits distributed in different cavities. Geometric-phase-based quantum computing has drawn much attention recently, which offers advantages against inaccuracies and local fluctuations. In addition, multiqubit gates are particularly appealing and play important roles in QIP. We here present a simple and efficient scheme for realizing a multi-target-qubit unconventional geometric phase gate in a multi-cavity system. This multiqubit phase gate has a common control qubit but different target qubits distributed in different cavities, which can be achieved using a single-step operation. The gate operation time is independent of the number of qubits and only two levels for each qubit are needed. This multiqubit gate is generic, e.g., by performing single-qubit operations, it can be converted into two types of significant multi-target-qubit phase gates useful in QIP. The proposal is quite general, which can be used to accomplish the same task for a general type of qubits such as atoms, NV centers, quantum dots, and superconducting qubits.

Decoherence dynamics of two charge qubits in vertically coupled quantum dots

International Nuclear Information System (INIS)

Ben Chouikha, W.; Bennaceur, R.; Jaziri, S.

2007-01-01

The decoherence dynamics of two charge qubits in a double quantum dot is investigated theoretically. We consider the quantum dynamics of two interacting electrons in a vertically coupled quantum dot driven by an external electric field. We derive the equations of motion for the density matrix, in which the presence of an electron confined in the double dot represents one qubit. A Markovian approach to the dynamical evolution of the reduced density matrix is adopted. We evaluate the concurrence of two qubits in order to study the effect of acoustic phonons on the entanglement. We also show that the disentanglement effect depends on the double dot parameters and increases with the temperature

Entanglement, purity, and energy: Two qubits versus two modes

International Nuclear Information System (INIS)

McHugh, Derek; Ziman, Mario; Buzek, Vladimir

2006-01-01

We study the relationship between the entanglement, mixedness, and energy of two-qubit and two-mode Gaussian quantum states. We parametrize the set of allowed states of these two fundamentally different physical systems using measures of entanglement, mixedness, and energy that allow us to compare and contrast the two systems using a phase diagram. This phase diagram enables one to clearly identify not only the physically allowed states, but the set of states connected under an arbitrary quantum operation. We pay particular attention to the maximally entangled mixed states of each system. Following this we investigate how efficiently one may transfer entanglement from two-mode to two-qubit states

Non-Bell-pair quantum channel for teleporting an arbitrary two-qubit state

International Nuclear Information System (INIS)

Zha Xinwei; Song Haiyang

2007-01-01

Recently, Yeo and Chua [Y. Yeo, W.K. Chua, Phys. Rev. Lett. 96 (2006) 060502] gave a protocol for faithfully teleporting an arbitrary two-qubit state via a genuine four-qubit entangled state, which is not reducible to a pair of Bell state. Here, we present a 'transformation operator' to give a criterion for faithful teleportation of an arbitrary two-qubit state via a four-qubit entangled state. The theoretical explanations of some quantum channels are given in term of transformation operators. The relation between the transformation operators and the Bell base measurement is also obtained. Furthermore, a new four-qubit entangled state quantum channel is presented

The two-qubit quantum Rabi model: inhomogeneous coupling

International Nuclear Information System (INIS)

Mao, Lijun; Huai, Sainan; Zhang, Yunbo

2015-01-01

We revisit the analytic solution of the two-qubit quantum Rabi model with inhomogeneous coupling and transition frequencies using a displaced oscillator basis. This approach enables us to apply the same truncation rules and techniques adopted in the Rabi model to the two qubits system. The derived analytical spectra match perfectly with the numerical solutions in the parameter regime where the qubits’ transition frequencies are far off-resonance with the field frequency and the interaction strengths reach the ultrastrong coupling regime. We further explore the dynamical behavior of the two qubits as well as the evolution of entanglement. The analytical methods provide unexpectedly accurate results in describing the dynamics of the two qubits in the present experimentally accessible coupling regime. The time evolutions of the probability for the qubits show that the collapse-revival phenomena emerge, survive and finally disappear when one coupling strength increases from weak to strong coupling regimes and the other coupling strength is well into the ultrastrong coupling regime. The inhomogeneous coupling system exhibits new dynamics, which are different from the homogeneous coupling case. (paper)

Simultaneous deterministic control of distant qubits in two semiconductor quantum dots.

Science.gov (United States)

Gamouras, A; Mathew, R; Freisem, S; Deppe, D G; Hall, K C

2013-10-09

In optimal quantum control (OQC), a target quantum state of matter is achieved by tailoring the phase and amplitude of the control Hamiltonian through femtosecond pulse-shaping techniques and powerful adaptive feedback algorithms. Motivated by recent applications of OQC in quantum information science as an approach to optimizing quantum gates in atomic and molecular systems, here we report the experimental implementation of OQC in a solid-state system consisting of distinguishable semiconductor quantum dots. We demonstrate simultaneous high-fidelity π and 2π single qubit gates in two different quantum dots using a single engineered infrared femtosecond pulse. These experiments enhance the scalability of semiconductor-based quantum hardware and lay the foundation for applications of pulse shaping to optimize quantum gates in other solid-state systems.

Gatemon Benchmarking and Two-Qubit Operation

Science.gov (United States)

Casparis, Lucas; Larsen, Thorvald; Olsen, Michael; Petersson, Karl; Kuemmeth, Ferdinand; Krogstrup, Peter; Nygard, Jesper; Marcus, Charles

Recent experiments have demonstrated superconducting transmon qubits with semiconductor nanowire Josephson junctions. These hybrid gatemon qubits utilize field effect tunability singular to semiconductors to allow complete qubit control using gate voltages, potentially a technological advantage over conventional flux-controlled transmons. Here, we present experiments with a two-qubit gatemon circuit. We characterize qubit coherence and stability and use randomized benchmarking to demonstrate single-qubit gate errors of ~0.5 % for all gates, including voltage-controlled Z rotations. We show coherent capacitive coupling between two gatemons and coherent SWAP operations. Finally, we perform a two-qubit controlled-phase gate with an estimated fidelity of ~91 %, demonstrating the potential of gatemon qubits for building scalable quantum processors. We acknowledge financial support from Microsoft Project Q and the Danish National Research Foundation.

Ultrafast optical control of individual quantum dot spin qubits.

Science.gov (United States)

De Greve, Kristiaan; Press, David; McMahon, Peter L; Yamamoto, Yoshihisa

2013-09-01

Single spins in semiconductor quantum dots form a promising platform for solid-state quantum information processing. The spin-up and spin-down states of a single electron or hole, trapped inside a quantum dot, can represent a single qubit with a reasonably long decoherence time. The spin qubit can be optically coupled to excited (charged exciton) states that are also trapped in the quantum dot, which provides a mechanism to quickly initialize, manipulate and measure the spin state with optical pulses, and to interface between a stationary matter qubit and a 'flying' photonic qubit for quantum communication and distributed quantum information processing. The interaction of the spin qubit with light may be enhanced by placing the quantum dot inside a monolithic microcavity. An entire system, consisting of a two-dimensional array of quantum dots and a planar microcavity, may plausibly be constructed by modern semiconductor nano-fabrication technology and could offer a path toward chip-sized scalable quantum repeaters and quantum computers. This article reviews the recent experimental developments in optical control of single quantum dot spins for quantum information processing. We highlight demonstrations of a complete set of all-optical single-qubit operations on a single quantum dot spin: initialization, an arbitrary SU(2) gate, and measurement. We review the decoherence and dephasing mechanisms due to hyperfine interaction with the nuclear-spin bath, and show how the single-qubit operations can be combined to perform spin echo sequences that extend the qubit decoherence from a few nanoseconds to several microseconds, more than 5 orders of magnitude longer than the single-qubit gate time. Two-qubit coupling is discussed, both within a single chip by means of exchange coupling of nearby spins and optically induced geometric phases, as well as over longer-distances. Long-distance spin-spin entanglement can be generated if each spin can emit a photon that is entangled

Computing prime factors with a Josephson phase qubit quantum processor

Science.gov (United States)

Lucero, Erik; Barends, R.; Chen, Y.; Kelly, J.; Mariantoni, M.; Megrant, A.; O'Malley, P.; Sank, D.; Vainsencher, A.; Wenner, J.; White, T.; Yin, Y.; Cleland, A. N.; Martinis, John M.

2012-10-01

A quantum processor can be used to exploit quantum mechanics to find the prime factors of composite numbers. Compiled versions of Shor's algorithm and Gauss sum factorizations have been demonstrated on ensemble quantum systems, photonic systems and trapped ions. Although proposed, these algorithms have yet to be shown using solid-state quantum bits. Using a number of recent qubit control and hardware advances, here we demonstrate a nine-quantum-element solid-state quantum processor and show three experiments to highlight its capabilities. We begin by characterizing the device with spectroscopy. Next, we produce coherent interactions between five qubits and verify bi- and tripartite entanglement through quantum state tomography. In the final experiment, we run a three-qubit compiled version of Shor's algorithm to factor the number 15, and successfully find the prime factors 48% of the time. Improvements in the superconducting qubit coherence times and more complex circuits should provide the resources necessary to factor larger composite numbers and run more intricate quantum algorithms.

Hybrid quantum systems: Outsourcing superconducting qubits

Science.gov (United States)

Cleland, Andrew

Superconducting qubits offer excellent prospects for manipulating quantum information, with good qubit lifetimes, high fidelity single- and two-qubit gates, and straightforward scalability (admittedly with multi-dimensional interconnect challenges). One interesting route for experimental development is the exploration of hybrid systems, i.e. coupling superconducting qubits to other systems. I will report on our group's efforts to develop approaches that will allow interfacing superconducting qubits in a quantum-coherent fashion to spin defects in solids, to optomechanical devices, and to resonant nanomechanical structures. The longer term goals of these efforts include transferring quantum states between different qubit systems; generating and receiving ``flying'' acoustic phonon-based as well as optical photon-based qubits; and ultimately developing systems that can be used for quantum memory, quantum computation and quantum communication, the last in both the microwave and fiber telecommunications bands. Work is supported by Grants from AFOSR, ARO, DOE and NSF.

Energy localization in maximally entangled two- and three-qubit phase space

International Nuclear Information System (INIS)

Pashaev, Oktay K; Gurkan, Zeynep N

2012-01-01

Motivated by the Möbius transformation for symmetric points under the generalized circle in the complex plane, the system of symmetric spin coherent states corresponding to antipodal qubit states is introduced. In terms of these states, we construct the maximally entangled complete set of two-qubit coherent states, which in the limiting cases reduces to the Bell basis. A specific property of our symmetric coherent states is that they never become unentangled for any value of ψ from the complex plane. Entanglement quantifications of our states are given by the reduced density matrix and the concurrence determinant, and it is shown that our basis is maximally entangled. Universal one- and two-qubit gates in these new coherent state basis are calculated. As an application, we find the Q symbol of the XY Z model Hamiltonian operator H as an average energy function in maximally entangled two- and three-qubit phase space. It shows regular finite-energy localized structure with specific local extremum points. The concurrence and fidelity of quantum evolution with dimerization of double periodic patterns are given. (paper)

Three-Party Quantum State Sharing of an Arbitrary Unknown Two-Qubit State Based on Entanglement Swapping and Bell-State Measurements

International Nuclear Information System (INIS)

Yuan Hao; Song Jun; Hou Kui; Hu Xiaoyuan; Shi Shouhua; Han Lianfang

2009-01-01

We propose a scheme for sharing an arbitrary unknown two-qubit state among three parties by using a four-qubit cluster-class state and a Bell state as a quantum channel. With a quantum controlled phase gate (QCPG) operation and a local unitary operation, any one of the two agents has the access to reconstruct the original state if he/she collaborates with the other one, whilst individual agent obtains no information. As all quantum resource can be used to carry the useful information, the intrinsic efficiency of qubits approaches the maximal value. Moreover, the present scheme is more feasible with present-day technique.

Coherent Coupled Qubits for Quantum Annealing

Science.gov (United States)

Weber, Steven J.; Samach, Gabriel O.; Hover, David; Gustavsson, Simon; Kim, David K.; Melville, Alexander; Rosenberg, Danna; Sears, Adam P.; Yan, Fei; Yoder, Jonilyn L.; Oliver, William D.; Kerman, Andrew J.

2017-07-01

Quantum annealing is an optimization technique which potentially leverages quantum tunneling to enhance computational performance. Existing quantum annealers use superconducting flux qubits with short coherence times limited primarily by the use of large persistent currents Ip. Here, we examine an alternative approach using qubits with smaller Ip and longer coherence times. We demonstrate tunable coupling, a basic building block for quantum annealing, between two flux qubits with small (approximately 50-nA) persistent currents. Furthermore, we characterize qubit coherence as a function of coupler setting and investigate the effect of flux noise in the coupler loop on qubit coherence. Our results provide insight into the available design space for next-generation quantum annealers with improved coherence.

Controllable conditional quantum oscillations and quantum gate operations in superconducting flux qubits

International Nuclear Information System (INIS)

Chen Aimin; Cho Samyoung

2011-01-01

Conditional quantum oscillations are investigated for quantum gate operations in superconducting flux qubits. We present an effective Hamiltonian which describes a conditional quantum oscillation in two-qubit systems. Rabi-type quantum oscillations are discussed in implementing conditional quantum oscillations to quantum gate operations. Two conditional quantum oscillations depending on the states of control qubit can be synchronized to perform controlled-gate operations by varying system parameters. It is shown that the conditional quantum oscillations with their frequency synchronization make it possible to operate the controlled-NOT and -U gates with a very accurate gate performance rate in interacting qubit systems. Further, this scheme can be applicable to realize a controlled multi-qubit operation in various solid-state qubit systems. (author)

Definition and evolution of quantum cellular automata with two qubits per cell

International Nuclear Information System (INIS)

Karafyllidis, Ioannis G.

2004-01-01

Studies of quantum computer implementations suggest cellular quantum computer architectures. These architectures can simulate the evolution of quantum cellular automata, which can possibly simulate both quantum and classical physical systems and processes. It is however known that except for the trivial case, unitary evolution of one-dimensional homogeneous quantum cellular automata with one qubit per cell is not possible. Quantum cellular automata that comprise two qubits per cell are defined and their evolution is studied using a quantum computer simulator. The evolution is unitary and its linearity manifests itself as a periodic structure in the probability distribution patterns

Silicon based quantum dot hybrid qubits

Science.gov (United States)

Kim, Dohun

2015-03-01

The charge and spin degrees of freedom of an electron constitute natural bases for constructing quantum two level systems, or qubits, in semiconductor quantum dots. The quantum dot charge qubit offers a simple architecture and high-speed operation, but generally suffers from fast dephasing due to strong coupling of the environment to the electron's charge. On the other hand, quantum dot spin qubits have demonstrated long coherence times, but their manipulation is often slower than desired for important future applications. This talk will present experimental progress of a `hybrid' qubit, formed by three electrons in a Si/SiGe double quantum dot, which combines desirable characteristics (speed and coherence) in the past found separately in qubits based on either charge or spin degrees of freedom. Using resonant microwaves, we first discuss qubit operations near the `sweet spot' for charge qubit operation. Along with fast (>GHz) manipulation rates for any rotation axis on the Bloch sphere, we implement two independent tomographic characterization schemes in the charge qubit regime: traditional quantum process tomography (QPT) and gate set tomography (GST). We also present resonant qubit operations of the hybrid qubit performed on the same device, DC pulsed gate operations of which were recently demonstrated. We demonstrate three-axis control and the implementation of dynamic decoupling pulse sequences. Performing QPT on the hybrid qubit, we show that AC gating yields π rotation process fidelities higher than 93% for X-axis and 96% for Z-axis rotations, which demonstrates efficient quantum control of semiconductor qubits using resonant microwaves. We discuss a path forward for achieving fidelities better than the threshold for quantum error correction using surface codes. This work was supported in part by ARO (W911NF-12-0607), NSF (PHY-1104660), DOE (DE-FG02-03ER46028), and by the Laboratory Directed Research and Development program at Sandia National Laboratories

Nonlinearities in the quantum measurement process of superconducting qubits

Energy Technology Data Exchange (ETDEWEB)

Serban, Ioana

2008-05-15

The work described in this thesis focuses on the investigation of decoherence and measurement backaction, on the theoretical description of measurement schemes and their improvement. The study presented here is centered around quantum computing implementations using superconducting devices and most important, the Josephson effect. The measured system is invariantly a qubit, i. e. a two-level system. The objective is to study detectors with increasing nonlinearity, e. g. coupling of the qubit to the frequency a driven oscillator, or to the bifurcation amplifier, to determine the performance and backaction of the detector on the measured system and to investigate the importance of a strong qubit-detector coupling for the achievement of a quantum non-demolition type of detection. The first part gives a very basic introduction to quantum information, briefly reviews some of the most promising physical implementations of a quantum computer before focusing on the superconducting devices. The second part presents a series of studies of different qubit measurements, describing the backaction of the measurement onto the measured system and the internal dynamics of the detector. Methodology adapted from quantum optics and chemical physics (master equations, phase-space analysis etc.) combined with the representation of a complex environment yielded a tool capable of describing a nonlinear, non-Markovian environment, which couples arbitrarily strongly to the measured system. This is described in chapter 3. Chapter 4 focuses on the backaction on the qubit and presents novel insights into the qubit dephasing in the strong coupling regime. Chapter 5 uses basically the same system and technical tools to explore the potential of a fast, strong, indirect measurement, and determine how close such a detection would ideally come to the quantum non-demolition regime. Chapter 6 focuses on the internal dynamics of a strongly driven Josephson junction. The analytical results are based on

Nonlinearities in the quantum measurement process of superconducting qubits

International Nuclear Information System (INIS)

Serban, Ioana

2008-05-01

The work described in this thesis focuses on the investigation of decoherence and measurement backaction, on the theoretical description of measurement schemes and their improvement. The study presented here is centered around quantum computing implementations using superconducting devices and most important, the Josephson effect. The measured system is invariantly a qubit, i. e. a two-level system. The objective is to study detectors with increasing nonlinearity, e. g. coupling of the qubit to the frequency a driven oscillator, or to the bifurcation amplifier, to determine the performance and backaction of the detector on the measured system and to investigate the importance of a strong qubit-detector coupling for the achievement of a quantum non-demolition type of detection. The first part gives a very basic introduction to quantum information, briefly reviews some of the most promising physical implementations of a quantum computer before focusing on the superconducting devices. The second part presents a series of studies of different qubit measurements, describing the backaction of the measurement onto the measured system and the internal dynamics of the detector. Methodology adapted from quantum optics and chemical physics (master equations, phase-space analysis etc.) combined with the representation of a complex environment yielded a tool capable of describing a nonlinear, non-Markovian environment, which couples arbitrarily strongly to the measured system. This is described in chapter 3. Chapter 4 focuses on the backaction on the qubit and presents novel insights into the qubit dephasing in the strong coupling regime. Chapter 5 uses basically the same system and technical tools to explore the potential of a fast, strong, indirect measurement, and determine how close such a detection would ideally come to the quantum non-demolition regime. Chapter 6 focuses on the internal dynamics of a strongly driven Josephson junction. The analytical results are based on

Detection of quantum critical points by a probe qubit.

Science.gov (United States)

Zhang, Jingfu; Peng, Xinhua; Rajendran, Nageswaran; Suter, Dieter

2008-03-14

Quantum phase transitions occur when the ground state of a quantum system undergoes a qualitative change when an external control parameter reaches a critical value. Here, we demonstrate a technique for studying quantum systems undergoing a phase transition by coupling the system to a probe qubit. It uses directly the increased sensibility of the quantum system to perturbations when it is close to a critical point. Using an NMR quantum simulator, we demonstrate this measurement technique for two different types of quantum phase transitions in an Ising spin chain.

Entanglement and Quantum Error Correction with Superconducting Qubits

Science.gov (United States)

Reed, Matthew

2015-03-01

Quantum information science seeks to take advantage of the properties of quantum mechanics to manipulate information in ways that are not otherwise possible. Quantum computation, for example, promises to solve certain problems in days that would take a conventional supercomputer the age of the universe to decipher. This power does not come without a cost however, as quantum bits are inherently more susceptible to errors than their classical counterparts. Fortunately, it is possible to redundantly encode information in several entangled qubits, making it robust to decoherence and control imprecision with quantum error correction. I studied one possible physical implementation for quantum computing, employing the ground and first excited quantum states of a superconducting electrical circuit as a quantum bit. These ``transmon'' qubits are dispersively coupled to a superconducting resonator used for readout, control, and qubit-qubit coupling in the cavity quantum electrodynamics (cQED) architecture. In this talk I will give an general introduction to quantum computation and the superconducting technology that seeks to achieve it before explaining some of the specific results reported in my thesis. One major component is that of the first realization of three-qubit quantum error correction in a solid state device, where we encode one logical quantum bit in three entangled physical qubits and detect and correct phase- or bit-flip errors using a three-qubit Toffoli gate. My thesis is available at arXiv:1311.6759.

Attacking quantum key distribution with single-photon two-qubit quantum logic

International Nuclear Information System (INIS)

Shapiro, Jeffrey H.; Wong, Franco N. C.

2006-01-01

The Fuchs-Peres-Brandt (FPB) probe realizes the most powerful individual attack on Bennett-Brassard 1984 quantum key distribution (BB84 QKD) by means of a single controlled-NOT (CNOT) gate. This paper describes a complete physical simulation of the FPB-probe attack on polarization-based BB84 QKD using a deterministic CNOT constructed from single-photon two-qubit quantum logic. Adding polarization-preserving quantum nondemolition measurements of photon number to this configuration converts the physical simulation into a true deterministic realization of the FPB attack

Circuit quantum electrodynamics with a spin qubit.

Science.gov (United States)

Petersson, K D; McFaul, L W; Schroer, M D; Jung, M; Taylor, J M; Houck, A A; Petta, J R

2012-10-18

Electron spins trapped in quantum dots have been proposed as basic building blocks of a future quantum processor. Although fast, 180-picosecond, two-quantum-bit (two-qubit) operations can be realized using nearest-neighbour exchange coupling, a scalable, spin-based quantum computing architecture will almost certainly require long-range qubit interactions. Circuit quantum electrodynamics (cQED) allows spatially separated superconducting qubits to interact via a superconducting microwave cavity that acts as a 'quantum bus', making possible two-qubit entanglement and the implementation of simple quantum algorithms. Here we combine the cQED architecture with spin qubits by coupling an indium arsenide nanowire double quantum dot to a superconducting cavity. The architecture allows us to achieve a charge-cavity coupling rate of about 30 megahertz, consistent with coupling rates obtained in gallium arsenide quantum dots. Furthermore, the strong spin-orbit interaction of indium arsenide allows us to drive spin rotations electrically with a local gate electrode, and the charge-cavity interaction provides a measurement of the resulting spin dynamics. Our results demonstrate how the cQED architecture can be used as a sensitive probe of single-spin physics and that a spin-cavity coupling rate of about one megahertz is feasible, presenting the possibility of long-range spin coupling via superconducting microwave cavities.

Multi-qubit parity measurement in circuit quantum electrodynamics

International Nuclear Information System (INIS)

DiVincenzo, David P; Solgun, Firat

2013-01-01

We present a concept for performing direct parity measurements on three or more qubits in microwave structures with superconducting resonators coupled to Josephson-junction qubits. We write the quantum-eraser conditions that must be fulfilled for the parity measurements as requirements for the scattering phase shift of our microwave structure. We show that these conditions can be fulfilled with present-day devices. We present one particular scheme, implemented with two-dimensional cavity techniques, in which each qubit should be coupled equally to two different microwave cavities. The magnitudes of the couplings that are needed are in the range that has been achieved in current experiments. A quantum calculation indicates that the measurement is optimal if the scattering signal can be measured with near single-photon sensitivity. A comparison with an extension of a related proposal from cavity optics is presented. We present a second scheme, for which a scalable implementation of the four-qubit parities of the surface quantum error correction code can be envisioned. It uses three-dimensional cavity structures, using cavity symmetries to achieve the necessary multiple resonant modes within a single resonant structure. (paper)

Efficient one- and two-qubit pulsed gates for an oscillator-stabilized Josephson qubit

International Nuclear Information System (INIS)

Brito, Frederico; DiVincenzo, David P; Koch, Roger H; Steffen, Matthias

2008-01-01

We present theoretical schemes for performing high-fidelity one- and two-qubit pulsed gates for a superconducting flux qubit. The 'IBM qubit' consists of three Josephson junctions, three loops and a superconducting transmission line. Assuming a fixed inductive qubit-qubit coupling, we show that the effective qubit-qubit interaction is tunable by changing the applied fluxes, and can be made negligible, allowing one to perform high-fidelity single qubit gates. Our schemes are tailored to alleviate errors due to 1/f noise; we find gates with only 1% loss of fidelity due to this source, for pulse times in the range of 20-30 ns for one-qubit gates (Z rotations, Hadamard) and 60 ns for a two-qubit gate (controlled-Z). Our relaxation and dephasing time estimates indicate a comparable loss of fidelity from this source. The control of leakage plays an important role in the design of our shaped pulses, preventing shorter pulse times. However, we have found that imprecision in the control of the quantum phase plays a major role in the limitation of the fidelity of our gates

Realization of quantum gates with multiple control qubits or multiple target qubits in a cavity

Science.gov (United States)

Waseem, Muhammad; Irfan, Muhammad; Qamar, Shahid

2015-06-01

We propose a scheme to realize a three-qubit controlled phase gate and a multi-qubit controlled NOT gate of one qubit simultaneously controlling n-target qubits with a four-level quantum system in a cavity. The implementation time for multi-qubit controlled NOT gate is independent of the number of qubit. Three-qubit phase gate is generalized to n-qubit phase gate with multiple control qubits. The number of steps reduces linearly as compared to conventional gate decomposition method. Our scheme can be applied to various types of physical systems such as superconducting qubits coupled to a resonator and trapped atoms in a cavity. Our scheme does not require adjustment of level spacing during the gate implementation. We also show the implementation of Deutsch-Joza algorithm. Finally, we discuss the imperfections due to cavity decay and the possibility of physical implementation of our scheme.

Multi-qubit circuit quantum electrodynamics

International Nuclear Information System (INIS)

Viehmann, Oliver

2013-01-01

Circuit QED systems are macroscopic, man-made quantum systems in which superconducting artificial atoms, also called Josephson qubits, interact with a quantized electromagnetic field. These systems have been devised to mimic the physics of elementary quantum optical systems with real atoms in a scalable and more flexible framework. This opens up a variety of possible applications of circuit QED systems. For instance, they provide a promising platform for processing quantum information. Recent years have seen rapid experimental progress on these systems, and experiments with multi-component circuit QED architectures are currently starting to come within reach. In this thesis, circuit QED systems with multiple Josephson qubits are studied theoretically. We focus on simple and experimentally realistic extensions of the currently operated circuit QED setups and pursue investigations in two main directions. First, we consider the equilibrium behavior of circuit QED systems containing a large number of mutually noninteracting Josephson charge qubits. The currently accepted standard description of circuit QED predicts the possibility of superradiant phase transitions in such systems. However, a full microscopic treatment shows that a no-go theorem for superradiant phase transitions known from atomic physics applies to circuit QED systems as well. This reveals previously unknown limitations of the applicability of the standard theory of circuit QED to multi-qubit systems. Second, we explore the potential of circuit QED for quantum simulations of interacting quantum many-body systems. We propose and analyze a circuit QED architecture that implements the quantum Ising chain in a time-dependent transverse magnetic field. Our setup can be used to study quench dynamics, the propagation of localized excitations, and other non-equilibrium features in this paradigmatic model in the theory of non-equilibrium thermodynamics and quantumcritical phenomena. The setup is based on a

Multi-qubit circuit quantum electrodynamics

Energy Technology Data Exchange (ETDEWEB)

Viehmann, Oliver

2013-09-03

Circuit QED systems are macroscopic, man-made quantum systems in which superconducting artificial atoms, also called Josephson qubits, interact with a quantized electromagnetic field. These systems have been devised to mimic the physics of elementary quantum optical systems with real atoms in a scalable and more flexible framework. This opens up a variety of possible applications of circuit QED systems. For instance, they provide a promising platform for processing quantum information. Recent years have seen rapid experimental progress on these systems, and experiments with multi-component circuit QED architectures are currently starting to come within reach. In this thesis, circuit QED systems with multiple Josephson qubits are studied theoretically. We focus on simple and experimentally realistic extensions of the currently operated circuit QED setups and pursue investigations in two main directions. First, we consider the equilibrium behavior of circuit QED systems containing a large number of mutually noninteracting Josephson charge qubits. The currently accepted standard description of circuit QED predicts the possibility of superradiant phase transitions in such systems. However, a full microscopic treatment shows that a no-go theorem for superradiant phase transitions known from atomic physics applies to circuit QED systems as well. This reveals previously unknown limitations of the applicability of the standard theory of circuit QED to multi-qubit systems. Second, we explore the potential of circuit QED for quantum simulations of interacting quantum many-body systems. We propose and analyze a circuit QED architecture that implements the quantum Ising chain in a time-dependent transverse magnetic field. Our setup can be used to study quench dynamics, the propagation of localized excitations, and other non-equilibrium features in this paradigmatic model in the theory of non-equilibrium thermodynamics and quantumcritical phenomena. The setup is based on a

Integrated devices for quantum information and quantum simulation with polarization encoded qubits

Science.gov (United States)

Sansoni, Linda; Sciarrino, Fabio; Mataloni, Paolo; Crespi, Andrea; Ramponi, Roberta; Osellame, Roberto

2012-06-01

The ability to manipulate quantum states of light by integrated devices may open new perspectives both for fundamental tests of quantum mechanics and for novel technological applications. The technology for handling polarization-encoded qubits, the most commonly adopted approach, was still missing in quantum optical circuits until the ultrafast laser writing (ULW) technique was adopted for the first time to realize integrated devices able to support and manipulate polarization encoded qubits.1 Thanks to this method, polarization dependent and independent devices can be realized. In particular the maintenance of polarization entanglement was demonstrated in a balanced polarization independent integrated beam splitter1 and an integrated CNOT gate for polarization qubits was realized and carachterized.2 We also exploited integrated optics for quantum simulation tasks: by adopting the ULW technique an integrated quantum walk circuit was realized3 and, for the first time, we investigate how the particle statistics, either bosonic or fermionic, influences a two-particle discrete quantum walk. Such experiment has been realized by adopting two-photon entangled states and an array of integrated symmetric directional couplers. The polarization entanglement was exploited to simulate the bunching-antibunching feature of non interacting bosons and fermions. To this scope a novel three-dimensional geometry for the waveguide circuit is introduced, which allows accurate polarization independent behaviour, maintaining a remarkable control on both phase and balancement of the directional couplers.

Single-photon three-qubit quantum logic using spatial light modulators.

Science.gov (United States)

Kagalwala, Kumel H; Di Giuseppe, Giovanni; Abouraddy, Ayman F; Saleh, Bahaa E A

2017-09-29

The information-carrying capacity of a single photon can be vastly expanded by exploiting its multiple degrees of freedom: spatial, temporal, and polarization. Although multiple qubits can be encoded per photon, to date only two-qubit single-photon quantum operations have been realized. Here, we report an experimental demonstration of three-qubit single-photon, linear, deterministic quantum gates that exploit photon polarization and the two-dimensional spatial-parity-symmetry of the transverse single-photon field. These gates are implemented using a polarization-sensitive spatial light modulator that provides a robust, non-interferometric, versatile platform for implementing controlled unitary gates. Polarization here represents the control qubit for either separable or entangling unitary operations on the two spatial-parity target qubits. Such gates help generate maximally entangled three-qubit Greenberger-Horne-Zeilinger and W states, which is confirmed by tomographical reconstruction of single-photon density matrices. This strategy provides access to a wide range of three-qubit states and operations for use in few-qubit quantum information processing protocols.Photons are essential for quantum information processing, but to date only two-qubit single-photon operations have been realized. Here the authors demonstrate experimentally a three-qubit single-photon linear deterministic quantum gate by exploiting polarization along with spatial-parity symmetry.

Sudden transitions and scaling behavior of geometric quantum correlation for two qubits in quantum critical environments at finite temperature

International Nuclear Information System (INIS)

Luo, Da-Wei; Xu, Jing-Bo

2014-01-01

We investigate the phenomenon of sudden transitions in geometric quantum correlation of two qubits in spin chain environments at finite temperature. It is shown that when only one qubit is coupled to the spin environment, the geometric discord exhibits a double sudden transition behavior, which is closely related to the quantum criticality of the spin chain environment. When two qubits are uniformly coupled to a common spin chain environment, the geometric discord is found to display a sudden transition behavior whereby the system transits from pure classical decoherence to pure quantum decoherence. Moreover, an interesting scaling behavior is revealed for the frozen time, and we also present a scheme to prolong the time during which the discord remains constant by applying bang–bang pulses. (paper)

Theory of the Quantum Dot Hybrid Qubit

Science.gov (United States)

Friesen, Mark

2015-03-01

The quantum dot hybrid qubit, formed from three electrons in two quantum dots, combines the desirable features of charge qubits (fast manipulation) and spin qubits (long coherence times). The hybridized spin and charge states yield a unique energy spectrum with several useful properties, including two different operating regimes that are relatively immune to charge noise due to the presence of optimal working points or ``sweet spots.'' In this talk, I will describe dc and ac-driven gate operations of the quantum dot hybrid qubit. I will analyze improvements in the dephasing that are enabled by the sweet spots, and I will discuss the outlook for quantum hybrid qubits in terms of scalability. This work was supported in part by ARO (W911NF-12-0607), NSF (PHY-1104660), the USDOD, and the Intelligence Community Postdoctoral Research Fellowship Program. The views and conclusions contained in this presentation are those of the authors and should not be interpreted as representing the official policies or endorsements, either expressed or implied, of the US government.

Direct method for measuring and witnessing quantum entanglement of arbitrary two-qubit states through Hong-Ou-Mandel interference

Science.gov (United States)

Bartkiewicz, Karol; Chimczak, Grzegorz; Lemr, Karel

2017-02-01

We describe a direct method for experimental determination of the negativity of an arbitrary two-qubit state with 11 measurements performed on multiple copies of the two-qubit system. Our method is based on the experimentally accessible sequences of singlet projections performed on up to four qubit pairs. In particular, our method permits the application of the Peres-Horodecki separability criterion to an arbitrary two-qubit state. We explicitly demonstrate that measuring entanglement in terms of negativity requires three measurements more than detecting two-qubit entanglement. The reported minimal set of interferometric measurements provides a complete description of bipartite quantum entanglement in terms of two-photon interference. This set is smaller than the set of 15 measurements needed to perform a complete quantum state tomography of an arbitrary two-qubit system. Finally, we demonstrate that the set of nine Makhlin's invariants needed to express the negativity can be measured by performing 13 multicopy projections. We demonstrate both that these invariants are a useful theoretical concept for designing specialized quantum interferometers and that their direct measurement within the framework of linear optics does not require performing complete quantum state tomography.

Distinguishing quantum from classical oscillations in a driven phase qubit

International Nuclear Information System (INIS)

Shevchenko, S N; Omelyanchouk, A N; Zagoskin, A M; Savel'ev, S; Nori, Franco

2008-01-01

Rabi oscillations are coherent transitions in a quantum two-level system under the influence of a resonant drive, with a much lower frequency dependent on the perturbation amplitude. These serve as one of the signatures of quantum coherent evolution in mesoscopic systems. It was shown recently (Groenbech-Jensen N and Cirillo M 2005 Phys. Rev. Lett. 95 067001) that in phase qubits (current-biased Josephson junctions) this effect can be mimicked by classical oscillations arising due to the anharmonicity of the effective potential. Nevertheless, we find qualitative differences between the classical and quantum effects. Firstly, while the quantum Rabi oscillations can be produced by the subharmonics of the resonant frequency ω 10 (multiphoton processes), the classical effect also exists when the system is excited at the overtones, nω 10 . Secondly, the shape of the resonance is, in the classical case, characteristically asymmetric, whereas quantum resonances are described by symmetric Lorentzians. Thirdly, the anharmonicity of the potential results in the negative shift of the resonant frequency in the classical case, in contrast to the positive Bloch-Siegert shift in the quantum case. We show that in the relevant range of parameters these features allow us to distinguish confidently the bona fide Rabi oscillations from their classical Doppelgaenger

Quantum discord dynamics of two qubits in single-mode cavities

International Nuclear Information System (INIS)

Wang Chen; Chen Qing-Hu

2013-01-01

The dynamics of quantum discord for two identical qubits in two independent single-mode cavities and a common single-mode cavity are discussed. For the initial Bell state with correlated spins, while the entanglement sudden death can occur, the quantum discord vanishes only at discrete moments in the independent cavities and never vanishes in the common cavity. Interestingly, quantum discord and entanglement show opposite behavior in the common cavity, unlike in the independent cavities. For the initial Bell state with anti-correlated spins, quantum discord and entanglement behave in the same way for both independent cavities and a common cavity. It is found that the detunings always stabilize the quantum discord. (general)

Reduced phase error through optimized control of a superconducting qubit

International Nuclear Information System (INIS)

Lucero, Erik; Kelly, Julian; Bialczak, Radoslaw C.; Lenander, Mike; Mariantoni, Matteo; Neeley, Matthew; O'Connell, A. D.; Sank, Daniel; Wang, H.; Weides, Martin; Wenner, James; Cleland, A. N.; Martinis, John M.; Yamamoto, Tsuyoshi

2010-01-01

Minimizing phase and other errors in experimental quantum gates allows higher fidelity quantum processing. To quantify and correct for phase errors, in particular, we have developed an experimental metrology - amplified phase error (APE) pulses - that amplifies and helps identify phase errors in general multilevel qubit architectures. In order to correct for both phase and amplitude errors specific to virtual transitions and leakage outside of the qubit manifold, we implement 'half derivative', an experimental simplification of derivative reduction by adiabatic gate (DRAG) control theory. The phase errors are lowered by about a factor of five using this method to ∼1.6 deg. per gate, and can be tuned to zero. Leakage outside the qubit manifold, to the qubit |2> state, is also reduced to ∼10 -4 for 20% faster gates.

Multiqubit quantum phase gate using four-level superconducting quantum interference devices coupled to superconducting resonator

Energy Technology Data Exchange (ETDEWEB)

Waseem, Muhammad; Irfan, Muhammad [Department of Physics and Applied Mathematics, Pakistan Institute of Engineering and Applied Sciences, Nilore, Islamabad 45650 (Pakistan); Qamar, Shahid, E-mail: shahid_qamar@pieas.edu.pk [Department of Physics and Applied Mathematics, Pakistan Institute of Engineering and Applied Sciences, Nilore, Islamabad 45650 (Pakistan)

2012-07-15

In this paper, we propose a scheme to realize three-qubit quantum phase gate of one qubit simultaneously controlling two target qubits using four-level superconducting quantum interference devices (SQUIDs) coupled to a superconducting resonator. The two lowest levels Divides 0 Right-Pointing-Angle-Bracket and Divides 1 Right-Pointing-Angle-Bracket of each SQUID are used to represent logical states while the higher energy levels Divides 2 Right-Pointing-Angle-Bracket and Divides 3 Right-Pointing-Angle-Bracket are utilized for gate realization. Our scheme does not require adiabatic passage, second order detuning, and the adjustment of the level spacing during gate operation which reduce the gate time significantly. The scheme is generalized for an arbitrary n-qubit quantum phase gate. We also apply the scheme to implement three-qubit quantum Fourier transform.

Quantum Privacy Amplification for a Sequence of Single Qubits

International Nuclear Information System (INIS)

Deng Fuguo; Long Guilu

2006-01-01

We present a scheme for quantum privacy amplification (QPA) for a sequence of single qubits. The QPA procedure uses a unitary operation with two controlled-not gates and a Hadamard gate. Every two qubits are performed with the unitary gate operation, and a measurement is made on one photon and the other one is retained. The retained qubit carries the state information of the discarded one. In this way, the information leakage is reduced. The procedure can be performed repeatedly so that the information leakage is reduced to any arbitrarily low level. With this QPA scheme, the quantum secure direct communication with single qubits can be implemented with arbitrarily high security. We also exploit this scheme to do privacy amplification on the single qubits in quantum information sharing for long-distance communication with quantum repeaters.

Controlled Quantum Operations of a Semiconductor Three-Qubit System

Science.gov (United States)

Li, Hai-Ou; Cao, Gang; Yu, Guo-Dong; Xiao, Ming; Guo, Guang-Can; Jiang, Hong-Wen; Guo, Guo-Ping

2018-02-01

In a specially designed semiconductor device consisting of three capacitively coupled double quantum dots, we achieve strong and tunable coupling between a target qubit and two control qubits. We demonstrate how to completely switch on and off the target qubit's coherent rotations by presetting two control qubits' states. A Toffoli gate is, therefore, possible based on these control effects. This research paves a way for realizing full quantum-logic operations in semiconductor multiqubit systems.

Electrically protected resonant exchange qubits in triple quantum dots.

Science.gov (United States)

Taylor, J M; Srinivasa, V; Medford, J

2013-08-02

We present a modulated microwave approach for quantum computing with qubits comprising three spins in a triple quantum dot. This approach includes single- and two-qubit gates that are protected against low-frequency electrical noise, due to an operating point with a narrowband response to high frequency electric fields. Furthermore, existing double quantum dot advances, including robust preparation and measurement via spin-to-charge conversion, are immediately applicable to the new qubit. Finally, the electric dipole terms implicit in the high frequency coupling enable strong coupling with superconducting microwave resonators, leading to more robust two-qubit gates.

Quantum control of finite-time disentanglement in qubit-qubit and qubit-qutrit systems

Energy Technology Data Exchange (ETDEWEB)

Ali, Mazhar

2009-07-13

This thesis is a theoretical study of entanglement dynamics and its control of qubit-qubit and qubit-qutrit systems. In particular, we focus on the decay of entanglement of quantum states interacting with dissipative environments. Qubit-qubit entanglement may vanish suddenly while interacting with statistically independent vacuum reservoirs. Such finite- time disentanglement is called sudden death of entanglement (ESD). We investigate entanglement sudden death of qubit-qubit and qubit-qutrit systems interacting with statistically independent reservoirs at zero- and finite-temperature. It is shown that for zero-temperature reservoirs, some entangled states exhibit sudden death while others lose their entanglement only after infinite time. Thus, there are two possible routes of entanglement decay, namely sudden death and asymptotic decay. We demonstrate that starting with an initial condition which leads to finite-time disentanglement, we can alter the future course of entanglement by local unitary actions. In other words, it is possible to put the quantum states on other track of decay once they are on a particular route of decay. We show that one can accelerate or delay sudden death. However, there is a critical time such that if local actions are taken before that critical time then sudden death can be delayed to infinity. Any local unitary action taken after that critical time can only accelerate or delay sudden death. In finite-temperature reservoirs, we demonstrate that a whole class of entangled states exhibit sudden death. This conclusion is valid if at least one of the reservoirs is at finite-temperature. However, we show that we can still hasten or delay sudden death by local unitary transformations up to some finite time. We also study sudden death for qubit-qutrit systems. Similar to qubit-qubit systems, some states exhibit sudden death while others do not. However, the process of disentanglement can be effected due to existence of quantum interference

Quantum control of finite-time disentanglement in qubit-qubit and qubit-qutrit systems

International Nuclear Information System (INIS)

Ali, Mazhar

2009-01-01

This thesis is a theoretical study of entanglement dynamics and its control of qubit-qubit and qubit-qutrit systems. In particular, we focus on the decay of entanglement of quantum states interacting with dissipative environments. Qubit-qubit entanglement may vanish suddenly while interacting with statistically independent vacuum reservoirs. Such finite- time disentanglement is called sudden death of entanglement (ESD). We investigate entanglement sudden death of qubit-qubit and qubit-qutrit systems interacting with statistically independent reservoirs at zero- and finite-temperature. It is shown that for zero-temperature reservoirs, some entangled states exhibit sudden death while others lose their entanglement only after infinite time. Thus, there are two possible routes of entanglement decay, namely sudden death and asymptotic decay. We demonstrate that starting with an initial condition which leads to finite-time disentanglement, we can alter the future course of entanglement by local unitary actions. In other words, it is possible to put the quantum states on other track of decay once they are on a particular route of decay. We show that one can accelerate or delay sudden death. However, there is a critical time such that if local actions are taken before that critical time then sudden death can be delayed to infinity. Any local unitary action taken after that critical time can only accelerate or delay sudden death. In finite-temperature reservoirs, we demonstrate that a whole class of entangled states exhibit sudden death. This conclusion is valid if at least one of the reservoirs is at finite-temperature. However, we show that we can still hasten or delay sudden death by local unitary transformations up to some finite time. We also study sudden death for qubit-qutrit systems. Similar to qubit-qubit systems, some states exhibit sudden death while others do not. However, the process of disentanglement can be effected due to existence of quantum interference

Experimental investigation of a four-qubit linear-optical quantum logic circuit.

Science.gov (United States)

Stárek, R; Mičuda, M; Miková, M; Straka, I; Dušek, M; Ježek, M; Fiurášek, J

2016-09-20

We experimentally demonstrate and characterize a four-qubit linear-optical quantum logic circuit. Our robust and versatile scheme exploits encoding of two qubits into polarization and path degrees of single photons and involves two crossed inherently stable interferometers. This approach allows us to design a complex quantum logic circuit that combines a genuine four-qubit C(3)Z gate and several two-qubit and single-qubit gates. The C(3)Z gate introduces a sign flip if and only if all four qubits are in the computational state |1〉. We verify high-fidelity performance of this central four-qubit gate using Hofmann bounds on quantum gate fidelity and Monte Carlo fidelity sampling. We also experimentally demonstrate that the quantum logic circuit can generate genuine multipartite entanglement and we certify the entanglement with the use of suitably tailored entanglement witnesses.

High-fidelity gates in quantum dot spin qubits.

Science.gov (United States)

Koh, Teck Seng; Coppersmith, S N; Friesen, Mark

2013-12-03

Several logical qubits and quantum gates have been proposed for semiconductor quantum dots controlled by voltages applied to top gates. The different schemes can be difficult to compare meaningfully. Here we develop a theoretical framework to evaluate disparate qubit-gating schemes on an equal footing. We apply the procedure to two types of double-dot qubits: the singlet-triplet and the semiconducting quantum dot hybrid qubit. We investigate three quantum gates that flip the qubit state: a DC pulsed gate, an AC gate based on logical qubit resonance, and a gate-like process known as stimulated Raman adiabatic passage. These gates are all mediated by an exchange interaction that is controlled experimentally using the interdot tunnel coupling g and the detuning [Symbol: see text], which sets the energy difference between the dots. Our procedure has two steps. First, we optimize the gate fidelity (f) for fixed g as a function of the other control parameters; this yields an f(opt)(g) that is universal for different types of gates. Next, we identify physical constraints on the control parameters; this yields an upper bound f(max) that is specific to the qubit-gate combination. We show that similar gate fidelities (~99:5%) should be attainable for singlet-triplet qubits in isotopically purified Si, and for hybrid qubits in natural Si. Considerably lower fidelities are obtained for GaAs devices, due to the fluctuating magnetic fields ΔB produced by nuclear spins.

How to implement a quantum algorithm on a large number of qubits by controlling one central qubit

Science.gov (United States)

Zagoskin, Alexander; Ashhab, Sahel; Johansson, J. R.; Nori, Franco

2010-03-01

It is desirable to minimize the number of control parameters needed to perform a quantum algorithm. We show that, under certain conditions, an entire quantum algorithm can be efficiently implemented by controlling a single central qubit in a quantum computer. We also show that the different system parameters do not need to be designed accurately during fabrication. They can be determined through the response of the central qubit to external driving. Our proposal is well suited for hybrid architectures that combine microscopic and macroscopic qubits. More details can be found in: A.M. Zagoskin, S. Ashhab, J.R. Johansson, F. Nori, Quantum two-level systems in Josephson junctions as naturally formed qubits, Phys. Rev. Lett. 97, 077001 (2006); and S. Ashhab, J.R. Johansson, F. Nori, Rabi oscillations in a qubit coupled to a quantum two-level system, New J. Phys. 8, 103 (2006).

Speed of quantum evolution of entangled two qubits states: Local vs. global evolution

International Nuclear Information System (INIS)

Curilef, S; Zander, C; Plastino, A R

2008-01-01

There is a lower bound for the 'speed' of quantum evolution as measured by the time needed to reach an orthogonal state. We show that, for two-qubits systems, states saturating the quantum speed limit tend to exhibit a small amount of local evolution, as measured by the fidelity between the initial and final single qubit states after the time τ required by the composite system to reach an orthogonal state. Consequently, a trade-off between the speed of global evolution and the amount of local evolution seems to be at work.

One-way quantum computation via manipulation of polarization and momentum qubits in two-photon cluster states

International Nuclear Information System (INIS)

Vallone, G; Pomarico, E; De Martini, F; Mataloni, P

2008-01-01

Four-qubit cluster states of two photons entangled in polarization and linear momentum have been used to realize a complete set of single qubit rotations and the C-NOT gate for equatorial qubits with high values of fidelity. By the computational equivalence of the two degrees of freedom our result demonstrate the suitability of two photon cluster states for rapid and efficient one-way quantum computing

Quantum measurement of a rapidly rotating spin qubit in diamond.

Science.gov (United States)

Wood, Alexander A; Lilette, Emmanuel; Fein, Yaakov Y; Tomek, Nikolas; McGuinness, Liam P; Hollenberg, Lloyd C L; Scholten, Robert E; Martin, Andy M

2018-05-01

A controlled qubit in a rotating frame opens new opportunities to probe fundamental quantum physics, such as geometric phases in physically rotating frames, and can potentially enhance detection of magnetic fields. Realizing a single qubit that can be measured and controlled during physical rotation is experimentally challenging. We demonstrate quantum control of a single nitrogen-vacancy (NV) center within a diamond rotated at 200,000 rpm, a rotational period comparable to the NV spin coherence time T 2 . We stroboscopically image individual NV centers that execute rapid circular motion in addition to rotation and demonstrate preparation, control, and readout of the qubit quantum state with lasers and microwaves. Using spin-echo interferometry of the rotating qubit, we are able to detect modulation of the NV Zeeman shift arising from the rotating NV axis and an external DC magnetic field. Our work establishes single NV qubits in diamond as quantum sensors in the physically rotating frame and paves the way for the realization of single-qubit diamond-based rotation sensors.

The quantum dynamics of two qubits inside two distant microcavities connected via a single-mode optical fiber

International Nuclear Information System (INIS)

Nguyen, Van Hieu; Nguyen, Bich Ha; Duong, Hai Trieu

2010-01-01

For application to studying the transmission of quantum information, also called quantum communication, between two identical qubits placed inside two identical single-mode microcavities connected via a single-mode optical fiber, the time evolution of this system is investigated. In the Markovian approximation, the von Neumann equation for its reduced density matrix contains a completely positive linear operator called the Liouvillian operator describing the decoherence of this system due to its interaction with the environment. By using the Linblad formula for the Liouvillian operator, a system of rate equations can be derived. In the special case of resonance between the energy difference of two states in each qubit and the energy of the fiber mode, the rate equations for the system excited up to the first level are solved in first order approximation with respect to the decoherence constants. It is shown that when there is no decoherence, the perfect quantum state transmission between two qubits can take place if the physical parameters of the system satisfy definite conditions. A possible extension to studying the system excited to high energy states is also discussed

Implementation of a two-qubit controlled-rotation gate based on unconventional geometric phase with a constant gating time

International Nuclear Information System (INIS)

Yabu-uti, B.F.C.; Roversi, J.A.

2011-01-01

We propose an alternative scheme to implement a two-qubit controlled-R (rotation) gate in the hybrid atom-CCA (coupled cavities array) system. Our scheme results in a constant gating time and, with an adjustable qubit-bus coupling (atom-resonator), one can specify a particular rotation R on the target qubit. We believe that this proposal may open promising perspectives for networking quantum information processors and implementing distributed and scalable quantum computation. -- Highlights: → We propose an alternative two-qubit controlled-rotation gate implementation. → Our gate is realized in a constant gating time for any rotation. → A particular rotation on the target qubit can be specified by an adjustable qubit-bus coupling. → Our proposal may open promising perspectives for implementing distributed and scalable quantum computation.

Preservation of Quantum Fisher Information and Geometric Phase of a Single Qubit System in a Dissipative Reservoir Through the Addition of Qubits

Science.gov (United States)

Guo, Y. N.; Tian, Q. L.; Mo, Y. F.; Zhang, G. L.; Zeng, K.

2018-04-01

In this paper, we have investigated the preservation of quantum Fisher information (QFI) of a single-qubit system coupled to a common zero temperature reservoir through the addition of noninteracting qubits. The results show that, the QFI is completely protected in both Markovian and non-Markovian regimes by increasing the number of additional qubits. Besides, the phenomena of QFI display monotonic decay or non-monotonic with revival oscillations depending on the number of additional qubits N - 1 in a common dissipative reservoir. If N revival oscillations. Moreover, we extend this model to investigate the effect of additional qubits and the initial conditions of the system on the geometric phase (GP). It is found that, the robustness of GP against the dissipative reservoir has been demonstrated by increasing gradually the number of additional qubits N - 1. Besides, the GP is sensitive to the initial parameter 𝜃, and possesses symmetric in a range regime [0,2 π].

Note on the quantum correlations of two qubits coupled to photon baths

International Nuclear Information System (INIS)

Quintana, Claudia; Rosas-Ortiz, Oscar

2015-01-01

The time-evolution of the quantum correlations between two qubits that are coupled to a pair of photon baths is studied. We show that conditioned transitions occurring in the entire system have influence on the time-evolution of the subsystems. Then, we show that the study of the population inversion of each of the qubits is a measure of the correlations between them that is in agreement with the notion of concurrence. (paper)

Assessment of a quantum phase-gate operation based on nonlinear optics

International Nuclear Information System (INIS)

Rebic, S.; Ottaviani, C.; Di Giuseppe, G.; Vitali, D.; Tombesi, P.

2006-01-01

We analyze in detail the proposal for a two-qubit gate for travelling single-photon qubits recently presented by Ottaviani et al. [Phys. Rev. A 73, 010301(R) (2006)]. The scheme is based on an ensemble of five-level atoms coupled to two quantum and two classical light fields. The two quantum fields undergo cross-phase modulation induced by electromagnetically induced transparency. The performance of this two-qubit quantum phase gate for travelling single-photon qubits is thoroughly examined in the steady-state and transient regimes, by means of a full quantum treatment of the system dynamics. In the steady-state regime, we find a general trade-off between the size of the conditional phase shift and the fidelity of the gate operation. However, this trade-off can be bypassed in the transient regime, where a satisfactory gate operation is found to be possible, significantly reducing the gate operation time

Metric Structure of the Space of Two-Qubit Gates, Perfect Entanglers and Quantum Control

Directory of Open Access Journals (Sweden)

Paul Watts

2013-05-01

Full Text Available We derive expressions for the invariant length element and measure for the simple compact Lie group SU(4 in a coordinate system particularly suitable for treating entanglement in quantum information processing. Using this metric, we compute the invariant volume of the space of two-qubit perfect entanglers. We find that this volume corresponds to more than 84% of the total invariant volume of the space of two-qubit gates. This same metric is also used to determine the effective target sizes that selected gates will present in any quantum-control procedure designed to implement them.

Superconducting Qubits as Mechanical Quantum Engines.

Science.gov (United States)

Sachtleben, Kewin; Mazon, Kahio T; Rego, Luis G C

2017-09-01

We propose the equivalence of superconducting qubits with a pistonlike mechanical quantum engine. The work reports a study on the nature of the nonequilibrium work exchanged with the quantum-nonadiabatic working medium, which is modeled as a multilevel coupled quantum well system subject to an external control parameter. The quantum dynamics is solved for arbitrary control protocols. It is shown that the work output has two components: one that depends instantaneously on the level populations and another that is due to the quantum coherences built in the system. The nonadiabatic coherent dynamics of the quantum engine gives rise to a resistance (friction) force that decreases the work output. We consider the functional equivalence of such a device and a rf-SQUID flux qubit.

The Quantum Socket: Wiring for Superconducting Qubits - Part 2

Science.gov (United States)

Bejanin, J. H.; McConkey, T. G.; Rinehart, J. R.; Bateman, J. D.; Earnest, C. T.; McRae, C. H.; Rohanizadegan, Y.; Shiri, D.; Mariantoni, M.; Penava, B.; Breul, P.; Royak, S.; Zapatka, M.; Fowler, A. G.

Quantum computing research has reached a level of maturity where quantum error correction (QEC) codes can be executed on linear arrays of superconducting quantum bits (qubits). A truly scalable quantum computing architecture, however, based on practical QEC algorithms, requires nearest neighbor interaction between qubits on a two-dimensional array. Such an arrangement is not possible with techniques that rely on wire bonding. To address this issue, we have developed the quantum socket, a device based on three-dimensional wires that enables the control of superconducting qubits on a two-dimensional grid. In this talk, we present experimental results characterizing this type of wiring. We will show that the quantum socket performs exceptionally well for the transmission and reflection of microwave signals up to 10 GHz, while minimizing crosstalk between adjacent wires. Under realistic conditions, we measured an S21 of -5 dB at 6 GHz and an average crosstalk of -60 dB. We also describe time domain reflectometry results and arbitrary pulse transmission tests, showing that the quantum socket can be used to control superconducting qubits.

Optimizing qubit resources for quantum chemistry simulations in second quantization on a quantum computer

International Nuclear Information System (INIS)

Moll, Nikolaj; Fuhrer, Andreas; Staar, Peter; Tavernelli, Ivano

2016-01-01

Quantum chemistry simulations on a quantum computer suffer from the overhead needed for encoding the Fermionic problem in a system of qubits. By exploiting the block diagonality of a Fermionic Hamiltonian, we show that the number of required qubits can be reduced while the number of terms in the Hamiltonian will increase. All operations for this reduction can be performed in operator space. The scheme is conceived as a pre-computational step that would be performed prior to the actual quantum simulation. We apply this scheme to reduce the number of qubits necessary to simulate both the Hamiltonian of the two-site Fermi–Hubbard model and the hydrogen molecule. Both quantum systems can then be simulated with a two-qubit quantum computer. Despite the increase in the number of Hamiltonian terms, the scheme still remains a useful tool to reduce the dimensionality of specific quantum systems for quantum simulators with a limited number of resources. (paper)

Entanglement-assisted quantum parameter estimation from a noisy qubit pair: A Fisher information analysis

Energy Technology Data Exchange (ETDEWEB)

Chapeau-Blondeau, François, E-mail: chapeau@univ-angers.fr

2017-04-25

Benefit from entanglement in quantum parameter estimation in the presence of noise or decoherence is investigated, with the quantum Fisher information to asses the performance. When an input probe experiences any (noisy) transformation introducing the parameter dependence, the performance is always maximized by a pure probe. As a generic estimation task, for estimating the phase of a unitary transformation on a qubit affected by depolarizing noise, the optimal separable probe and its performance are characterized as a function of the level of noise. By entangling qubits in pairs, enhancements of performance over that of the optimal separable probe are quantified, in various settings of the entangled pair. In particular, in the presence of the noise, enhancement over the performance of the one-qubit optimal probe can always be obtained with a second entangled qubit although never interacting with the process to be estimated. Also, enhancement over the performance of the two-qubit optimal separable probe can always be achieved by a two-qubit entangled probe, either partially or maximally entangled depending on the level of the depolarizing noise. - Highlights: • Quantum parameter estimation from a noisy qubit pair is investigated. • The quantum Fisher information is used to assess the ultimate best performance. • Theoretical expressions are established and analyzed for the Fisher information. • Enhanced performances are quantified with various entanglements of the pair. • Enhancement is shown even with one entangled qubit noninteracting with the process.

Adaptive homodyne phase discrimination and qubit measurement

International Nuclear Information System (INIS)

Sarovar, Mohan; Whaley, K. Birgitta

2007-01-01

Fast and accurate measurement is a highly desirable, if not vital, feature of quantum computing architectures. In this work we investigate the usefulness of adaptive measurements in improving the speed and accuracy of qubit measurement. We examine a particular class of quantum computing architectures, ones based on qubits coupled to well-controlled harmonic oscillator modes (reminiscent of cavity QED), where adaptive schemes for measurement are particularly appropriate. In such architectures, qubit measurement is equivalent to phase discrimination for a mode of the electromagnetic field, and we examine adaptive techniques for doing this. In the final section we present a concrete example of applying adaptive measurement to the particularly well-developed circuit-QED architecture

Scheme for implementing N-qubit controlled phase gate of photons assisted by quantum-dot-microcavity coupled system: optimal probability of success

International Nuclear Information System (INIS)

Cui, Wen-Xue; Hu, Shi; Wang, Hong-Fu; Zhu, Ai-Dong; Zhang, Shou

2015-01-01

The direct implementation of multiqubit controlled phase gate of photons is appealing and important for reducing the complexity of the physical realization of linear-optics-based practical quantum computer and quantum algorithms. In this letter we propose a nondestructive scheme for implementing an N-qubit controlled phase gate of photons with a high success probability. The gate can be directly implemented with the self-designed quantum encoder circuits, which are probabilistic optical quantum entangler devices and can be achieved using linear optical elements, single-photon superposition state, and quantum dot coupled to optical microcavity. The calculated results indicate that both the success probabilities of the quantum encoder circuit and the N-qubit controlled phase gate in our scheme are higher than those in the previous schemes. We also consider the effects of the side leakage and cavity loss on the success probability and the fidelity of the quantum encoder circuit for a realistic quantum-dot-microcavity coupled system. (letter)

Measurement and Quantum State Transfer in Superconducting Qubits

Science.gov (United States)

Mlinar, Eric

The potential of superconducting qubits as the medium for a scalable quantum computer has motivated the pursuit of improved interactions within this system. Two challenges for the field of superconducting qubits are measurement fidelity, to accurately determine the state of the qubit, and the efficient transfer of quantum states. In measurement, the current state-of-the-art method employs dispersive readout, by coupling the qubit to a cavity and reading the resulting shift in cavity frequency to infer the qubit's state; however, this is vulnerable to Purcell relaxation, as well as being modeled off a simplified two-level abstraction of the qubit. In state transfer, the existing proposal for moving quantum states is mostly untested against non-idealities that will likely be present in an experiment. In this dissertation, we examine three problems within these two areas. We first describe a new scheme for fast and high-fidelity dispersive measurement specifically designed to circumvent the Purcell Effect. To do this, the qubit-resonator interaction is turned on only when the resonator is decoupled from the environment; then, after the resonator state has shifted enough to infer the qubit state, the qubit-resonator interaction is turned off before the resonator and environment are recoupled. We also show that the effectiveness of this "Catch-Disperse-Release'' procedure partly originates from quadrature squeezing of the resonator state induced by the Jaynes-Cummings nonlinearity. The Catch-Disperse-Release measurement scheme treats the qubit as a two-level system, which is a common simplification used in theoretical works. However, the most promising physical candidate for a superconducting qubit, the transmon, is a multi-level system. In the second work, we examine the effects of including the higher energy levels of the transmon. Specifically, we expand the eigenstate picture developed in the first work to encompass multiple qubit levels, and examine the resulting

Geometric manipulation of the quantum states of two-level atoms

International Nuclear Information System (INIS)

Tian, Mingzhen; Barber, Zeb W.; Fischer, Joe A.; Babbitt, Wm. Randall

2004-01-01

Manipulation of the quantum states of two-level atoms has been investigated using laser-controlled geometric phase change, which has the potential to build robust quantum logic gates for quantum computing. For a qubit based on two electronic transition levels of an atom, two basic quantum operations that can make any universal single qubit gate have been designed employing resonant laser pulses. An operation equivalent to a phase gate has been demonstrated using Tm 3+ doped in a yttrium aluminum garnet crystal

Two-axis control of a coupled quantum dot - donor qubit in Si-MOS

Science.gov (United States)

Rudolph, Martin; Harvey-Collard, Patrick; Jacobson, Tobias; Wendt, Joel; Pluym, Tammy; Dominguez, Jason; Ten-Eyck, Greg; Lilly, Mike; Carroll, Malcolm

Si-MOS based QD qubits are attractive due to their similarity to the current semiconductor industry. We introduce a highly tunable MOS foundry compatible qubit design that couples an electrostatic quantum dot (QD) with an implanted donor. We show for the first time coherent two-axis control of a two-electron spin logical qubit that evolves under the QD-donor exchange interaction and the hyperfine interaction with the donor nucleus. The two interactions are tuned electrically with surface gate voltages to provide control of both qubit axes. Qubit decoherence is influenced by charge noise, which is of similar strength as epitaxial systems like GaAs and Si/SiGe. This work was performed, in part, at the Center for Integrated Nanotechnologies, a U.S. DOE, Office of Basic Energy Sciences user facility. The work was supported by the Sandia National Laboratories Directed Research and Development Program. Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a Lockheed-Martin Company, for the U. S. Department of Energy under Contract No. DE-AC04-94AL85000.

Quantum qubit measurement by a quantum point contact with a quantum Langevin equation approach

International Nuclear Information System (INIS)

Dong, Bing; Lei, X.L.; Horing, N.J.M.; Cui, H.L.

2007-01-01

We employ a microscopic quantum Heisenberg-Langevin equation approach to establish a set of quantum Bloch equations for a two-level system (coupled quantum dots) capacitively coupled to a quantum point contact (QPC). The resulting Bloch equations facilitate our analysis of qubit relaxation and decoherence in coupled quantum dots induced by measurement processes at arbitrary bias-voltage and temperature. We also examine the noise spectrum of the meter output current for a symmetric qubit. These results help resolve a recent debate about a quantum oscillation peak in the noise spectrum. (copyright 2007 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

Quantum logic gates generated by SC-charge qubits coupled to a resonator

International Nuclear Information System (INIS)

Obada, A-S F; Hessian, H A; Mohamed, A-B A; Homid, Ali H

2012-01-01

We propose some quantum logic gates by using SC-charge qubits coupled to a resonator to study two types of quantum operation. By applying a classical magnetic field with the flux, a simple rotation on the target qubit is generated. Single and two-qubit gates of quantum logic gates are realized. Two-qubit joint operations are firstly generated by applying a classical magnetic field with the flux, and secondly by applying a classical magnetic field with the flux when qubits are placed a quarter of the distance along the resonator. A short discussion of fidelity is given to prove the success of the operations in implementing these gates. (paper)

Efficient amplification of photonic qubits by optimal quantum cloning

Czech Academy of Sciences Publication Activity Database

Bartkiewicz, K.; Černoch, A.; Lemr, K.; Soubusta, Jan; Stobińska, M.

2014-01-01

Roč. 89, č. 6 (2014), "062322-1"-"062322-10" ISSN 1050-2947 Institutional support: RVO:68378271 Keywords : optimal quantum cloning * cryptography * qubit * phase-independent quantum amplifier Subject RIV: BH - Optics, Masers, Lasers Impact factor: 2.808, year: 2014

Adiabatic quantum computing with spin qubits hosted by molecules.

Science.gov (United States)

Yamamoto, Satoru; Nakazawa, Shigeaki; Sugisaki, Kenji; Sato, Kazunobu; Toyota, Kazuo; Shiomi, Daisuke; Takui, Takeji

2015-01-28

A molecular spin quantum computer (MSQC) requires electron spin qubits, which pulse-based electron spin/magnetic resonance (ESR/MR) techniques can afford to manipulate for implementing quantum gate operations in open shell molecular entities. Importantly, nuclear spins, which are topologically connected, particularly in organic molecular spin systems, are client qubits, while electron spins play a role of bus qubits. Here, we introduce the implementation for an adiabatic quantum algorithm, suggesting the possible utilization of molecular spins with optimized spin structures for MSQCs. We exemplify the utilization of an adiabatic factorization problem of 21, compared with the corresponding nuclear magnetic resonance (NMR) case. Two molecular spins are selected: one is a molecular spin composed of three exchange-coupled electrons as electron-only qubits and the other an electron-bus qubit with two client nuclear spin qubits. Their electronic spin structures are well characterized in terms of the quantum mechanical behaviour in the spin Hamiltonian. The implementation of adiabatic quantum computing/computation (AQC) has, for the first time, been achieved by establishing ESR/MR pulse sequences for effective spin Hamiltonians in a fully controlled manner of spin manipulation. The conquered pulse sequences have been compared with the NMR experiments and shown much faster CPU times corresponding to the interaction strength between the spins. Significant differences are shown in rotational operations and pulse intervals for ESR/MR operations. As a result, we suggest the advantages and possible utilization of the time-evolution based AQC approach for molecular spin quantum computers and molecular spin quantum simulators underlain by sophisticated ESR/MR pulsed spin technology.

Quantum Discord in Two-Qubit System Constructed from the Yang—Baxter Equation

International Nuclear Information System (INIS)

Gou Li-Dan; Wang Xiao-Qian; Sun Yuan-Yuan; Xu Yu-Mei

2014-01-01

Quantum correlations among parts of a composite quantum system are a fundamental resource for several applications in quantum information. In general, quantum discord can measure quantum correlations. In that way, we investigate the quantum discord of the two-qubit system constructed from the Yang—Baxter Equation. The density matrix of this system is generated through the unitary Yang—Baxter matrix R. The analytical expression and numerical result of quantum discord and geometric measure of quantum discord are obtained for the Yang—Baxter system. These results show that quantum discord and geometric measure of quantum discord are only connect with the parameter θ, which is the important spectral parameter in Yang—Baxter equation. (general)

Fast quantum logic gates with trapped-ion qubits

Science.gov (United States)

Schäfer, V. M.; Ballance, C. J.; Thirumalai, K.; Stephenson, L. J.; Ballance, T. G.; Steane, A. M.; Lucas, D. M.

2018-03-01

Quantum bits (qubits) based on individual trapped atomic ions are a promising technology for building a quantum computer. The elementary operations necessary to do so have been achieved with the required precision for some error-correction schemes. However, the essential two-qubit logic gate that is used to generate quantum entanglement has hitherto always been performed in an adiabatic regime (in which the gate is slow compared with the characteristic motional frequencies of the ions in the trap), resulting in logic speeds of the order of 10 kilohertz. There have been numerous proposals of methods for performing gates faster than this natural ‘speed limit’ of the trap. Here we implement one such method, which uses amplitude-shaped laser pulses to drive the motion of the ions along trajectories designed so that the gate operation is insensitive to the optical phase of the pulses. This enables fast (megahertz-rate) quantum logic that is robust to fluctuations in the optical phase, which would otherwise be an important source of experimental error. We demonstrate entanglement generation for gate times as short as 480 nanoseconds—less than a single oscillation period of an ion in the trap and eight orders of magnitude shorter than the memory coherence time measured in similar calcium-43 hyperfine qubits. The power of the method is most evident at intermediate timescales, at which it yields a gate error more than ten times lower than can be attained using conventional techniques; for example, we achieve a 1.6-microsecond-duration gate with a fidelity of 99.8 per cent. Faster and higher-fidelity gates are possible at the cost of greater laser intensity. The method requires only a single amplitude-shaped pulse and one pair of beams derived from a continuous-wave laser. It offers the prospect of combining the unrivalled coherence properties, operation fidelities and optical connectivity of trapped-ion qubits with the submicrosecond logic speeds that are usually

Hybrid spin and valley quantum computing with singlet-triplet qubits.

Science.gov (United States)

Rohling, Niklas; Russ, Maximilian; Burkard, Guido

2014-10-24

The valley degree of freedom in the electronic band structure of silicon, graphene, and other materials is often considered to be an obstacle for quantum computing (QC) based on electron spins in quantum dots. Here we show that control over the valley state opens new possibilities for quantum information processing. Combining qubits encoded in the singlet-triplet subspace of spin and valley states allows for universal QC using a universal two-qubit gate directly provided by the exchange interaction. We show how spin and valley qubits can be separated in order to allow for single-qubit rotations.

Quantum Gate Operations in Decoherence-Free Subspace with Superconducting Charge Qubits inside a Cavity

International Nuclear Information System (INIS)

Yi-Min, Wang; Yan-Li, Zhou; Lin-Mei, Liang; Cheng-Zu, Li

2009-01-01

We propose a feasible scheme to achieve universal quantum gate operations in decoherence-free subspace with superconducting charge qubits placed in a microwave cavity. Single-logic-qubit gates can be realized with cavity assisted interaction, which possesses the advantages of unconventional geometric gate operation. The two-logic-qubit controlled-phase gate between subsystems can be constructed with the help of a variable electrostatic transformer. The collective decoherence can be successfully avoided in our well-designed system. Moreover, GHZ state for logical qubits can also be easily produced in this system

Experimental quantum multimeter and one-qubit fingerprinting

International Nuclear Information System (INIS)

Du Jiangfeng; Zou Ping; Peng Xinhua; Oi, Daniel K. L.; Ekert, Artur; Kwek, L. C.; Oh, C. H.

2006-01-01

There has been much recent effort to realize quantum devices in many different physical systems. Among them, nuclear magnetic resonance (NMR) has been the first to demonstrate nontrivial quantum algorithms with small numbers of qubits and hence is a prototype for the key ingredients needed to build quantum computers. An important building block in many quantum applications is the scattering circuit, which can be used as a quantum multimeter to perform various quantum information processing tasks directly without recourse to quantum tomography. We implement in NMR a three-qubit version of the multimeter and also demonstrate a single-qubit fingerprinting

A CMOS silicon spin qubit

Science.gov (United States)

Maurand, R.; Jehl, X.; Kotekar-Patil, D.; Corna, A.; Bohuslavskyi, H.; Laviéville, R.; Hutin, L.; Barraud, S.; Vinet, M.; Sanquer, M.; de Franceschi, S.

2016-11-01

Silicon, the main constituent of microprocessor chips, is emerging as a promising material for the realization of future quantum processors. Leveraging its well-established complementary metal-oxide-semiconductor (CMOS) technology would be a clear asset to the development of scalable quantum computing architectures and to their co-integration with classical control hardware. Here we report a silicon quantum bit (qubit) device made with an industry-standard fabrication process. The device consists of a two-gate, p-type transistor with an undoped channel. At low temperature, the first gate defines a quantum dot encoding a hole spin qubit, the second one a quantum dot used for the qubit read-out. All electrical, two-axis control of the spin qubit is achieved by applying a phase-tunable microwave modulation to the first gate. The demonstrated qubit functionality in a basic transistor-like device constitutes a promising step towards the elaboration of scalable spin qubit geometries in a readily exploitable CMOS platform.

Classical-processing and quantum-processing signal separation methods for qubit uncoupling

Science.gov (United States)

Deville, Yannick; Deville, Alain

2012-12-01

The Blind Source Separation problem consists in estimating a set of unknown source signals from their measured combinations. It was only investigated in a non-quantum framework up to now. We propose its first quantum extensions. We thus introduce the Quantum Source Separation field, investigating both its blind and non-blind configurations. More precisely, we show how to retrieve individual quantum bits (qubits) only from the global state resulting from their undesired coupling. We consider cylindrical-symmetry Heisenberg coupling, which e.g. occurs when two electron spins interact through exchange. We first propose several qubit uncoupling methods which typically measure repeatedly the coupled quantum states resulting from individual qubits preparations, and which then statistically process the classical data provided by these measurements. Numerical tests prove the effectiveness of these methods. We then derive a combination of quantum gates for performing qubit uncoupling, thus avoiding repeated qubit preparations and irreversible measurements.

Quantum teleportation via a two-qubit Heisenberg XY chain-effects of anisotropy and magnetic field

Energy Technology Data Exchange (ETDEWEB)

Yeo Ye [Centre for Mathematical Sciences, Wilberforce Road, Cambridge CB3 0WB (United Kingdom); Liu Tongqi [Department of Engineering, Trumpington Street, Cambridge CB3 1PZ (United Kingdom); Lu Yuen [Computer Laboratory, William Gates Building, 15 J J Thomson Avenue, Cambridge CB3 0FD (United Kingdom); Yang Qizhong [Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE (United Kingdom)

2005-04-08

In this paper we study the influence of anisotropy on the usefulness of the entanglement in a two-qubit Heisenberg XY chain at thermal equilibrium in the presence of an external magnetic field, as a resource for quantum teleportation via the standard teleportation protocol. We show that the nonzero thermal entanglement produced by adjusting the external magnetic field beyond some critical strength is a useful resource. We also consider entanglement teleportation via two two-qubit Heisenberg XY chains.

Quantum teleportation via a two-qubit Heisenberg XY chain-effects of anisotropy and magnetic field

International Nuclear Information System (INIS)

Yeo Ye; Liu Tongqi; Lu Yuen; Yang Qizhong

2005-01-01

In this paper we study the influence of anisotropy on the usefulness of the entanglement in a two-qubit Heisenberg XY chain at thermal equilibrium in the presence of an external magnetic field, as a resource for quantum teleportation via the standard teleportation protocol. We show that the nonzero thermal entanglement produced by adjusting the external magnetic field beyond some critical strength is a useful resource. We also consider entanglement teleportation via two two-qubit Heisenberg XY chains

Entanglement routers via a wireless quantum network based on arbitrary two qubit systems

International Nuclear Information System (INIS)

Metwally, N

2014-01-01

A wireless quantum network is generated between multi-hops, where each hop consists of two entangled nodes. These nodes share a finite number of entangled two-qubit systems randomly. Different types of wireless quantum bridges (WQBS) are generated between the non-connected nodes. The efficiency of these WQBS to be used as quantum channels between its terminals to perform quantum teleportation is investigated. We suggest a theoretical wireless quantum communication protocol to teleport unknown quantum signals from one node to another, where the more powerful WQBS are used as quantum channels. It is shown that, by increasing the efficiency of the sources that emit the initial partial entangled states, one can increase the efficiency of the wireless quantum communication protocol. (paper)

PREFACE: Nobel Symposium 141: Qubits for Future Quantum Information Nobel Symposium 141: Qubits for Future Quantum Information

Science.gov (United States)

Claeson, Tord; Delsing, Per; Wendin, Göran

2009-12-01

Quantum mechanics is the most ground-breaking and fascinating theoretical concept developed in physics during the past century. Much of our present understanding of the microscopic world and its extension into the macroscopic world, including modern technical applications, is based upon quantum mechanics. We have experienced a remarkable development of information and communication technology during the past two decades, to a large extent depending upon successful fabrication of smaller and smaller components and circuits. However, we are finally approaching the physical limits of component miniaturization as we enter a microscopic world ruled by quantum mechanics. Present technology is mainly based upon classical physics such as mechanics and electromagnetism. We now face a similar paradigm shift as was experienced two hundred years ago, at the time of the industrial revolution. Engineered construction of systems is currently increasingly based on quantum physics instead of classical physics, and quantum information is replacing much of classical communication. Quantum computing is one of the most exciting sub-fields of this revolution. Individual quantum systems can be used to store and process information. They are called quantum bits, or qubits for short. A quantum computer could eventually be constructed by combining a number of qubits that act coherently. Important computations can be performed much more quickly than by classical computers. However, while we control and measure a qubit, it must be sufficiently isolated from its environment to avoid noise that causes decoherence at the same time. Currently, low temperature is generally needed to obtain sufficiently long decoherence times. Single qubits of many different kinds can be built and manipulated; some research groups have managed to successfully couple qubits and perform rudimentary logic operations. However, the fundamental problems, such as decoherence, entanglement, quantum measurements and error

Coupled Qubits for Next Generation Quantum Annealing: Improving Coherence

Science.gov (United States)

Weber, Steven; Samach, Gabriel; Hover, David; Rosenberg, Danna; Yoder, Jonilyn; Kim, David K.; Kerman, Andrew; Oliver, William D.

Quantum annealing is an optimization technique which potentially leverages quantum tunneling to enhance computational performance. Existing quantum annealers use superconducting flux qubits with short coherence times, limited primarily by the use of large persistent currents. Here, we examine an alternative approach, using flux qubits with smaller persistent currents and longer coherence times. We demonstrate tunable coupling, a basic building-block for quantum annealing, between two such qubits. Furthermore, we characterize qubit coherence as a function of coupler setting and investigate the effect of flux noise in the coupler loop on qubit coherence. Our results provide insight into the available design space for next-generation quantum annealers with improved coherence. This research was funded by the Office of the Director of National Intelligence (ODNI), Intelligence Advanced Research Projects Activity (IARPA) and by the Assistant Secretary of Defense for Research & Engineering under Air Force Contract No. FA8721-05-C-0002. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of ODNI, IARPA, or the US Government.

Scalable quantum computing based on stationary spin qubits in coupled quantum dots inside double-sided optical microcavities.

Science.gov (United States)

Wei, Hai-Rui; Deng, Fu-Guo

2014-12-18

Quantum logic gates are the key elements in quantum computing. Here we investigate the possibility of achieving a scalable and compact quantum computing based on stationary electron-spin qubits, by using the giant optical circular birefringence induced by quantum-dot spins in double-sided optical microcavities as a result of cavity quantum electrodynamics. We design the compact quantum circuits for implementing universal and deterministic quantum gates for electron-spin systems, including the two-qubit CNOT gate and the three-qubit Toffoli gate. They are compact and economic, and they do not require additional electron-spin qubits. Moreover, our devices have good scalability and are attractive as they both are based on solid-state quantum systems and the qubits are stationary. They are feasible with the current experimental technology, and both high fidelity and high efficiency can be achieved when the ratio of the side leakage to the cavity decay is low.

Entangling distant resonant exchange qubits via circuit quantum electrodynamics

Science.gov (United States)

Srinivasa, V.; Taylor, J. M.; Tahan, Charles

2016-11-01

We investigate a hybrid quantum system consisting of spatially separated resonant exchange qubits, defined in three-electron semiconductor triple quantum dots, that are coupled via a superconducting transmission line resonator. Drawing on methods from circuit quantum electrodynamics and Hartmann-Hahn double resonance techniques, we analyze three specific approaches for implementing resonator-mediated two-qubit entangling gates in both dispersive and resonant regimes of interaction. We calculate entangling gate fidelities as well as the rate of relaxation via phonons for resonant exchange qubits in silicon triple dots and show that such an implementation is particularly well suited to achieving the strong coupling regime. Our approach combines the favorable coherence properties of encoded spin qubits in silicon with the rapid and robust long-range entanglement provided by circuit QED systems.

Rotations of a logical qubit using the quantum Zeno effect extended to a manifold

Science.gov (United States)

Touzard, S.; Grimm, A.; Leghtas, Z.; Mundhada, S. O.; Reinhold, P.; Heeres, R.; Axline, C.; Reagor, M.; Chou, K.; Blumoff, J.; Sliwa, K. M.; Shankar, S.; Frunzio, L.; Schoelkopf, R. J.; Mirrahimi, M.; Devoret, M. H.

Encoding Quantum Information in the large Hilbert space of a harmonic oscillator has proven to have advantages over encoding in a register of physical qubits, but has also provided new challenges. While recent experiments have demonstrated quantum error correction using such an encoding based on superpositions of coherent states, these codes are still susceptible to non-corrected errors and lack controllability: compared to physical qubits it is hard to make arbitrary states and to perform operations on them. Our approach is to engineer the dynamics and the dissipation of a microwave cavity to implement a continuous dissipative measurement yielding two degenerate outcomes. This extends the quantum Zeno effect to a manifold, which in our case is spanned by two coherent states of opposite phases. In this second talk we present the result and analysis of an experiment that performs rotations on a logical qubit encoded in this protected manifold. Work supported by: ARO, ONR, AFOSR and YINQE.

Quantum acoustics with superconducting qubits

Science.gov (United States)

Chu, Yiwen

2017-04-01

The ability to engineer and manipulate different types of quantum mechanical objects allows us to take advantage of their unique properties and create useful hybrid technologies. Thus far, complex quantum states and exquisite quantum control have been demonstrated in systems ranging from trapped ions to superconducting resonators. Recently, there have been many efforts to extend these demonstrations to the motion of complex, macroscopic objects. These mechanical objects have important applications as quantum memories or transducers for measuring and connecting different types of quantum systems. In particular, there have been a few experiments that couple motion to nonlinear quantum objects such as superconducting qubits. This opens up the possibility of creating, storing, and manipulating non-Gaussian quantum states in mechanical degrees of freedom. However, before sophisticated quantum control of mechanical motion can be achieved, we must realize systems with long coherence times while maintaining a sufficient interaction strength. These systems should be implemented in a simple and robust manner that allows for increasing complexity and scalability in the future. In this talk, I will describe our recent experiments demonstrating a high frequency bulk acoustic wave resonator that is strongly coupled to a superconducting qubit using piezoelectric transduction. In contrast to previous experiments with qubit-mechanical systems, our device requires only simple fabrication methods, extends coherence times to many microseconds, and provides controllable access to a multitude of phonon modes. We use this system to demonstrate basic quantum operations on the coupled qubit-phonon system. Straightforward improvements to the current device will allow for advanced protocols analogous to what has been shown in optical and microwave resonators, resulting in a novel resource for implementing hybrid quantum technologies.

Qubits in phase space: Wigner-function approach to quantum-error correction and the mean-king problem

International Nuclear Information System (INIS)

Paz, Juan Pablo; Roncaglia, Augusto Jose; Saraceno, Marcos

2005-01-01

We analyze and further develop a method to represent the quantum state of a system of n qubits in a phase-space grid of NxN points (where N=2 n ). The method, which was recently proposed by Wootters and co-workers (Gibbons et al., Phys. Rev. A 70, 062101 (2004).), is based on the use of the elements of the finite field GF(2 n ) to label the phase-space axes. We present a self-contained overview of the method, we give insights into some of its features, and we apply it to investigate problems which are of interest for quantum-information theory: We analyze the phase-space representation of stabilizer states and quantum error-correction codes and present a phase-space solution to the so-called mean king problem

Dissipation, dephasing and quantum Darwinism in qubit systems with random unitary interactions

Science.gov (United States)

Balaneskovic, Nenad; Mendler, Marc

2016-09-01

We investigate the influence of dissipation and decoherence on quantum Darwinism by generalizing Zurek's original qubit model of decoherence and the establishment of pointer states [W.H. Zurek, Nat. Phys. 5, 181 (2009); see also arXiv: quant-ph/0707.2832v1, pp. 14-19.]. Our model allows for repeated multiple qubit-qubit couplings between system and environment which are described by randomly applied two-qubit quantum operations inducing entanglement, dissipation and dephasing. The resulting stationary qubit states of system and environment are investigated. They exhibit the intricate influence of entanglement generation, dissipation and dephasing on this characteristic quantum phenomenon.

Negative inductance SQUID qubit operating in a quantum regime

Science.gov (United States)

Liu, W. Y.; Su, F. F.; Xu, H. K.; Li, Z. Y.; Tian, Ye; Zhu, X. B.; Lu, Li; Han, Siyuan; Zhao, S. P.

2018-04-01

Two-junction SQUIDs with negative mutual inductance between their two arms, called nSQUIDs, have been proposed for significantly improving quantum information transfer but their quantum nature has not been experimentally demonstrated. We have designed, fabricated, and characterized superconducting nSQUID qubits. Our results provide clear evidence of the quantum coherence of the device, whose properties are well described by theoretical calculations using parameters determined from spectroscopic measurement. In addition to their future application for fast quantum information transfer, the nSQUID qubits exhibit rich characteristics in their tunable two-dimensional (2D) potentials, energy levels, wave function symmetries, and dipole matrix elements, which are essential to the study of a wide variety of macroscopic quantum phenomena such as tunneling in 2D potential landscapes.

Quantum state transfer via a two-qubit Heisenberg XXZ spin model

Energy Technology Data Exchange (ETDEWEB)

Liu Jia; Zhang Guofeng [Department of Physics, Beijing University of Aeronautics and Astronautics, Beijing 100083 (China); Chen Ziyu [Department of Physics, Beijing University of Aeronautics and Astronautics, Beijing 100083 (China)], E-mail: chenzy@buaa.edu.cn

2008-04-14

Transfer of quantum states through a two-qubit Heisenberg XXZ spin model with a nonuniform magnetic field b is investigated by means of quantum theory. The influences of b, the spin exchange coupling J and the effective transfer time T=Jt on the fidelity have been studied for some different initial states. Results show that fidelity of the transferred state is determined not only by J, T and b but also by the initial state of this quantum system. Ideal information transfer can be realized for some kinds of initial states. We also found that the interactions of the z-component J{sub z} and uniform magnetic field B do not have any contribution to the fidelity. These results may be useful for quantum information processing.

Quantum state transfer via a two-qubit Heisenberg XXZ spin model

International Nuclear Information System (INIS)

Liu Jia; Zhang Guofeng; Chen Ziyu

2008-01-01

Transfer of quantum states through a two-qubit Heisenberg XXZ spin model with a nonuniform magnetic field b is investigated by means of quantum theory. The influences of b, the spin exchange coupling J and the effective transfer time T=Jt on the fidelity have been studied for some different initial states. Results show that fidelity of the transferred state is determined not only by J, T and b but also by the initial state of this quantum system. Ideal information transfer can be realized for some kinds of initial states. We also found that the interactions of the z-component J z and uniform magnetic field B do not have any contribution to the fidelity. These results may be useful for quantum information processing

Quantum dynamics of spin qubits in optically active quantum dots

International Nuclear Information System (INIS)

Bechtold, Alexander

2017-01-01

lead to a relatively fast but incomplete nonmonotonic relaxation of the qubit's spin polarization. This observation changes our understanding of spin qubit decoherence mechanisms, as quadrupolar effects were already known but as yet not associated to an additional source of decoherence. It is found that the strength of the quadrupolar coupling is approximately twice as large as the hyperfine coupling. At microsecond timescales and low external magnetic fields (<1.5 T), the combined effect of quadrupolar coherent nuclear spin dynamics and incoherent hyperfine interaction induce a stage with monotonic relaxation and with almost complete loss of coherence, in contrast to earlier theoretical predictions where quadrupolar effects were not taken into account. By applying an in-plane magnetic field we also implemented an optical phase gate that enabled the coherent control of the qubit's state. This is used to perform spin echo experiments in order to remove the inhomogeneous dephasing and uncover the electron spin's intrinsic coherence time T 2 . By performing such spin echo measurements, it is found that at low magnetic fields the intrinsic coherence time of the electron is drastically reduced due to combined effects of hyperfine and quadrupolar interactions, reducing T 2 to a value of √(T * 2 T Q ) corresponding to ∝40 ns. Only by applying a magnetic field exceeding the quadrupolar interaction strength in the quantum dot (>1.5 T) these effects could be removed and an increase of the coherence time to T 2 =1.3 μs is observed. A long spin echo lifetime T 2 , however, does not necessarily predicate the ability of quantum dot spin qubits to process quantum information. Spin echo is a classical effect in the sense that it can be fully explained in terms of a classical measurement and the behavior of classical spins changing the axis of precession under the action of a properly applied control pulse. How long such an electron spin qubit can store quantum

The Quantum Socket: Wiring for Superconducting Qubits - Part 3

Science.gov (United States)

Mariantoni, M.; Bejianin, J. H.; McConkey, T. G.; Rinehart, J. R.; Bateman, J. D.; Earnest, C. T.; McRae, C. H.; Rohanizadegan, Y.; Shiri, D.; Penava, B.; Breul, P.; Royak, S.; Zapatka, M.; Fowler, A. G.

The implementation of a quantum computer requires quantum error correction codes, which allow to correct errors occurring on physical quantum bits (qubits). Ensemble of physical qubits will be grouped to form a logical qubit with a lower error rate. Reaching low error rates will necessitate a large number of physical qubits. Thus, a scalable qubit architecture must be developed. Superconducting qubits have been used to realize error correction. However, a truly scalable qubit architecture has yet to be demonstrated. A critical step towards scalability is the realization of a wiring method that allows to address qubits densely and accurately. A quantum socket that serves this purpose has been designed and tested at microwave frequencies. In this talk, we show results where the socket is used at millikelvin temperatures to measure an on-chip superconducting resonator. The control electronics is another fundamental element for scalability. We will present a proposal based on the quantum socket to interconnect a classical control hardware to a superconducting qubit hardware, where both are operated at millikelvin temperatures.

The Quantum Socket: Wiring for Superconducting Qubits - Part 1

Science.gov (United States)

McConkey, T. G.; Bejanin, J. H.; Rinehart, J. R.; Bateman, J. D.; Earnest, C. T.; McRae, C. H.; Rohanizadegan, Y.; Shiri, D.; Mariantoni, M.; Penava, B.; Breul, P.; Royak, S.; Zapatka, M.; Fowler, A. G.

Quantum systems with ten superconducting quantum bits (qubits) have been realized, making it possible to show basic quantum error correction (QEC) algorithms. However, a truly scalable architecture has not been developed yet. QEC requires a two-dimensional array of qubits, restricting any interconnection to external classical systems to the third axis. In this talk, we introduce an interconnect solution for solid-state qubits: The quantum socket. The quantum socket employs three-dimensional wires and makes it possible to connect classical electronics with quantum circuits more densely and accurately than methods based on wire bonding. The three-dimensional wires are based on spring-loaded pins engineered to insure compatibility with quantum computing applications. Extensive design work and machining was required, with focus on material quality to prevent magnetic impurities. Microwave simulations were undertaken to optimize the design, focusing on the interface between the micro-connector and an on-chip coplanar waveguide pad. Simulations revealed good performance from DC to 10 GHz and were later confirmed against experimental measurements.

Quantum state engineering with flux-biased Josephson phase qubits by rapid adiabatic passages

International Nuclear Information System (INIS)

Nie, W.; Huang, J. S.; Shi, X.; Wei, L. F.

2010-01-01

In this article, the scheme of quantum computing based on the Stark-chirped rapid adiabatic passage (SCRAP) technique [L. F. Wei, J. R. Johansson, L. X. Cen, S. Ashhab, and F. Nori, Phys. Rev. Lett. 100, 113601 (2008)] is extensively applied to implement quantum state manipulations in flux-biased Josephson phase qubits. The broken-parity symmetries of bound states in flux-biased Josephson junctions are utilized to conveniently generate the desirable Stark shifts. Then, assisted by various transition pulses, universal quantum logic gates as well as arbitrary quantum state preparations can be implemented. Compared with the usual π-pulse operations widely used in experiments, the adiabatic population passages proposed here are insensitive to the details of the applied pulses and thus the desirable population transfers can be satisfyingly implemented. The experimental feasibility of the proposal is also discussed.

Realization of deterministic quantum teleportation with solid state qubits

International Nuclear Information System (INIS)

Andreas Wallfraff

2014-01-01

Using modern micro and nano-fabrication techniques combined with superconducting materials we realize electronic circuits the dynamics of which are governed by the laws of quantum mechanics. Making use of the strong interaction of photons with superconducting quantum two-level systems realized in these circuits we investigate both fundamental quantum effects of light and applications in quantum information processing. In this talk I will discuss the deterministic teleportation of a quantum state in a macroscopic quantum system. Teleportation may be used for distributing entanglement between distant qubits in a quantum network and for realizing universal and fault-tolerant quantum computation. Previously, we have demonstrated the implementation of a teleportation protocol, up to the single-shot measurement step, with three superconducting qubits coupled to a single microwave resonator. Using full quantum state tomography and calculating the projection of the measured density matrix onto the basis of two qubits has allowed us to reconstruct the teleported state with an average output state fidelity of 86%. Now we have realized a new device in which four qubits are coupled pair-wise to three resonators. Making use of parametric amplifiers coupled to the output of two of the resonators we are able to perform high-fidelity single-shot read-out. This has allowed us to demonstrate teleportation by individually post-selecting on any Bell-state and by deterministically distinguishing between all four Bell states measured by the sender. In addition, we have recently implemented fast feed-forward to complete the teleportation process. In all instances, we demonstrate that the fidelity of the teleported states are above the threshold imposed by classical physics. The presented experiments are expected to contribute towards realizing quantum communication with microwave photons in the foreseeable future. (author)

A precise error bound for quantum phase estimation.

Directory of Open Access Journals (Sweden)

James M Chappell

Full Text Available Quantum phase estimation is one of the key algorithms in the field of quantum computing, but up until now, only approximate expressions have been derived for the probability of error. We revisit these derivations, and find that by ensuring symmetry in the error definitions, an exact formula can be found. This new approach may also have value in solving other related problems in quantum computing, where an expected error is calculated. Expressions for two special cases of the formula are also developed, in the limit as the number of qubits in the quantum computer approaches infinity and in the limit as the extra added qubits to improve reliability goes to infinity. It is found that this formula is useful in validating computer simulations of the phase estimation procedure and in avoiding the overestimation of the number of qubits required in order to achieve a given reliability. This formula thus brings improved precision in the design of quantum computers.

Remote one-qubit information concentration and decoding of operator quantum error-correction codes

International Nuclear Information System (INIS)

Hsu Liyi

2007-01-01

We propose the general scheme of remote one-qubit information concentration. To achieve the task, the Bell-correlated mixed states are exploited. In addition, the nonremote one-qubit information concentration is equivalent to the decoding of the quantum error-correction code. Here we propose how to decode the stabilizer codes. In particular, the proposed scheme can be used for the operator quantum error-correction codes. The encoded state can be recreated on the errorless qubit, regardless how many bit-flip errors and phase-flip errors have occurred

Scaling of the local quantum uncertainty at quantum phase transitions

International Nuclear Information System (INIS)

Coulamy, I.B.; Warnes, J.H.; Sarandy, M.S.; Saguia, A.

2016-01-01

We investigate the local quantum uncertainty (LQU) between a block of L qubits and one single qubit in a composite system of n qubits driven through a quantum phase transition (QPT). A first-order QPT is analytically considered through a Hamiltonian implementation of the quantum search. In the case of second-order QPTs, we consider the transverse-field Ising chain via a numerical analysis through density matrix renormalization group. For both cases, we compute the LQU for finite-sizes as a function of L and of the coupling parameter, analyzing its pronounced behavior at the QPT. - Highlights: • LQU is suitable for the analysis of block correlations. • LQU exhibits pronounced behavior at quantum phase transitions. • LQU exponentially saturates in the quantum search. • Concavity of LQU indicates criticality in the Ising chain.

Dynamically protected cat-qubits: a new paradigm for universal quantum computation

International Nuclear Information System (INIS)

Mirrahimi, Mazyar; Leghtas, Zaki; Albert, Victor V; Touzard, Steven; Schoelkopf, Robert J; Jiang, Liang; Devoret, Michel H

2014-01-01

We present a new hardware-efficient paradigm for universal quantum computation which is based on encoding, protecting and manipulating quantum information in a quantum harmonic oscillator. This proposal exploits multi-photon driven dissipative processes to encode quantum information in logical bases composed of Schrödinger cat states. More precisely, we consider two schemes. In a first scheme, a two-photon driven dissipative process is used to stabilize a logical qubit basis of two-component Schrödinger cat states. While such a scheme ensures a protection of the logical qubit against the photon dephasing errors, the prominent error channel of single-photon loss induces bit-flip type errors that cannot be corrected. Therefore, we consider a second scheme based on a four-photon driven dissipative process which leads to the choice of four-component Schrödinger cat states as the logical qubit. Such a logical qubit can be protected against single-photon loss by continuous photon number parity measurements. Next, applying some specific Hamiltonians, we provide a set of universal quantum gates on the encoded qubits of each of the two schemes. In particular, we illustrate how these operations can be rendered fault-tolerant with respect to various decoherence channels of participating quantum systems. Finally, we also propose experimental schemes based on quantum superconducting circuits and inspired by methods used in Josephson parametric amplification, which should allow one to achieve these driven dissipative processes along with the Hamiltonians ensuring the universal operations in an efficient manner

Dynamically protected cat-qubits: a new paradigm for universal quantum computation

Science.gov (United States)

Mirrahimi, Mazyar; Leghtas, Zaki; Albert, Victor V.; Touzard, Steven; Schoelkopf, Robert J.; Jiang, Liang; Devoret, Michel H.

2014-04-01

We present a new hardware-efficient paradigm for universal quantum computation which is based on encoding, protecting and manipulating quantum information in a quantum harmonic oscillator. This proposal exploits multi-photon driven dissipative processes to encode quantum information in logical bases composed of Schrödinger cat states. More precisely, we consider two schemes. In a first scheme, a two-photon driven dissipative process is used to stabilize a logical qubit basis of two-component Schrödinger cat states. While such a scheme ensures a protection of the logical qubit against the photon dephasing errors, the prominent error channel of single-photon loss induces bit-flip type errors that cannot be corrected. Therefore, we consider a second scheme based on a four-photon driven dissipative process which leads to the choice of four-component Schrödinger cat states as the logical qubit. Such a logical qubit can be protected against single-photon loss by continuous photon number parity measurements. Next, applying some specific Hamiltonians, we provide a set of universal quantum gates on the encoded qubits of each of the two schemes. In particular, we illustrate how these operations can be rendered fault-tolerant with respect to various decoherence channels of participating quantum systems. Finally, we also propose experimental schemes based on quantum superconducting circuits and inspired by methods used in Josephson parametric amplification, which should allow one to achieve these driven dissipative processes along with the Hamiltonians ensuring the universal operations in an efficient manner.

Deterministic nonlinear phase gates induced by a single qubit

Science.gov (United States)

Park, Kimin; Marek, Petr; Filip, Radim

2018-05-01

We propose deterministic realizations of nonlinear phase gates by repeating a finite sequence of non-commuting Rabi interactions between a harmonic oscillator and only a single two-level ancillary qubit. We show explicitly that the key nonclassical features of the ideal cubic phase gate and the quartic phase gate are generated in the harmonic oscillator faithfully by our method. We numerically analyzed the performance of our scheme under realistic imperfections of the oscillator and the two-level system. The methodology is extended further to higher-order nonlinear phase gates. This theoretical proposal completes the set of operations required for continuous-variable quantum computation.

Logical Qubit in a Linear Array of Semiconductor Quantum Dots

Directory of Open Access Journals (Sweden)

Cody Jones

2018-06-01

Full Text Available We design a logical qubit consisting of a linear array of quantum dots, we analyze error correction for this linear architecture, and we propose a sequence of experiments to demonstrate components of the logical qubit on near-term devices. To avoid the difficulty of fully controlling a two-dimensional array of dots, we adapt spin control and error correction to a one-dimensional line of silicon quantum dots. Control speed and efficiency are maintained via a scheme in which electron spin states are controlled globally using broadband microwave pulses for magnetic resonance, while two-qubit gates are provided by local electrical control of the exchange interaction between neighboring dots. Error correction with two-, three-, and four-qubit codes is adapted to a linear chain of qubits with nearest-neighbor gates. We estimate an error correction threshold of 10^{-4}. Furthermore, we describe a sequence of experiments to validate the methods on near-term devices starting from four coupled dots.

Volume of the space of qubit-qubit channels and state transformations under random quantum channels

OpenAIRE

Lovas, Attila; Andai, Attila

2017-01-01

The simplest building blocks for quantum computations are the qubit-qubit quantum channels. In this paper, we analyze the structure of these channels via their Choi representation. The restriction of a quantum channel to the space of classical states (i.e. probability distributions) is called the underlying classical channel. The structure of quantum channels over a fixed classical channel is studied, the volume of general and unital qubit channels with respect to the Lebesgue measure is comp...

Quantum information storage using tunable flux qubits

Energy Technology Data Exchange (ETDEWEB)

Steffen, Matthias; Brito, Frederico; DiVincenzo, David; Farinelli, Matthew; Keefe, George; Ketchen, Mark; Kumar, Shwetank; Milliken, Frank; Rothwell, Mary Beth; Rozen, Jim; Koch, Roger H, E-mail: msteffe@us.ibm.co [IBM Watson Research Center, Yorktown Heights, NY 10598 (United States)

2010-02-10

We present details and results for a superconducting quantum bit (qubit) design in which a tunable flux qubit is coupled strongly to a transmission line. Quantum information storage in the transmission line is demonstrated with a dephasing time of T{sub 2}approx2.5 mus. However, energy lifetimes of the qubit are found to be short (approx10 ns) and not consistent with predictions. Several design and material changes do not affect qubit coherence times. In order to determine the cause of these short coherence times, we fabricated standard flux qubits based on a design which was previously successfully used by others. Initial results show significantly improved coherence times, possibly implicating losses associated with the large size of our qubit. (topical review)

Flux qubits on semiconducting quantum ring

International Nuclear Information System (INIS)

Szopa, M; Zipper, E

2010-01-01

The ability to control the quantum state of a single electrons in a quantum ring made of a semiconductor is at the heart of recent developments towards a scalable quantum computer. A peculiar dispersion relation of quantum rings allows to steer the ground state properties by the magnetic flux and offers spin and orbital degrees of freedom for quantum manipulations. We show that such ring can be effectively reduced to the two-state system forming a qubit on orbital or spin degrees of freedom.

Coupled qubits as a quantum heat switch

Science.gov (United States)

Karimi, B.; Pekola, J. P.; Campisi, M.; Fazio, R.

2017-12-01

We present a quantum heat switch based on coupled superconducting qubits, connected to two LC resonators that are terminated by resistors providing two heat baths. To describe the system, we use a standard second order master equation with respect to coupling to the baths. We find that this system can act as an efficient heat switch controlled by the applied magnetic flux. The flux influences the energy level separations of the system, and under some conditions, the finite coupling of the qubits enhances the transmitted power between the two baths, by an order of magnitude under realistic conditions. At the same time, the bandwidth at maximum power of the switch formed of the coupled qubits is narrowed.

Quantum state engineering with flux-biased Josephson phase qubits by rapid adiabatic passages

Science.gov (United States)

Nie, W.; Huang, J. S.; Shi, X.; Wei, L. F.

2010-09-01

In this article, the scheme of quantum computing based on the Stark-chirped rapid adiabatic passage (SCRAP) technique [L. F. Wei, J. R. Johansson, L. X. Cen, S. Ashhab, and F. Nori, Phys. Rev. Lett.PRLTAO0031-900710.1103/PhysRevLett.100.113601 100, 113601 (2008)] is extensively applied to implement quantum state manipulations in flux-biased Josephson phase qubits. The broken-parity symmetries of bound states in flux-biased Josephson junctions are utilized to conveniently generate the desirable Stark shifts. Then, assisted by various transition pulses, universal quantum logic gates as well as arbitrary quantum state preparations can be implemented. Compared with the usual π-pulse operations widely used in experiments, the adiabatic population passages proposed here are insensitive to the details of the applied pulses and thus the desirable population transfers can be satisfyingly implemented. The experimental feasibility of the proposal is also discussed.

On a formulation of qubits in quantum field theory

Energy Technology Data Exchange (ETDEWEB)

Calmet, Jacques, E-mail: calmet@ira.uka.de [Karlsruhe Institute of Technology (KIT), Institute for Cryptography and Security, Am Fasanengarten 5, 76131 Karlsruhe (Germany); Calmet, Xavier, E-mail: x.calmet@sussex.ac.uk [Physics and Astronomy, University of Sussex, Falmer, Brighton, BN1 9QH (United Kingdom)

2012-01-30

Qubits have been designed in the framework of quantum mechanics. Attempts to formulate the problem in the language of quantum field theory have been proposed already. In this short Letter we refine the meaning of qubits within the framework of quantum field theory. We show that the notion of gauge invariance naturally leads to a generalization of qubits to QFTbits which are then the fundamental carriers of information from the quantum field theoretical point of view. The goal of this Letter is to stress the availability of such a generalized concept of QFTbits. -- Highlights: ► Gauge invariant qubits are proposed. ► Non-linear QFT effects are discussed. ► Entanglement of qubits in QFT.

Quasiparticle-induced decoherence of microscopic two-level-systems in superconducting qubits

Energy Technology Data Exchange (ETDEWEB)

Bilmes, Alexander; Lisenfeld, Juergen; Zanker, Sebastian; Weiss, Georg; Ustinov, Alexey V. [PHI, KIT, Karlsruhe (Germany); Marthaler, Michael; Schoen, Gerd [TFP, KIT, Karlsruhe (Germany)

2016-07-01

Parasitic Two-Level-Systems (TLS) are one of the main sources of decoherence in superconducting nano-scale devices such as SQUIDs, resonators and quantum bits (qubits), although the TLS' microscopic nature remains unclear. We use a superconducting phase qubit to detect TLS contained within the tunnel barrier of the qubit's Al/AlOx/Al Josephson junction. If the TLS transition frequency lies within the 6-10 GHz range, we can coherently drive it by resonant microwave pulses and access its quantum state by utilizing the strong coupling to the qubit. Our previous measurements of TLS coherence in dependence of the temperature indicate that quasiparticles (QPs), which diffuse from the superconducting Al electrodes into the oxide layer, may give rise to TLS energy loss and dephasing. Here, we probe the TLS-QP interaction using a reliable method of in-situ QP injection via an on-chip dc-SQUID that is pulse-biased beyond its switching current. The QP density is calibrated by measuring associated characteristic changes to the qubit's energy relaxation rate. We will present experimental data which show the QP-induced TLS decoherence in good agreement to theoretical predictions.

Rotations of a logical qubit using the quantum Zeno effect extended to a manifold - Part 1

Science.gov (United States)

Grimm, A.; Touzard, S.; Leghtas, Z.; Mundhada, S. O.; Reinhold, P.; Heeres, R.; Axline, C.; Reagor, M.; Chou, K.; Blumoff, J.; Sliwa, K. M.; Shankar, S.; Frunzio, L.; Schoelkopf, R. J.; Mirrahimi, M.; Devoret, M. H.

Encoding Quantum Information in the large Hilbert space of a harmonic oscillator has proven to have advantages over encoding in a register of physical qubits, but has also provided new challenges. While recent experiments have demonstrated quantum error correction using such an encoding based on superpositions of coherent states, these codes are still susceptible to non-corrected errors and lack controllability: compared to physical qubits it is hard to make arbitrary states and to perform operations on them. Our approach is to engineer the dynamics and the dissipation of a microwave cavity to implement a continuous dissipative measurement yielding two degenerate outcomes. This extends the quantum Zeno effect to a manifold, which in our case is spanned by two coherent states of opposite phases. In this first talk we present the concept and architecture of an experiment that performs rotations on a logical qubit encoded in this protected manifold. Work supported by: ARO, ONR, AFOSR and YINQE.

Faithful qubit transmission in a quantum communication network with heterogeneous channels

Science.gov (United States)

Chen, Na; Zhang, Lin Xi; Pei, Chang Xing

2018-04-01

Quantum communication networks enable long-distance qubit transmission and distributed quantum computation. In this paper, a quantum communication network with heterogeneous quantum channels is constructed. A faithful qubit transmission scheme is presented. Detailed calculations and performance analyses show that even in a low-quality quantum channel with serious decoherence, only modest number of locally prepared target qubits are required to achieve near-deterministic qubit transmission.

Experimental realization of quantum cheque using a five-qubit quantum computer

Science.gov (United States)

Behera, Bikash K.; Banerjee, Anindita; Panigrahi, Prasanta K.

2017-12-01

Quantum cheques could be a forgery-free way to make transaction in a quantum networked banking system with perfect security against any no-signalling adversary. Here, we demonstrate the implementation of quantum cheque, proposed by Moulick and Panigrahi (Quantum Inf Process 15:2475-2486, 2016), using the five-qubit IBM quantum computer. Appropriate single qubit, CNOT and Fredkin gates are used in an optimized configuration. The accuracy of implementation is checked and verified through quantum state tomography by comparing results from the theoretical and experimental density matrices.

Silicon quantum processor with robust long-distance qubit couplings

Energy Technology Data Exchange (ETDEWEB)

Tosi, Guilherme; Mohiyaddin, Fahd A.; Schmitt, Vivien; Tenberg, Stefanie; Rahman, Rajib; Klimeck, Gerhard; Morello, Andrea

2017-09-06

Practical quantum computers require a large network of highly coherent qubits, interconnected in a design robust against errors. Donor spins in silicon provide state-of-the-art coherence and quantum gate fidelities, in a platform adapted from industrial semiconductor processing. Here we present a scalable design for a silicon quantum processor that does not require precise donor placement and leaves ample space for the routing of interconnects and readout devices. We introduce the flip-flop qubit, a combination of the electron-nuclear spin states of a phosphorus donor that can be controlled by microwave electric fields. Two-qubit gates exploit a second-order electric dipole-dipole interaction, allowing selective coupling beyond the nearest-neighbor, at separations of hundreds of nanometers, while microwave resonators can extend the entanglement to macroscopic distances. We predict gate fidelities within fault-tolerance thresholds using realistic noise models. This design provides a realizable blueprint for scalable spin-based quantum computers in silicon.

Topological networks for quantum communication between distant qubits

Science.gov (United States)

Lang, Nicolai; Büchler, Hans Peter

2017-11-01

Efficient communication between qubits relies on robust networks, which allow for fast and coherent transfer of quantum information. It seems natural to harvest the remarkable properties of systems characterized by topological invariants to perform this task. Here, we show that a linear network of coupled bosonic degrees of freedom, characterized by topological bands, can be employed for the efficient exchange of quantum information over large distances. Important features of our setup are that it is robust against quenched disorder, all relevant operations can be performed by global variations of parameters, and the time required for communication between distant qubits approaches linear scaling with their distance. We demonstrate that our concept can be extended to an ensemble of qubits embedded in a two-dimensional network to allow for communication between all of them.

Non-stoquastic Hamiltonians in quantum annealing via geometric phases

Science.gov (United States)

Vinci, Walter; Lidar, Daniel A.

2017-09-01

We argue that a complete description of quantum annealing implemented with continuous variables must take into account the non-adiabatic Aharonov-Anandan geometric phase that arises when the system Hamiltonian changes during the anneal. We show that this geometric effect leads to the appearance of non-stoquasticity in the effective quantum Ising Hamiltonians that are typically used to describe quantum annealing with flux qubits. We explicitly demonstrate the effect of this geometric non-stoquasticity when quantum annealing is performed with a system of one and two coupled flux qubits. The realization of non-stoquastic Hamiltonians has important implications from a computational complexity perspective, since it is believed that in many cases quantum annealing with stoquastic Hamiltonians can be efficiently simulated via classical algorithms such as Quantum Monte Carlo. It is well known that the direct implementation of non-stoquastic Hamiltonians with flux qubits is particularly challenging. Our results suggest an alternative path for the implementation of non-stoquasticity via geometric phases that can be exploited for computational purposes.

Genetic algorithm based on qubits and quantum gates

International Nuclear Information System (INIS)

Silva, Joao Batista Rosa; Ramos, Rubens Viana

2003-01-01

Full text: Genetic algorithm, a computational technique based on the evolution of the species, in which a possible solution of the problem is coded in a binary string, called chromosome, has been used successfully in several kinds of problems, where the search of a minimal or a maximal value is necessary, even when local minima are present. A natural generalization of a binary string is a qubit string. Hence, it is possible to use the structure of a genetic algorithm having a sequence of qubits as a chromosome and using quantum operations in the reproduction in order to find the best solution in some problems of quantum information. For example, given a unitary matrix U what is the pair of qubits that, when applied at the input, provides the output state with maximal entanglement? In order to solve this problem, a population of chromosomes of two qubits was created. The crossover was performed applying the quantum gates CNOT and SWAP at the pair of qubits, while the mutation was performed applying the quantum gates Hadamard, Z and Not in a single qubit. The result was compared with a classical genetic algorithm used to solve the same problem. A hundred simulations using the same U matrix was performed. Both algorithms, hereafter named by CGA (classical) and QGA (using qu bits), reached good results close to 1 however, the number of generations needed to find the best result was lower for the QGA. Another problem where the QGA can be useful is in the calculation of the relative entropy of entanglement. We have tested our algorithm using 100 pure states chosen randomly. The stop criterion used was the error lower than 0.01. The main advantages of QGA are its good precision, robustness and very easy implementation. The main disadvantage is its low velocity, as happen for all kind of genetic algorithms. (author)

Universal set of quantum gates for double-dot exchange-only spin qubits with intradot coupling

International Nuclear Information System (INIS)

Michielis, M De; Ferraro, E; Fanciulli, M; Prati, E

2015-01-01

We present a universal set of quantum gate operations based on exchange-only spin qubits in a double quantum dot, where each qubit is obtained by three electrons in the (2,1) filling. Gate operations are addressed by modulating electrostatically the tunneling barrier and the energy offset between the two dots, singly and doubly occupied respectively. We propose explicit gate sequences of single qubit operations for arbitrary rotations, and the two-qubit controlled NOT gate, to complete the universal set. The unswitchable interaction between the two electrons of the doubly occupied quantum dot is taken into account. Short gate times are obtained by employing spin density functional theory simulations. (paper)

Quantum Key Distribution Using Four-Qubit W State

International Nuclear Information System (INIS)

Cai Haijing; Song Heshan

2006-01-01

A new theoretical quantum key distribution scheme based on entanglement swapping is proposed, where four-qubit symmetric W state functions as quantum channel. It is shown that two legitimate users can secretly share a series of key bits by using Bell-state measurements and classical communication.

Quantum gambling using mesoscopic ring qubits

Energy Technology Data Exchange (ETDEWEB)

Pakula, Ireneusz [University of Silesia, Institute of Physics, ul. Uniwersytecka 4, 40-007 Katowice (Poland)

2007-07-15

Quantum Game Theory provides us with new tools for practising games and some other risk related enterprices like, for example, gambling. The two party gambling protocol presented by Goldenberg et al. is one of the simplest yet still hard to implementapplications of Quantum Game Theory. We propose potential physical realisation of the quantum gambling protocol with use of three mesoscopic ring qubits. We point out problems in implementation of such game. (copyright 2007 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

Quantum gambling using mesoscopic ring qubits

International Nuclear Information System (INIS)

Pakula, Ireneusz

2007-01-01

Quantum Game Theory provides us with new tools for practising games and some other risk related enterprices like, for example, gambling. The two party gambling protocol presented by Goldenberg et al. is one of the simplest yet still hard to implementapplications of Quantum Game Theory. We propose potential physical realisation of the quantum gambling protocol with use of three mesoscopic ring qubits. We point out problems in implementation of such game. (copyright 2007 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

A fault-tolerant addressable spin qubit in a natural silicon quantum dot

Science.gov (United States)

Takeda, Kenta; Kamioka, Jun; Otsuka, Tomohiro; Yoneda, Jun; Nakajima, Takashi; Delbecq, Matthieu R.; Amaha, Shinichi; Allison, Giles; Kodera, Tetsuo; Oda, Shunri; Tarucha, Seigo

2016-01-01

Fault-tolerant quantum computing requires high-fidelity qubits. This has been achieved in various solid-state systems, including isotopically purified silicon, but is yet to be accomplished in industry-standard natural (unpurified) silicon, mainly as a result of the dephasing caused by residual nuclear spins. This high fidelity can be achieved by speeding up the qubit operation and/or prolonging the dephasing time, that is, increasing the Rabi oscillation quality factor Q (the Rabi oscillation decay time divided by the π rotation time). In isotopically purified silicon quantum dots, only the second approach has been used, leaving the qubit operation slow. We apply the first approach to demonstrate an addressable fault-tolerant qubit using a natural silicon double quantum dot with a micromagnet that is optimally designed for fast spin control. This optimized design allows access to Rabi frequencies up to 35 MHz, which is two orders of magnitude greater than that achieved in previous studies. We find the optimum Q = 140 in such high-frequency range at a Rabi frequency of 10 MHz. This leads to a qubit fidelity of 99.6% measured via randomized benchmarking, which is the highest reported for natural silicon qubits and comparable to that obtained in isotopically purified silicon quantum dot–based qubits. This result can inspire contributions to quantum computing from industrial communities. PMID:27536725

Deterministic Assisted Clone of an Arbitrary Two- and Three-qubit States via Multi-qubit Brown State

Science.gov (United States)

Hou, Kui; Zhu, Cheng-Jie; Yang, Ya-Ping

2017-08-01

We present two schemes for deterministic assisted clone(DAC) of an unknown two- and three-qubit entangled states with assistance via muti-qubit Brown state. In the schemes, the sender wish to teleport an unknown original entangled state which from the state preparer, and then create a perfect copy of the unknown state at her place. The DAC schemes include two stages. The first stage requires teleportation with Bell-state measurements via a five-qubit Brown state(or seven-qubit Brown state) as the quantum channel. In the second stage, to help the sender realize the quantum cloning, the state preparer performs projective measurements on their own particles which from the sender, then the sender can acquire a perfect copy of the unknown state by means of some appropriate unitary operations. Furthermore, the total success probability for assisted cloning a perfect copy of the unknown state can reach 1 in our schemes.

Simulation of n-qubit quantum systems. III. Quantum operations

Science.gov (United States)

Radtke, T.; Fritzsche, S.

2007-05-01

During the last decade, several quantum information protocols, such as quantum key distribution, teleportation or quantum computation, have attracted a lot of interest. Despite the recent success and research efforts in quantum information processing, however, we are just at the beginning of understanding the role of entanglement and the behavior of quantum systems in noisy environments, i.e. for nonideal implementations. Therefore, in order to facilitate the investigation of entanglement and decoherence in n-qubit quantum registers, here we present a revised version of the FEYNMAN program for working with quantum operations and their associated (Jamiołkowski) dual states. Based on the implementation of several popular decoherence models, we provide tools especially for the quantitative analysis of quantum operations. Apart from the implementation of different noise models, the current program extension may help investigate the fragility of many quantum states, one of the main obstacles in realizing quantum information protocols today. Program summaryTitle of program: Feynman Catalogue identifier: ADWE_v3_0 Program summary URL:http://cpc.cs.qub.ac.uk/summaries/ADWE_v3_0 Program obtainable from: CPC Program Library, Queen's University of Belfast, N. Ireland Licensing provisions: None Operating systems: Any system that supports MAPLE; tested under Microsoft Windows XP, SuSe Linux 10 Program language used:MAPLE 10 Typical time and memory requirements: Most commands that act upon quantum registers with five or less qubits take ⩽10 seconds of processor time (on a Pentium 4 processor with ⩾2 GHz or equivalent) and 5-20 MB of memory. Especially when working with symbolic expressions, however, the memory and time requirements critically depend on the number of qubits in the quantum registers, owing to the exponential dimension growth of the associated Hilbert space. For example, complex (symbolic) noise models (with several Kraus operators) for multi-qubit systems

Single-photon two-qubit entangled states: Preparation and measurement

International Nuclear Information System (INIS)

Kim, Yoon-Ho

2003-01-01

We implement experimentally a deterministic method to prepare and measure the so-called single-photon two-qubit entangled states or single-photon Bell states, in which the polarization and the spatial modes of a single photon each represent a quantum bit. All four single-photon Bell states can be easily prepared and measured deterministically using linear optical elements alone. We also discuss how this method can be used for the recently proposed single-photon two-qubit quantum cryptography scheme

Two-photon interference at telecom wavelengths for time-bin-encoded single photons from quantum-dot spin qubits.

Science.gov (United States)

Yu, Leo; Natarajan, Chandra M; Horikiri, Tomoyuki; Langrock, Carsten; Pelc, Jason S; Tanner, Michael G; Abe, Eisuke; Maier, Sebastian; Schneider, Christian; Höfling, Sven; Kamp, Martin; Hadfield, Robert H; Fejer, Martin M; Yamamoto, Yoshihisa

2015-11-24

Practical quantum communication between remote quantum memories rely on single photons at telecom wavelengths. Although spin-photon entanglement has been demonstrated in atomic and solid-state qubit systems, the produced single photons at short wavelengths and with polarization encoding are not suitable for long-distance communication, because they suffer from high propagation loss and depolarization in optical fibres. Establishing entanglement between remote quantum nodes would further require the photons generated from separate nodes to be indistinguishable. Here, we report the observation of correlations between a quantum-dot spin and a telecom single photon across a 2-km fibre channel based on time-bin encoding and background-free frequency downconversion. The downconverted photon at telecom wavelengths exhibits two-photon interference with another photon from an independent source, achieving a mean wavepacket overlap of greater than 0.89 despite their original wavelength mismatch (900 and 911 nm). The quantum-networking operations that we demonstrate will enable practical communication between solid-state spin qubits across long distances.

Adaptive recurrence quantum entanglement distillation for two-Kraus-operator channels

Science.gov (United States)

Ruan, Liangzhong; Dai, Wenhan; Win, Moe Z.

2018-05-01

Quantum entanglement serves as a valuable resource for many important quantum operations. A pair of entangled qubits can be shared between two agents by first preparing a maximally entangled qubit pair at one agent, and then sending one of the qubits to the other agent through a quantum channel. In this process, the deterioration of entanglement is inevitable since the noise inherent in the channel contaminates the qubit. To address this challenge, various quantum entanglement distillation (QED) algorithms have been developed. Among them, recurrence algorithms have advantages in terms of implementability and robustness. However, the efficiency of recurrence QED algorithms has not been investigated thoroughly in the literature. This paper puts forth two recurrence QED algorithms that adapt to the quantum channel to tackle the efficiency issue. The proposed algorithms have guaranteed convergence for quantum channels with two Kraus operators, which include phase-damping and amplitude-damping channels. Analytical results show that the convergence speed of these algorithms is improved from linear to quadratic and one of the algorithms achieves the optimal speed. Numerical results confirm that the proposed algorithms significantly improve the efficiency of QED.

Computing prime factors using a Josephson phase-qubit architecture: 15 = 3 x 5

Science.gov (United States)

Lucero, Erik Anthony

Josephson phase-quantum-bits, (“qubits”), together with superconducting resonators, comprise the essential quantum elements in a state-of-the-art quantum processor (QuP). A QuP can be used to exploit quantum mechanics to find the prime factors of composite numbers by running Shor's algorithm[57]. In this thesis, I describe the first solid-state demonstration of a compiled version of Shor's algorithm. To meet this challenge, I designed a QuP so that I could map the problem of factoring the number N = 15 onto a quantum circuit that is compatible with our technological capabilities. The QuP is composed of nine quantum elements: four phase qubits and five superconducting coplanar waveguide (CPW) resonators. Using this device, I ran a three-qubit complied version of Shor's algorithm and successfully found the prime factors 48% of the time (compared to the ideal success rate of 50 %). In addition, the QuP produced coherent interactions between five quantum elements, and bi- and tripartite entanglement, which was verified via quantum state tomography (QST). Scaling up to nine quantum elements and performing these experiments represent key milestones to realizing a quantum computer. Continued improvements in the superconducting qubit coherence times and more complex circuits should provide the resources necessary to factor larger composite numbers and run more intricate quantum algorithms in the near future.

Enhancing non-local correlations in the bipartite partitions of two qubit-system with non-mutual interaction

Energy Technology Data Exchange (ETDEWEB)

Mohamed, A.-B.A., E-mail: abdelbastm@yahoo.com [College of Sciences and Humanities, Prince Sattam Bin Abdulaziz University, Al-Aflaj (Saudi Arabia); Faculty of Science, Assiut University, Assiut (Egypt); Joshi, A., E-mail: mcbamji@gmail.com [Physics Department, Adelphi University Garden City, NY 11530 (United States); Department of Physics and Optical Engineering, RHIT, Terra Haute IN 47803 (United States); Hassan, S.S., E-mail: shoukryhassan@hotmail.com [Department of Mathematics, College of Science, University of Bahrain, P.O. Box 32038 (Bahrain)

2016-03-15

Several quantum-mechanical correlations, notably, quantum entanglement, measurement-induced nonlocality and Bell nonlocality are studied for a two qubit-system having no mutual interaction. Analytical expressions for the measures of these quantum-mechanical correlations of different bipartite partitions of the system are obtained, for initially two entangled qubits and the two photons are in their vacuum states. It is found that the qubits-fields interaction leads to the loss and gain of the initial quantum correlations. The lost initial quantum correlations transfer from the qubits to the cavity fields. It is found that the maximal violation of Bell’s inequality is occurring when the quantum correlations of both the logarithmic negativity and measurement-induced nonlocality reach particular values. The maximal violation of Bell’s inequality occurs only for certain bipartite partitions of the system. The frequency detuning leads to quick oscillations of the quantum correlations and inhibits their transfer from the qubits to the cavity modes. It is also found that the dynamical behavior of the quantum correlation clearly depends on the qubit distribution angle.

Two Superconducting Charge Qubits Coupled by a Josephson Inductance

Science.gov (United States)

Watanabe, Michio; Yamamoto, Tsuyoshi; Pashkin, Yuri A.; Astafiev, Oleg; Nakamura, Yasunobu; Tsai, Jaw-Shen

2007-03-01

When the quantum oscillations [Pashkin et al., Nature 421, 823 (2003)] and the conditional gate operation [Yamamoto et al., Nature 425, 941 (2003)] were demonstrated using superconducting charge qubits, the charge qubits were coupled capacitively, where the coupling was always on and the coupling strength was not tunable. This fixed coupling, however, is not ideal because for example, it makes unconditional gate operations difficult. In this work, we aimed to tunably couple two charge qubits. We fabricated circuits based on the theoretical proposal by You, Tsai, and Nori [PRB 68, 024510 (2003)], where the inductance of a Josephson junction, which has a much larger junction area than the qubit junctions, couples the qubits and the coupling strength is controlled by the external magnetic flux. We confirmed by spectroscopy that the large Josephson junction was indeed coupled to the qubits and that the coupling was turned on and off by the external magnetic flux. In the talk, we will also discuss the quantum oscillations in the circuits.

Signifying quantum benchmarks for qubit teleportation and secure quantum communication using Einstein-Podolsky-Rosen steering inequalities

Science.gov (United States)

Reid, M. D.

2013-12-01

The demonstration of quantum teleportation of a photonic qubit from Alice to Bob usually relies on data conditioned on detection at Bob's location. I show that Bohm's Einstein-Podolsky-Rosen (EPR) paradox can be used to verify that the quantum benchmark for qubit teleportation has been reached, without postselection. This is possible for scenarios insensitive to losses at the generation station, and with efficiencies of ηB>1/3 for the teleportation process. The benchmark is obtained if it is shown that Bob can “steer” Alice's record of the qubit as stored by Charlie. EPR steering inequalities involving m measurement settings can also be used to confirm quantum teleportation, for efficiencies ηB>1/m, if one assumes trusted detectors for Charlie and Alice. Using proofs of monogamy, I show that two-setting EPR steering inequalities can signify secure teleportation of the qubit state.

High-Fidelity Quantum Logic Gates Using Trapped-Ion Hyperfine Qubits.

Science.gov (United States)

Ballance, C J; Harty, T P; Linke, N M; Sepiol, M A; Lucas, D M

2016-08-05

We demonstrate laser-driven two-qubit and single-qubit logic gates with respective fidelities 99.9(1)% and 99.9934(3)%, significantly above the ≈99% minimum threshold level required for fault-tolerant quantum computation, using qubits stored in hyperfine ground states of calcium-43 ions held in a room-temperature trap. We study the speed-fidelity trade-off for the two-qubit gate, for gate times between 3.8  μs and 520  μs, and develop a theoretical error model which is consistent with the data and which allows us to identify the principal technical sources of infidelity.

Distributed quantum information processing via quantum dot spins

International Nuclear Information System (INIS)

Jun, Liu; Qiong, Wang; Le-Man, Kuang; Hao-Sheng, Zeng

2010-01-01

We propose a scheme to engineer a non-local two-qubit phase gate between two remote quantum-dot spins. Along with one-qubit local operations, one can in principal perform various types of distributed quantum information processing. The scheme employs a photon with linearly polarisation interacting one after the other with two remote quantum-dot spins in cavities. Due to the optical spin selection rule, the photon obtains a Faraday rotation after the interaction process. By measuring the polarisation of the final output photon, a non-local two-qubit phase gate between the two remote quantum-dot spins is constituted. Our scheme may has very important applications in the distributed quantum information processing

Building logical qubits in a superconducting quantum computing system

Science.gov (United States)

Gambetta, Jay M.; Chow, Jerry M.; Steffen, Matthias

2017-01-01

The technological world is in the midst of a quantum computing and quantum information revolution. Since Richard Feynman's famous `plenty of room at the bottom' lecture (Feynman, Engineering and Science23, 22 (1960)), hinting at the notion of novel devices employing quantum mechanics, the quantum information community has taken gigantic strides in understanding the potential applications of a quantum computer and laid the foundational requirements for building one. We believe that the next significant step will be to demonstrate a quantum memory, in which a system of interacting qubits stores an encoded logical qubit state longer than the incorporated parts. Here, we describe the important route towards a logical memory with superconducting qubits, employing a rotated version of the surface code. The current status of technology with regards to interconnected superconducting-qubit networks will be described and near-term areas of focus to improve devices will be identified. Overall, the progress in this exciting field has been astounding, but we are at an important turning point, where it will be critical to incorporate engineering solutions with quantum architectural considerations, laying the foundation towards scalable fault-tolerant quantum computers in the near future.

Four-level and two-qubit systems, subalgebras, and unitary integration

International Nuclear Information System (INIS)

Rau, A.R.P.; Selvaraj, G.; Uskov, D.

2005-01-01

Four-level systems in quantum optics, and for representing two qubits in quantum computing, are difficult to solve for general time-dependent Hamiltonians. A systematic procedure is presented which combines analytical handling of the algebraic operator aspects with simple solutions of classical, first-order differential equations. In particular, by exploiting su(2)+su(2) and su(2)+su(2)+u(1) subalgebras of the full SU(4) dynamical group of the system, the nontrivial part of the final calculation is reduced to a single Riccati (first-order, quadratically nonlinear) equation, itself simply solved. Examples are provided of two-qubit problems from the recent literature, including implementation of two-qubit gates with Josephson junctions

Entangling capabilities of symmetric two-qubit gates

Indian Academy of Sciences (India)

Com- putational investigation of entanglement of such ensembles is therefore impractical for ... the computational complexity. Pairs of spin-1 ... tensor operators which can also provide different symmetric logic gates for quantum pro- ... that five of the eight, two-qubit symmetric quantum gates expressed in terms of our newly.

Quantum information density scaling and qubit operation time constraints of CMOS silicon-based quantum computer architectures

Science.gov (United States)

Rotta, Davide; Sebastiano, Fabio; Charbon, Edoardo; Prati, Enrico

2017-06-01

range of a silicon complementary metal-oxide-semiconductor quantum processor to be within 1 and 100 GHz. Such constraint limits the feasibility of fault-tolerant quantum information processing with complementary metal-oxide-semiconductor technology only to the most advanced nodes. The compatibility with classical complementary metal-oxide-semiconductor control circuitry is discussed, focusing on the cryogenic complementary metal-oxide-semiconductor operation required to bring the classical controller as close as possible to the quantum processor and to enable interfacing thousands of qubits on the same chip via time-division, frequency-division, and space-division multiplexing. The operation time range prospected for cryogenic control electronics is found to be compatible with the operation time expected for qubits. By combining the forecast of the development of scaled technology nodes with operation time and classical circuitry constraints, we derive a maximum quantum information density for logical qubits of 2.8 and 4 Mqb/cm2 for the 10 and 7-nm technology nodes, respectively, for the Steane code. The density is one and two orders of magnitude less for surface codes and for concatenated codes, respectively. Such values provide a benchmark for the development of fault-tolerant quantum algorithms by circuital quantum information based on silicon platforms and a guideline for other technologies in general.

Quantum-dot cluster-state computing with encoded qubits

International Nuclear Information System (INIS)

Weinstein, Yaakov S.; Hellberg, C. Stephen; Levy, Jeremy

2005-01-01

A class of architectures is advanced for cluster-state quantum computation using quantum dots. These architectures include using single and multiple dots as logical qubits. Special attention is given to supercoherent qubits introduced by Bacon et al. [Phys. Rev. Lett. 87, 247902 (2001)] for which we discuss the effects of various errors and present a means of error protection

Coherence properties in superconducting flux qubits

Energy Technology Data Exchange (ETDEWEB)

Spilla, Samuele

2015-02-16

The research work discussed in this thesis deals with the study of superconducting Josephson qubits. Superconducting qubits are solid-state artificial atoms which are based on lithographically defined Josephson tunnel junctions properties. When sufficiently cooled, these superconducting devices exhibit quantized states of charge, flux or junction phase depending on their design parameters. This allows to observe coherent evolutions of their states. The results presented can be divided into two parts. In a first part we investigate operations of superconducting qubits based on the quantum coherence in superconducting quantum interference devices (SQUID). We explain experimental data which has been observed in a SQUID subjected to fast, large-amplitude modifications of its effective potential shape. The motivations for this work come from the fact that in the past few years there have been attempts to interpret the supposed quantum behavior of physical systems, such as Josephson devices, within a classical framework. Moreover, we analyze the possibility of generating GHZ states, namely maximally entangled states, in a quantum system made out of three Josephson qubits. In particular, we investigate the possible limitations of the GHZ state generation due to coupling to bosonic baths. In the second part of the thesis we address a particular cause of decoherence of flux qubits which has been disregarded until now: thermal gradients, which can arise due to accidental non equilibrium quasiparticle distributions. The reason for these detrimental effects is that heat currents flowing through Josephson tunnel junctions in response to a temperature gradient are periodic functions of the phase difference between the electrodes. The phase dependence of the heat current comes from Andreev reflection, namely an interplay between the quasiparticles which carry heat and the superconducting condensate which is sensitive to the superconducting phase difference. Generally speaking

Visualization of the Invisible: The Qubit as Key to Quantum Physics

Science.gov (United States)

Dür, Wolfgang; Heusler, Stefan

2014-11-01

Quantum mechanics is one of the pillars of modern physics, however rather difficult to teach at the introductory level due to the conceptual difficulties and the required advanced mathematics. Nevertheless, attempts to identify relevant features of quantum mechanics and to put forward concepts of how to teach it have been proposed.1-8 Here we present an approach to quantum physics based on the simplest quantum mechanical system—the quantum bit (qubit).1 Like its classical counterpart—the bit—a qubit corresponds to a two-level system, i.e., some system with a physical property that can admit two possible values. While typically a physical system has more than just one property or the property can admit more than just two values, in many situations most degrees of freedom can be considered to be fixed or frozen. Hence a variety of systems can be effectively described as a qubit. For instance, one may consider the spin of an electron or atom, with spin up and spin down as two possible values, and where other properties of the particle such as its mass or its position are fixed. Further examples include the polarization degree of freedom of a photon (horizontal and vertical polarization), two electronic degrees of freedom (i.e., two energy levels) of an atom, or the position of an atom in a double well potential (atom in left or right well). In all cases, only two states are relevant to describe the system.

Synthetic Topological Qubits in Conventional Bilayer Quantum Hall Systems

Directory of Open Access Journals (Sweden)

Maissam Barkeshli

2014-11-01

Full Text Available The idea of topological quantum computation is to build powerful and robust quantum computers with certain macroscopic quantum states of matter called topologically ordered states. These systems have degenerate ground states that can be used as robust “topological qubits” to store and process quantum information. In this paper, we propose a new experimental setup that can realize topological qubits in a simple bilayer fractional quantum Hall system with proper electric gate configurations. Our proposal is accessible with current experimental techniques, involves well-established topological states, and, moreover, can realize a large class of topological qubits, generalizing the Majorana zero modes studied in recent literature to more computationally powerful possibilities. We propose three tunneling and interferometry experiments to detect the existence and nonlocal topological properties of the topological qubits.

A blueprint for demonstrating quantum supremacy with superconducting qubits

Science.gov (United States)

Neill, C.; Roushan, P.; Kechedzhi, K.; Boixo, S.; Isakov, S. V.; Smelyanskiy, V.; Megrant, A.; Chiaro, B.; Dunsworth, A.; Arya, K.; Barends, R.; Burkett, B.; Chen, Y.; Chen, Z.; Fowler, A.; Foxen, B.; Giustina, M.; Graff, R.; Jeffrey, E.; Huang, T.; Kelly, J.; Klimov, P.; Lucero, E.; Mutus, J.; Neeley, M.; Quintana, C.; Sank, D.; Vainsencher, A.; Wenner, J.; White, T. C.; Neven, H.; Martinis, J. M.

2018-04-01

A key step toward demonstrating a quantum system that can address difficult problems in physics and chemistry will be performing a computation beyond the capabilities of any classical computer, thus achieving so-called quantum supremacy. In this study, we used nine superconducting qubits to demonstrate a promising path toward quantum supremacy. By individually tuning the qubit parameters, we were able to generate thousands of distinct Hamiltonian evolutions and probe the output probabilities. The measured probabilities obey a universal distribution, consistent with uniformly sampling the full Hilbert space. As the number of qubits increases, the system continues to explore the exponentially growing number of states. Extending these results to a system of 50 qubits has the potential to address scientific questions that are beyond the capabilities of any classical computer.

Thermal quantum and classical correlations in a two-qubit XX model in a nonuniform external magnetic field

Energy Technology Data Exchange (ETDEWEB)

Hassan, Ali Saif M [Department of Physics, University of Amran, Amran (Yemen); Lari, Behzad; Joag, Pramod S, E-mail: alisaif73@gmail.co, E-mail: behzadlari1979@yahoo.co, E-mail: pramod@physics.unipune.ac.i [Department of Physics, University of Pune, Pune 411007 (India)

2010-12-03

We investigate how thermal quantum discord (QD) and classical correlations (CC) of a two-qubit one-dimensional XX Heisenberg chain in thermal equilibrium depend on the temperature of the bath as well as on nonuniform external magnetic fields applied to two qubits and varied separately. We show that the behavior of QD differs in many unexpected ways from the thermal entanglement (EOF). For the nonuniform case (B{sub 1} = -B{sub 2}), we find that QD and CC are equal for all values of (B{sub 1} = -B{sub 2}) and for different temperatures. We show that, in this case, the thermal states of the system belong to a class of mixed states and satisfy certain conditions under which QD and CC are equal. The specification of this class and the corresponding conditions are completely general and apply to any quantum system in a state in this class satisfying these conditions. We further find that the relative contributions of QD and CC can be controlled easily by changing the relative magnitudes of B{sub 1} and B{sub 2}. Finally, we connect our results with the monogamy relations between the EOF, CC and the QD of two qubits and the environment.

Notes on qubit phase space and discrete symplectic structures

International Nuclear Information System (INIS)

Livine, Etera R

2010-01-01

We start from Wootter's construction of discrete phase spaces and Wigner functions for qubits and more generally for finite-dimensional Hilbert spaces. We look at this framework from a non-commutative space perspective and we focus on the Moyal product and the differential calculus on these discrete phase spaces. In particular, the qubit phase space provides the simplest example of a four-point non-commutative phase space. We give an explicit expression of the Moyal bracket as a differential operator. We then compare the quantum dynamics encoded by the Moyal bracket to the classical dynamics: we show that the classical Poisson bracket does not satisfy the Jacobi identity thus leaving the Moyal bracket as the only consistent symplectic structure. We finally generalize our analysis to Hilbert spaces of prime dimensions d and their associated d x d phase spaces.

Exact non-Markovian master equations for multiple qubit systems: Quantum-trajectory approach

Science.gov (United States)

Chen, Yusui; You, J. Q.; Yu, Ting

2014-11-01

A wide class of exact master equations for a multiple qubit system can be explicitly constructed by using the corresponding exact non-Markovian quantum-state diffusion equations. These exact master equations arise naturally from the quantum decoherence dynamics of qubit system as a quantum memory coupled to a collective colored noisy source. The exact master equations are also important in optimal quantum control, quantum dissipation, and quantum thermodynamics. In this paper, we show that the exact non-Markovian master equation for a dissipative N -qubit system can be derived explicitly from the statistical average of the corresponding non-Markovian quantum trajectories. We illustrated our general formulation by an explicit construction of a three-qubit system coupled to a non-Markovian bosonic environment. This multiple qubit master equation offers an accurate time evolution of quantum systems in various domains, and paves the way to investigate the memory effect of an open system in a non-Markovian regime without any approximation.

The two Josephson junction flux qubit with large tunneling amplitude

International Nuclear Information System (INIS)

Shnurkov, V.I.; Soroka, A.A.; Mel'nik, S.I.

2008-01-01

In this paper we discuss solid-state nanoelectronic realizations of Josephson flux qubits with large tunneling amplitude between the two macroscopic states. The latter can be controlled via the height and form of the potential barrier, which is determined by quantum-state engineering of the flux qubit circuit. The simplest circuit of the flux qubit is a superconducting loop interrupted by a Josephson nanoscale tunnel junction. The tunneling amplitude between two macroscopically different states can be increased substantially by engineering of the qubit circuit if the tunnel junction is replaced by a ScS contact. However, only Josephson tunnel junctions are particularly suitable for large-scale integration circuits and quantum detectors with present-day technology. To overcome this difficulty we consider here a flux qubit with high energy-level separation between the 'ground' and 'excited' states, consisting of a superconducting loop with two low-capacitance Josephson tunnel junctions in series. We demonstrate that for real parameters of resonant superposition between the two macroscopic states the tunneling amplitude can reach values greater than 1 K. Analytical results for the tunneling amplitude obtained within the semiclassical approximation by the instanton technique show good correlation with a numerical solution

Fungible dynamics: There are only two types of entangling multiple-qubit interactions

International Nuclear Information System (INIS)

Bremner, Michael J.; Dodd, Jennifer L.; Nielsen, Michael A.; Bacon, Dave

2004-01-01

What interactions are sufficient to simulate arbitrary quantum dynamics in a composite quantum system? It has been shown that all two-body Hamiltonian evolutions can be simulated using any fixed two-body entangling n-qubit Hamiltonian and fast local unitaries. By entangling we mean that every qubit is coupled to every other qubit, if not directly, then indirectly via intermediate qubits. We extend this study to the case where interactions may involve more than two qubits at a time. We find necessary and sufficient conditions for an arbitrary n-qubit Hamiltonian to be dynamically universal, that is, able to simulate any other Hamiltonian acting on n qubits, possibly in an inefficient manner. We prove that an entangling Hamiltonian is dynamically universal if and only if it contains at least one coupling term involving an even number of interacting qubits. For odd entangling Hamiltonians, i.e., Hamiltonians with couplings that involve only an odd number of qubits, we prove that dynamic universality is possible on an encoded set of n-1 logical qubits. We further prove that an odd entangling Hamiltonian can simulate any other odd Hamiltonian and classify the algebras that such Hamiltonians generate. Thus, our results show that up to local unitary operations, there are only two fundamentally different types of entangling Hamiltonian on n qubits. We also demonstrate that, provided the number of qubits directly coupled by the Hamiltonian is bounded above by a constant, our techniques can be made efficient

Adaptive phase measurements in linear optical quantum computation

International Nuclear Information System (INIS)

Ralph, T C; Lund, A P; Wiseman, H M

2005-01-01

Photon counting induces an effective non-linear optical phase shift in certain states derived by linear optics from single photons. Although this non-linearity is non-deterministic, it is sufficient in principle to allow scalable linear optics quantum computation (LOQC). The most obvious way to encode a qubit optically is as a superposition of the vacuum and a single photon in one mode-so-called 'single-rail' logic. Until now this approach was thought to be prohibitively expensive (in resources) compared to 'dual-rail' logic where a qubit is stored by a photon across two modes. Here we attack this problem with real-time feedback control, which can realize a quantum-limited phase measurement on a single mode, as has been recently demonstrated experimentally. We show that with this added measurement resource, the resource requirements for single-rail LOQC are not substantially different from those of dual-rail LOQC. In particular, with adaptive phase measurements an arbitrary qubit state α vertical bar 0>+β vertical bar 1> can be prepared deterministically

Multi-qubit compensation sequences

International Nuclear Information System (INIS)

Tomita, Y; Merrill, J T; Brown, K R

2010-01-01

The Hamiltonian control of n qubits requires precision control of both the strength and timing of interactions. Compensation pulses relax the precision requirements by reducing unknown but systematic errors. Using composite pulse techniques designed for single qubits, we show that systematic errors for n-qubit systems can be corrected to arbitrary accuracy given either two non-commuting control Hamiltonians with identical systematic errors or one error-free control Hamiltonian. We also examine composite pulses in the context of quantum computers controlled by two-qubit interactions. For quantum computers based on the XY interaction, single-qubit composite pulse sequences naturally correct systematic errors. For quantum computers based on the Heisenberg or exchange interaction, the composite pulse sequences reduce the logical single-qubit gate errors but increase the errors for logical two-qubit gates.

QUANTUM INFORMATION. Coherent coupling between a ferromagnetic magnon and a superconducting qubit.

Science.gov (United States)

Tabuchi, Yutaka; Ishino, Seiichiro; Noguchi, Atsushi; Ishikawa, Toyofumi; Yamazaki, Rekishu; Usami, Koji; Nakamura, Yasunobu

2015-07-24

Rigidity of an ordered phase in condensed matter results in collective excitation modes spatially extending to macroscopic dimensions. A magnon is a quantum of such collective excitation modes in ordered spin systems. Here, we demonstrate the coherent coupling between a single-magnon excitation in a millimeter-sized ferromagnetic sphere and a superconducting qubit, with the interaction mediated by the virtual photon excitation in a microwave cavity. We obtain the coupling strength far exceeding the damping rates, thus bringing the hybrid system into the strong coupling regime. Furthermore, we use a parametric drive to realize a tunable magnon-qubit coupling scheme. Our approach provides a versatile tool for quantum control and measurement of the magnon excitations and may lead to advances in quantum information processing. Copyright © 2015, American Association for the Advancement of Science.

Quantum information processing beyond ten ion-qubits

International Nuclear Information System (INIS)

Monz, T.

2011-01-01

Successful processing of quantum information is, to a large degree, based on two aspects: a) the implementation of high-fidelity quantum gates, as well as b) avoiding or suppressing decoherence processes that destroy quantum information. The presented work shows our progress in the field of experimental quantum information processing over the last years: the implementation and characterisation of several quantum operations, amongst others the first realisation of the quantum Toffoli gate in an ion-trap based quantum computer. The creation of entangled states with up to 14 qubits serves as basis for investigations of decoherence processes. Based on the realised quantum operations as well as the knowledge about dominant noise processes in the employed apparatus, entanglement swapping as well as quantum operations within a decoherence-free subspace are demonstrated. (author) [de

Error-Transparent Quantum Gates for Small Logical Qubit Architectures

Science.gov (United States)

Kapit, Eliot

2018-02-01

One of the largest obstacles to building a quantum computer is gate error, where the physical evolution of the state of a qubit or group of qubits during a gate operation does not match the intended unitary transformation. Gate error stems from a combination of control errors and random single qubit errors from interaction with the environment. While great strides have been made in mitigating control errors, intrinsic qubit error remains a serious problem that limits gate fidelity in modern qubit architectures. Simultaneously, recent developments of small error-corrected logical qubit devices promise significant increases in logical state lifetime, but translating those improvements into increases in gate fidelity is a complex challenge. In this Letter, we construct protocols for gates on and between small logical qubit devices which inherit the parent device's tolerance to single qubit errors which occur at any time before or during the gate. We consider two such devices, a passive implementation of the three-qubit bit flip code, and the author's own [E. Kapit, Phys. Rev. Lett. 116, 150501 (2016), 10.1103/PhysRevLett.116.150501] very small logical qubit (VSLQ) design, and propose error-tolerant gate sets for both. The effective logical gate error rate in these models displays superlinear error reduction with linear increases in single qubit lifetime, proving that passive error correction is capable of increasing gate fidelity. Using a standard phenomenological noise model for superconducting qubits, we demonstrate a realistic, universal one- and two-qubit gate set for the VSLQ, with error rates an order of magnitude lower than those for same-duration operations on single qubits or pairs of qubits. These developments further suggest that incorporating small logical qubits into a measurement based code could substantially improve code performance.

Scheme for realizing quantum computation and quantum information transfer with superconducting qubits coupling to a 1D transmission line resonator

International Nuclear Information System (INIS)

Zhen-Gang, Shi; Xiong-Wen, Chen; Xi-Xiang, Zhu; Ke-Hui, Song

2009-01-01

This paper proposes a simple scheme for realizing one-qubit and two-qubit quantum gates as well as multiqubit entanglement based on dc-SQUID charge qubits through the control of their coupling to a 1D transmission line resonator (TLR). The TLR behaves effectively as a quantum data-bus mode of a harmonic oscillator, which has several practical advantages including strong coupling strength, reproducibility, immunity to 1/f noise, and suppressed spontaneous emission. In this protocol, the data-bus does not need to stay adiabatically in its ground state, which results in not only fast quantum operation, but also high-fidelity quantum information processing. Also, it elaborates the transfer process with the 1D transmission line. (general)

Teleportation of a two-qubit arbitrary unknown state using a four-qubit genuine entangled state with the combination of bell-state measurements

Energy Technology Data Exchange (ETDEWEB)

Dong, Li; Xiu, Xiao-Ming, E-mail: xiuxiaomingdl@126.com [Dalian University of Technology, School of Physics and Optoelectronic Technology (China); Ren, Yuan-Peng [Bohai University, Higher Professional Technical Institute (China); Gao, Ya-Jun [Bohai University, College of Mathematics and Physics (China); Yi, X. X. [Dalian University of Technology, School of Physics and Optoelectronic Technology (China)

2013-01-15

We propose a protocol transferring an arbitrary unknown two-qubit state using the quantum channel of a four-qubit genuine entangled state. Simplifying the four-qubit joint measurement to the combination of Bell-state measurements, it can be realized more easily with currently available technologies.

High-capacity quantum secure direct communication with two-photon six-qubit hyperentangled states

Science.gov (United States)

Wu, FangZhou; Yang, GuoJian; Wang, HaiBo; Xiong, Jun; Alzahrani, Faris; Hobiny, Aatef; Deng, FuGuo

2017-12-01

This study proposes the first high-capacity quantum secure direct communication (QSDC) with two-photon six-qubit hyper-entangled Bell states in two longitudinal momentum and polarization degrees of freedom (DOFs) of photon pairs, which can be generated using two 0.5 mm-thick type-I β barium borate crystal slabs aligned one behind the other and an eight-hole screen. The secret message can be independently encoded on the photon pairs with 64 unitary operations in all three DOFs. This protocol has a higher capacity than previous QSDC protocols because each photon pair can carry 6 bits of information, not just 2 or 4 bits. Our QSDC protocol decreases the influence of decoherence from environment noise by exploiting the decoy photons to check the security of the transmission of the first photon sequence. Compared with two-way QSDC protocols, our QSDC protocol is immune to an attack by an eavesdropper using Trojan horse attack strategies because it is a one-way quantum communication. The QSDC protocol has good applications in the future quantum communication because of all these features.

A Blueprint for Demonstrating Quantum Supremacy with Superconducting Qubits

Science.gov (United States)

Kechedzhi, Kostyantyn

2018-01-01

Long coherence times and high fidelity control recently achieved in scalable superconducting circuits paved the way for the growing number of experimental studies of many-qubit quantum coherent phenomena in these devices. Albeit full implementation of quantum error correction and fault tolerant quantum computation remains a challenge the near term pre-error correction devices could allow new fundamental experiments despite inevitable accumulation of errors. One such open question foundational for quantum computing is achieving the so called quantum supremacy, an experimental demonstration of a computational task that takes polynomial time on the quantum computer whereas the best classical algorithm would require exponential time and/or resources. It is possible to formulate such a task for a quantum computer consisting of less than a 100 qubits. The computational task we consider is to provide approximate samples from a non-trivial quantum distribution. This is a generalization for the case of superconducting circuits of ideas behind boson sampling protocol for quantum optics introduced by Arkhipov and Aaronson. In this presentation we discuss a proof-of-principle demonstration of such a sampling task on a 9-qubit chain of superconducting gmon qubits developed by Google. We discuss theoretical analysis of the driven evolution of the device resulting in output approximating samples from a uniform distribution in the Hilbert space, a quantum chaotic state. We analyze quantum chaotic characteristics of the output of the circuit and the time required to generate a sufficiently complex quantum distribution. We demonstrate that the classical simulation of the sampling output requires exponential resources by connecting the task of calculating the output amplitudes to the sign problem of the Quantum Monte Carlo method. We also discuss the detailed theoretical modeling required to achieve high fidelity control and calibration of the multi-qubit unitary evolution in the

Josephson phase qubit circuit for the evaluation of advanced tunnel barrier materials

Energy Technology Data Exchange (ETDEWEB)

Kline, Jeffrey S; Oh, Seongshik; Pappas, David P [National Institute of Standards and Technology, Boulder, CO 80305 (United States); Wang Haohua; Martinis, John M [Department of Physics, University of California, Santa Barbara, CA 93106 (United States)], E-mail: klinej@nist.gov

2009-01-15

We have found that crystalline Josephson junctions have problems with the control of critical current density that decrease the circuit yield. We present a superconducting quantum bit circuit designed to accommodate a factor of five variation in critical current density from one fabrication run to the next. The new design enables the evaluation of advanced tunnel barrier materials for superconducting quantum bits. Using this circuit design, we compare the performance of Josephson phase qubits fabricated with MgO and Al{sub 2}O{sub 3} advanced crystalline tunnel barriers to AlO{sub x} amorphous tunnel barrier qubits.

Quantum routing of single optical photons with a superconducting flux qubit

Science.gov (United States)

Xia, Keyu; Jelezko, Fedor; Twamley, Jason

2018-05-01

Interconnecting optical photons with superconducting circuits is a challenging problem but essential for building long-range superconducting quantum networks. We propose a hybrid quantum interface between the microwave and optical domains where the propagation of a single-photon pulse along a nanowaveguide is controlled in a coherent way by tuning the electromagnetically induced transparency window with the quantum state of a flux qubit mediated by the spin in a nanodiamond. The qubit can route a single-photon pulse using the nanodiamond into a quantum superposition of paths without the aid of an optical cavity—simplifying the setup. By preparing the flux qubit in a superposition state our cavityless scheme creates a hybrid state-path entanglement between a flying single optical photon and a static superconducting qubit.

Duality, phase structures, and dilemmas in symmetric quantum games

International Nuclear Information System (INIS)

Ichikawa, Tsubasa; Tsutsui, Izumi

2007-01-01

Symmetric quantum games for 2-player, 2-qubit strategies are analyzed in detail by using a scheme in which all pure states in the 2-qubit Hilbert space are utilized for strategies. We consider two different types of symmetric games exemplified by the familiar games, the Battle of the Sexes (BoS) and the Prisoners' Dilemma (PD). These two types of symmetric games are shown to be related by a duality map, which ensures that they share common phase structures with respect to the equilibria of the strategies. We find eight distinct phase structures possible for the symmetric games, which are determined by the classical payoff matrices from which the quantum games are defined. We also discuss the possibility of resolving the dilemmas in the classical BoS, PD, and the Stag Hunt (SH) game based on the phase structures obtained in the quantum games. It is observed that quantization cannot resolve the dilemma fully for the BoS, while it generically can for the PD and SH if appropriate correlations for the strategies of the players are provided

State determination for composite systems of two spatial qubits

International Nuclear Information System (INIS)

Lima, G; Torres-Ruiz, F A; Neves, L; Delgado, A; Saavedra, C; Padua, S

2007-01-01

In a recent letter [Phys. Rev. Lett. 94, 100501 (2005)], we presented a scheme for generating pure entangled states of spatial qudits using transverse correlations of parametric down-converted photons. Here we show how the modication of this scheme can be used to generate mixed states and we investigate the state determination for composite systems of two spatial qubits, motivated by the fact that quantum information protocols may be easier to be implemented for this case. By means of local operations on the twin photons we were able to perform the quantum tomography process to reconstruct the density matrix of a mixed state of two spatial qubits

Entanglement of mixed quantum states for qubits and qudit in double photoionization of atoms

Energy Technology Data Exchange (ETDEWEB)

Chakraborty, M., E-mail: bminakshi@yahoo.com [Department of Physics, Asansol Girls’ College, Asansol 713304 (India); Sen, S. [Department of Physics, Triveni Devi Bhalotia College, Raniganj 713347 (India)

2015-08-15

Highlights: • We study tripartite entanglement between two electronic qubits and an ionic qudit. • We study bipartite entanglement between any two subsystems of a tripartite system. • We have presented a quantitative application of entangled properties in Neon atom. - Abstract: Quantum entanglement and its paradoxical properties are genuine physical resources for various quantum information tasks like quantum teleportation, quantum cryptography, and quantum computer technology. The physical characteristic of the entanglement of quantum-mechanical states, both for pure and mixed, has been recognized as a central resource in various aspects of quantum information processing. In this article, we study the bipartite entanglement of one electronic qubit along with the ionic qudit and also entanglement between two electronic qubits. The tripartite entanglement properties also have been investigated between two electronic qubits and an ionic qudit. All these studies have been done for the single-step double photoionization from an atom following the absorption of a single photon without observing spin orbit interaction. The dimension of the Hilbert space of the qudit depends upon the electronic state of the residual photoion A{sup 2+}. In absence of SOI, when Russell–Saunders coupling (L–S coupling) is applicable, dimension of the qudit is equal to the spin multiplicity of A{sup 2+}. For estimations of entanglement and mixedness, we consider the Peres–Horodecki condition, concurrence, entanglement of formation, negativity, linear and von Neumann entropies. In case of L–S coupling, all the properties of a qubit–qudit system can be predicted merely with the knowledge of the spins of the target atom and the residual photoion.

Flipping qubits

International Nuclear Information System (INIS)

Martini De, F.; Sciarrino, F.; Sias, C.; Buzek, V.

2003-01-01

On a classical level the information can be represented by bits, each of which can be either 0 or 1. Quantum information, on the other hand, consists of qubits which can be represented as two-level quantum systems with one level labeled |0> and the other |1>. Unlike bits, qubits cannot only be in one of the two levels, but in any superposition of them as well. This superposition principle makes quantum information fundamentally different from its classical counterpart. One of the most striking difference between the classical and quantum information is as follows: it is not a problem to flip a classical bit, i.e., to change the value of a bit, a 0 to a 1 and vice versa. This is accomplished by a NOT gate. Flipping a qubit, however, is another matter: there exists the fundamental bound which prohibits to flip a qubit prepared in an arbitrary state |Ψ>=α|0> and to obtain the state |Ψ T >=β*|0>-α*|1> which is orthogonal to it, i.e., T |Ψ>=0. We experimentally realize the best possible approximation of the qubit flipping that achieves bounds imposed by complete positivity of quantum mechanics

A blueprint for demonstrating quantum supremacy with superconducting qubits.

Science.gov (United States)

Neill, C; Roushan, P; Kechedzhi, K; Boixo, S; Isakov, S V; Smelyanskiy, V; Megrant, A; Chiaro, B; Dunsworth, A; Arya, K; Barends, R; Burkett, B; Chen, Y; Chen, Z; Fowler, A; Foxen, B; Giustina, M; Graff, R; Jeffrey, E; Huang, T; Kelly, J; Klimov, P; Lucero, E; Mutus, J; Neeley, M; Quintana, C; Sank, D; Vainsencher, A; Wenner, J; White, T C; Neven, H; Martinis, J M

2018-04-13

A key step toward demonstrating a quantum system that can address difficult problems in physics and chemistry will be performing a computation beyond the capabilities of any classical computer, thus achieving so-called quantum supremacy. In this study, we used nine superconducting qubits to demonstrate a promising path toward quantum supremacy. By individually tuning the qubit parameters, we were able to generate thousands of distinct Hamiltonian evolutions and probe the output probabilities. The measured probabilities obey a universal distribution, consistent with uniformly sampling the full Hilbert space. As the number of qubits increases, the system continues to explore the exponentially growing number of states. Extending these results to a system of 50 qubits has the potential to address scientific questions that are beyond the capabilities of any classical computer. Copyright © 2018 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.

Scheme for secure swapping two unknown states of a photonic qubit and an electron-spin qubit using simultaneous quantum transmission and teleportation via quantum dots inside single-sided optical cavities

Energy Technology Data Exchange (ETDEWEB)

Heo, Jino [College of Electrical and Computer Engineering, Chungbuk National University, Chungdae-ro 1, Seowon-Gu, Cheongju (Korea, Republic of); Kang, Min-Sung [Center for Quantum Information, Korea Institute of Science and Technology (KIST), Seoul, 136-791 (Korea, Republic of); Hong, Chang-Ho [National Security Research Institute, P.O.Box 1, Yuseong, Daejeon, 34188 (Korea, Republic of); Choi, Seong-Gon [College of Electrical and Computer Engineering, Chungbuk National University, Chungdae-ro 1, Seowon-Gu, Cheongju (Korea, Republic of); Hong, Jong-Phil, E-mail: jongph@cbnu.ac.kr [College of Electrical and Computer Engineering, Chungbuk National University, Chungdae-ro 1, Seowon-Gu, Cheongju (Korea, Republic of)

2017-06-15

We propose a scheme for swapping two unknown states of a photon and electron spin confined to a charged quantum dot (QD) between two users by transferring a single photon. This scheme simultaneously transfers and teleports an unknown state (electron spin) between two users. For this bidirectional quantum communication, we utilize the interactions between a photonic and an electron-spin qubits of a QD located inside a single-sided optical cavity. Thus, our proposal using QD-cavity systems can obtain a certain success probability with high fidelity. Furthermore, compared to a previous scheme using cross-Kerr nonlinearities and homodyne detections, our scheme (using QD-cavity systems) can improve the feasibility under the decoherence effect in practice. - Highlights: • Design of Simultaneous quantum transmission and teleportation scheme via quantum dots and cavities. • We have developed the experimental feasibility of this scheme compared with the existing scheme. • Analysis of some benefits when our scheme is experimentally implemented using quantum dots and single-sided cavities.

Coherent quantum phase slip in two-component bosonic atomtronic circuits

International Nuclear Information System (INIS)

Gallemí, A; Mateo, A Muñoz; Mayol, R; Guilleumas, M

2016-01-01

Coherent quantum phase slip consists in the coherent transfer of vortices in superfluids. We investigate this phenomenon in two miscible coherently coupled components of a spinor Bose gas confined in a toroidal trap. After imprinting different vortex states, i.e. states with quantized circulation, on each component, we demonstrate that during the whole dynamics the system remains in a linear superposition of two current states in spite of the nonlinearity, and can be mapped onto a linear Josephson problem. We propose this system as a good candidate for the realization of a Mooij–Harmans qubit and remark its feasibility for implementation in current experiments with 87 Rb, since we have used values for the physical parameters currently available in laboratories. (paper)

Probing the quantum coherence of a nanomechanical resonator using a superconducting qubit: II. Implementation

Science.gov (United States)

Blencowe, M. P.; Armour, A. D.

2008-09-01

We describe a possible implementation of the nanomechanical quantum superposition generation and detection scheme described in the preceding, companion paper (Armour A D and Blencowe M P 2008 New. J. Phys. 10 095004). The implementation is based on the circuit quantum electrodynamics (QED) set-up, with the addition of a mechanical degree of freedom formed out of a suspended, doubly-clamped segment of the superconducting loop of a dc SQUID located directly opposite the centre conductor of a coplanar waveguide (CPW). The relative merits of two SQUID based qubit realizations are addressed, in particular a capacitively coupled charge qubit and inductively coupled flux qubit. It is found that both realizations are equally promising, with comparable qubit-mechanical resonator mode as well as qubit-microwave resonator mode coupling strengths.

Probing the quantum coherence of a nanomechanical resonator using a superconducting qubit: II. Implementation

International Nuclear Information System (INIS)

Blencowe, M P; Armour, A D

2008-01-01

We describe a possible implementation of the nanomechanical quantum superposition generation and detection scheme described in the preceding, companion paper (Armour A D and Blencowe M P 2008 New. J. Phys. 10 095004). The implementation is based on the circuit quantum electrodynamics (QED) set-up, with the addition of a mechanical degree of freedom formed out of a suspended, doubly-clamped segment of the superconducting loop of a dc SQUID located directly opposite the centre conductor of a coplanar waveguide (CPW). The relative merits of two SQUID based qubit realizations are addressed, in particular a capacitively coupled charge qubit and inductively coupled flux qubit. It is found that both realizations are equally promising, with comparable qubit-mechanical resonator mode as well as qubit-microwave resonator mode coupling strengths.

A quantum-dot spin qubit with coherence limited by charge noise and fidelity higher than 99.9%

Science.gov (United States)

Yoneda, Jun; Takeda, Kenta; Otsuka, Tomohiro; Nakajima, Takashi; Delbecq, Matthieu R.; Allison, Giles; Honda, Takumu; Kodera, Tetsuo; Oda, Shunri; Hoshi, Yusuke; Usami, Noritaka; Itoh, Kohei M.; Tarucha, Seigo

2018-02-01

The isolation of qubits from noise sources, such as surrounding nuclear spins and spin-electric susceptibility1-4, has enabled extensions of quantum coherence times in recent pivotal advances towards the concrete implementation of spin-based quantum computation. In fact, the possibility of achieving enhanced quantum coherence has been substantially doubted for nanostructures due to the characteristic high degree of background charge fluctuations5-7. Still, a sizeable spin-electric coupling will be needed in realistic multiple-qubit systems to address single-spin and spin-spin manipulations8-10. Here, we realize a single-electron spin qubit with an isotopically enriched phase coherence time (20 μs)11,12 and fast electrical control speed (up to 30 MHz) mediated by extrinsic spin-electric coupling. Using rapid spin rotations, we reveal that the free-evolution dephasing is caused by charge noise—rather than conventional magnetic noise—as highlighted by a 1/f spectrum extended over seven decades of frequency. The qubit exhibits superior performance with single-qubit gate fidelities exceeding 99.9% on average, offering a promising route to large-scale spin-qubit systems with fault-tolerant controllability.

Quantum correlations of coupled superconducting two-qubit system in various cavity environments

International Nuclear Information System (INIS)

Yu, Yanxia; Fu, Guolan; Guo, L.P.; Pan, Hui; Wang, Z.S.

2013-01-01

Highlights: •We investigate dynamic evolutions of quantum and classical correlations for coupled superconducting system with various cavity environments. •We show that the quantum discord continues to reflect quantum information. •A transition of quantum discord is founded between classical loss and quantum increasing of correlations for a purely dephasing mode. •We show that the environment-dependent models can delay the loss of quantum discord. •We find that the results depend strongly on the initial angle. -- Abstract: Dynamic evolutions of quantum discord, concurrence, and classical correlation are investigated in coupled superconducting system with various cavity environments, focusing on the two-qubit system at an initially entangling X-state and Y-state. We find that for a smaller photon number, the quantum discord, concurrence and classical correlation show damped oscillations for all different decay modes. Differently from the sudden death or the dark and bright periods emerging in evolving processing of the concurrence and classical correlation, however, the quantum discord decreases gradually to zero. The results reveal that the quantum entanglement and classical correlation are lost, but the quantum discord continues to reflect quantum information in the same evolving period. For a larger photon number, the oscillations disappear. It is surprised that there exists a transition of quantum discord between classical loss and quantum increasing of correlations for a purely dephasing mode. For a larger photon number in the Y-state, the transition disappears. Moreover, we show that the environment-dependent models can delay the loss of quantum discord. The results depend strongly on the initial angle, which provide a clue to control the quantum gate of superconducting circuit

Interfacing spin qubits in quantum dots and donors—hot, dense, and coherent

Science.gov (United States)

Vandersypen, L. M. K.; Bluhm, H.; Clarke, J. S.; Dzurak, A. S.; Ishihara, R.; Morello, A.; Reilly, D. J.; Schreiber, L. R.; Veldhorst, M.

2017-09-01

Semiconductor spins are one of the few qubit realizations that remain a serious candidate for the implementation of large-scale quantum circuits. Excellent scalability is often argued for spin qubits defined by lithography and controlled via electrical signals, based on the success of conventional semiconductor integrated circuits. However, the wiring and interconnect requirements for quantum circuits are completely different from those for classical circuits, as individual direct current, pulsed and in some cases microwave control signals need to be routed from external sources to every qubit. This is further complicated by the requirement that these spin qubits currently operate at temperatures below 100 mK. Here, we review several strategies that are considered to address this crucial challenge in scaling quantum circuits based on electron spin qubits. Key assets of spin qubits include the potential to operate at 1 to 4 K, the high density of quantum dots or donors combined with possibilities to space them apart as needed, the extremely long-spin coherence times, and the rich options for integration with classical electronics based on the same technology.

Qubits and quantum Hamiltonian computing performances for operating a digital Boolean 1/2-adder

Science.gov (United States)

Dridi, Ghassen; Faizy Namarvar, Omid; Joachim, Christian

2018-04-01

Quantum Boolean (1 + 1) digits 1/2-adders are designed with 3 qubits for the quantum computing (Qubits) and 4 quantum states for the quantum Hamiltonian computing (QHC) approaches. Detailed analytical solutions are provided to analyse the time operation of those different 1/2-adder gates. QHC is more robust to noise than Qubits and requires about the same amount of energy for running its 1/2-adder logical operations. QHC is faster in time than Qubits but its logical output measurement takes longer.

Multihop teleportation of two-qubit state via the composite GHZ–Bell channel

Energy Technology Data Exchange (ETDEWEB)

Zou, Zhen-Zhen [State Key Lab. of Millimeter Waves, Southeast University, Nanjing, 210096 (China); Yu, Xu-Tao, E-mail: yuxutao@seu.edu.cn [State Key Lab. of Millimeter Waves, Southeast University, Nanjing, 210096 (China); Gong, Yan-Xiao [Department of Physics, Southeast University, Nanjing, 211189 (China); Zhang, Zai-Chen [National Mobile Communications Research Lab, Southeast University, Nanjing, 210096 (China)

2017-01-15

A multihop teleportation protocol in quantum communication network is introduced to teleport an arbitrary two-qubit state, between two nodes without directly sharing entanglement pairs. Quantum channels are built among neighbor nodes based on a five-qubit entangled system composed of GHZ and Bell pairs. The von Neumann measurements in all intermediate nodes and the source node are implemented, and then the measurement outcomes are sent to the destination node independently. After collecting all the measurement outcomes at the destination node, an efficient method is proposed to calculate the unitary operations for transforming the receiver's states to the state teleported. Therefore, only adopting the proper unitary operations at the destination node, the desired quantum state can be recovered perfectly. The transmission flexibility and efficiency of quantum network with composite GHZ–Bell channel are improved by transmitting measurement outcomes of all nodes in parallelism and reducing hop-by-hop teleportation delay. - Highlights: • A multihop teleportation protocol is introduced to teleport two-qubit state. • Quantum channels are built by composite of GHZ and Bell pairs. • Measurement outcomes are sent to the destination node independently. • Destination node calculates and adopts unitary operations to recover initial state.

Quantum State Transmission in a Superconducting Charge Qubit-Atom Hybrid

Science.gov (United States)

Yu, Deshui; Valado, María Martínez; Hufnagel, Christoph; Kwek, Leong Chuan; Amico, Luigi; Dumke, Rainer

2016-01-01

Hybrids consisting of macroscopic superconducting circuits and microscopic components, such as atoms and spins, have the potential of transmitting an arbitrary state between different quantum species, leading to the prospective of high-speed operation and long-time storage of quantum information. Here we propose a novel hybrid structure, where a neutral-atom qubit directly interfaces with a superconducting charge qubit, to implement the qubit-state transmission. The highly-excited Rydberg atom located inside the gate capacitor strongly affects the behavior of Cooper pairs in the box while the atom in the ground state hardly interferes with the superconducting device. In addition, the DC Stark shift of the atomic states significantly depends on the charge-qubit states. By means of the standard spectroscopic techniques and sweeping the gate voltage bias, we show how to transfer an arbitrary quantum state from the superconducting device to the atom and vice versa. PMID:27922087

Spinor Slow Light and Two-Color Qubits

Science.gov (United States)

Yu, Ite; Lee, Meng-Jung; Ruseckas, Julius; Lee, Chin-Yuan; Kudriasov, Viaceslav; Chang, Kao-Fang; Cho, Hung-Wen; Juzeliunas, Gediminas; Yu, Ite A.

2015-05-01

We report the first experimental demonstration of two-component or spinor slow light (SSL) using a double tripod (DT) atom-light coupling scheme. The scheme involves three atomic ground states coupled to two excited states by six light fields. The oscillation due to the interaction between the two components was observed. SSL can be used to achieve high conversion efficiencies in the sum frequency generation and is a better method than the widely-used double- Λ scheme. On the basis of the stored light, our data showed that the DT scheme behaves like the two outcomes of an interferometer enabling precision measurements of frequency detuning. Furthermore, the single-photon SSL can be considered as the qubit with the superposition state of two frequency modes or, simply, as the two-color qubit. We experimentally demonstrated a possible application of the DT scheme as quantum memory/rotator for the two-color qubit. This work opens up a new direction in the EIT/slow light research. yu@phys.nthu.edu.tw

Generation of an N-qubit phase gate via atom—cavity nonidentical coupling

International Nuclear Information System (INIS)

Ying-Qiao, Zhang; Shou, Zhang

2009-01-01

A scheme for approximate generation of an N-qubit phase gate is proposed in cavity QED based on nonidentical coupling between the atoms and the cavity. The atoms interact with a highly detuned cavity-field mode, but quantum information does not transfer between the atoms and cavity field, and thus the cavity decay is negligible. The gate time does not rise with an increase in the number of qubits. With the choice of a smaller odd number l (related to atom–cavity coupling constants), the phase gate can be generated with a higher fidelity and a higher success probability in a shorter time (the gate time is much shorter than the atomic radiative lifetime and photon lifetime). When the number of qubits N exceeds certain small values, the fidelity and success probability rise slowly with an increase in the number of qubits N. When N → ∞, the fidelity and success probability infinitely approach 1, but never exceed 1. (general)

Quantum logic as superbraids of entangled qubit world lines

International Nuclear Information System (INIS)

Yepez, Jeffrey

2010-01-01

Presented is a topological representation of quantum logic that views entangled qubit spacetime histories (or qubit world lines) as a generalized braid, referred to as a superbraid. The crossing of world lines can be quantum-mechanical in nature, most conveniently expressed analytically with ladder-operator-based quantum gates. At a crossing, independent world lines can become entangled. Complicated superbraids are systematically reduced by recursively applying quantum skein relations. If the superbraid is closed (e.g., representing quantum circuits with closed-loop feedback, quantum lattice gas algorithms, loop or vacuum diagrams in quantum field theory), then one can decompose the resulting superlink into an entangled superposition of classical links. Thus, one can compute a superlink invariant, for example, the Jones polynomial for the square root of a classical knot.

Coupled Qubits for Next Generation Quantum Annealing: Novel Interactions

Science.gov (United States)

Samach, Gabriel; Weber, Steven; Hover, David; Rosenberg, Danna; Yoder, Jonilyn; Kim, David; Oliver, William D.; Kerman, Andrew J.

While the first generation of quantum annealers based on Josephson junction technology have been successfully engineered to represent arrays of spins in the quantum transverse-field Ising model, no circuit architecture to date has succeeded in emulating the more complicated non-stoquastic Hamiltonians of interest for next generation quantum annealing. Here, we present our recent results for tunable ZZ- and XX-coupling between high coherence superconducting flux qubits. We discuss the larger architectures these coupled two-qubit building blocks will enable, as well as comment on the limitations of such architectures. This research was funded by the Office of the Director of National Intelligence (ODNI), Intelligence Advanced Research Projects Activity (IARPA) and by the Assistant Secretary of Defense for Research & Engineering under Air Force Contract No. FA8721-05-C-0002. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of ODNI, IARPA, or the US Government.

Six-qubit two-photon hyperentangled cluster states: Characterization and application to quantum computation

International Nuclear Information System (INIS)

Vallone, Giuseppe; Donati, Gaia; Ceccarelli, Raino; Mataloni, Paolo

2010-01-01

Six-qubit cluster states built on the simultaneous entanglement of two photons in three independent degrees of freedom, that is, polarization and a double longitudinal momentum, have been recently demonstrated. We present here the peculiar entanglement properties of the linear cluster state |L-tildeC 6 > related to the three degrees of freedom. This state has been adopted to realize various kinds of controlled not (cnot) gates, obtaining high values of the fidelity of the expected output states for all considered cases. Our results demonstrate that these states may represent a promising approach toward scalable quantum computation in a medium-term time scale. The future perspectives of a hybrid approach to one-way quantum computing based on multiple degrees of freedom and multiphoton cluster states are also discussed in the conclusion of this article.

The Influences of Quantum Coherence on the Positive Work and the Efficiency of Quantum Heat Engine with Working Substance of Two-Qubit Heisenberg XXX Model

Science.gov (United States)

Peng, Hu-Ping; Fang, Mao-Fa; Yu, Min; Zou, Hong-Mei

2018-03-01

We study the influences of quantum coherence on the positive work and the efficiency of quantum heat engine (QHE) based on working substance of two-qubit Heisenberg model under a constant external magnetic field. By using analytical and numerical solution, we give the relation expressions for both the positive work and the efficiency with quantum coherence, and in detail discuss the effects of the quantum coherence on the positive work and the efficiency of QHE in the absence or presence of external magnetic field, respectively.

The Influences of Quantum Coherence on the Positive Work and the Efficiency of Quantum Heat Engine with Working Substance of Two-Qubit Heisenberg XXX Model

Science.gov (United States)

Peng, Hu-Ping; Fang, Mao-Fa; Yu, Min; Zou, Hong-Mei

2018-06-01

We study the influences of quantum coherence on the positive work and the efficiency of quantum heat engine (QHE) based on working substance of two-qubit Heisenberg model under a constant external magnetic field. By using analytical and numerical solution, we give the relation expressions for both the positive work and the efficiency with quantum coherence, and in detail discuss the effects of the quantum coherence on the positive work and the efficiency of QHE in the absence or presence of external magnetic field, respectively.

Controlling electron quantum dot qubits by spin-orbit interactions

International Nuclear Information System (INIS)

Stano, P.

2007-01-01

Single electron confined in a quantum dot is studied. A special emphasis is laid on the spin properties and the influence of spin-orbit interactions on the system. The study is motivated by a perspective exploitation of the spin of the confined electron as a qubit, a basic building block of in a foreseen quantum computer. The electron is described using the single band effective mass approximation, with parameters typical for a lateral electrostatically defined quantum dot in a GaAs/AlGaAs heterostructure. The stemming data for the analysis are obtained by numerical methods of exact diagonalization, however, all important conclusions are explained analytically. The work focuses on three main areas -- electron spectrum, phonon induced relaxation and electrically and magnetically induced Rabi oscillations. It is shown, how spin-orbit interactions influence the energy spectrum, cause finite spin relaxation and allow for all-electrical manipulation of the spin qubit. Among the main results is the discovery of easy passages, where the spin relaxation is unusually slow and the qubit is protected against parasitic electrical fields connected with manipulation by resonant electromagnetic fields. The results provide direct guide for manufacturing quantum dots with much improved properties, suitable for realizing single electron spin qubits. (orig.)

Controlling electron quantum dot qubits by spin-orbit interactions

Energy Technology Data Exchange (ETDEWEB)

Stano, P.

2007-01-15

Single electron confined in a quantum dot is studied. A special emphasis is laid on the spin properties and the influence of spin-orbit interactions on the system. The study is motivated by a perspective exploitation of the spin of the confined electron as a qubit, a basic building block of in a foreseen quantum computer. The electron is described using the single band effective mass approximation, with parameters typical for a lateral electrostatically defined quantum dot in a GaAs/AlGaAs heterostructure. The stemming data for the analysis are obtained by numerical methods of exact diagonalization, however, all important conclusions are explained analytically. The work focuses on three main areas -- electron spectrum, phonon induced relaxation and electrically and magnetically induced Rabi oscillations. It is shown, how spin-orbit interactions influence the energy spectrum, cause finite spin relaxation and allow for all-electrical manipulation of the spin qubit. Among the main results is the discovery of easy passages, where the spin relaxation is unusually slow and the qubit is protected against parasitic electrical fields connected with manipulation by resonant electromagnetic fields. The results provide direct guide for manufacturing quantum dots with much improved properties, suitable for realizing single electron spin qubits. (orig.)

Experiments on two-resonator circuit quantum electrodynamics. A superconducting quantum switch

International Nuclear Information System (INIS)

Hoffmann, Elisabeth Christiane Maria

2013-01-01

The field of cavity quantum electrodynamics (QED) studies the interaction between light and matter on a fundamental level. In typical experiments individual natural atoms are interacting with individual photons trapped in three-dimensional cavities. Within the last decade the prospering new field of circuit QED has been developed. Here, the natural atoms are replaced by artificial solid state quantum circuits offering large dipole moments which are coupled to quasi-onedimensional cavities providing a small mode volume and hence a large vacuum field strength. In our experiments Josephson junction based superconducting quantum bits are coupled to superconducting microwave resonators. In circuit QED the number of parameters that can be varied is increased and regimes that are not accessible using natural atoms can be entered and investigated. Apart from design flexibility and tunability of system parameters a particular advantage of circuit QED is the scalability to larger system size enabled by well developed micro- and nanofabrication tools. When scaling up the resonator-qubit systems beyond a few coupled circuits, the rapidly increasing number of interacting subsystems requires an active control and directed transmission of quantum signals. This can, for example, be achieved by implementing switchable coupling between two microwave resonators. To this end, a superconducting flux qubit is used to realize a suitable coupling between two microwave resonators, all working in the Gigahertz regime. The resulting device is called quantum switch. The flux qubit mediates a second order tunable and switchable coupling between the resonators. Depending on the qubit state, this coupling can compensate for the direct geometric coupling of the two resonators. As the qubit may also be in a quantum superposition state, the switch itself can be ''quantum'': it can be a superposition of ''on'' and ''off''. This work presents the theoretical background, the fabrication techniques and

Continuous-variable geometric phase and its manipulation for quantum computation in a superconducting circuit.

Science.gov (United States)

Song, Chao; Zheng, Shi-Biao; Zhang, Pengfei; Xu, Kai; Zhang, Libo; Guo, Qiujiang; Liu, Wuxin; Xu, Da; Deng, Hui; Huang, Keqiang; Zheng, Dongning; Zhu, Xiaobo; Wang, H

2017-10-20

Geometric phase, associated with holonomy transformation in quantum state space, is an important quantum-mechanical effect. Besides fundamental interest, this effect has practical applications, among which geometric quantum computation is a paradigm, where quantum logic operations are realized through geometric phase manipulation that has some intrinsic noise-resilient advantages and may enable simplified implementation of multi-qubit gates compared to the dynamical approach. Here we report observation of a continuous-variable geometric phase and demonstrate a quantum gate protocol based on this phase in a superconducting circuit, where five qubits are controllably coupled to a resonator. Our geometric approach allows for one-step implementation of n-qubit controlled-phase gates, which represents a remarkable advantage compared to gate decomposition methods, where the number of required steps dramatically increases with n. Following this approach, we realize these gates with n up to 4, verifying the high efficiency of this geometric manipulation for quantum computation.

Generation of three-qubit Greenberger-Horne-Zeilinger state of superconducting qubits via transitionless quantum driving

Science.gov (United States)

Zhang, Xu; Chen, Ye-Hong; Wu, Qi-Cheng; Shi, Zhi-Cheng; Song, Jie; Xia, Yan

2017-01-01

We present an efficient scheme to quickly generate three-qubit Greenberger-Horne-Zeilinger (GHZ) states by using three superconducting qubits (SQs) separated by two coplanar waveguide resonators (CPWRs) capacitively. The scheme is based on quantum Zeno dynamics and the approach of transitionless quantum driving to construct shortcuts to adiabatic passage. In order to highlight the advantages, we compare the present scheme with the traditional one with adiabatic passage. The comparison result shows the shortcut scheme is closely related to the adiabatic scheme but is better than it. Moreover, we discuss the influence of various decoherences with numerical simulation. The result proves that the present scheme is less sensitive to the energy relaxation, the decay of CPWRs and the deviations of the experimental parameters the same as the adiabatic passage. However, the shortcut scheme is effective and robust against the dephasing of SQs in comparison with the adiabatic scheme.

Hybrid quantum circuit with a superconducting qubit coupled to an electron spin ensemble

Energy Technology Data Exchange (ETDEWEB)

Kubo, Yuimaru; Grezes, Cecile; Vion, Denis; Esteve, Daniel; Bertet, Patrice [Quantronics Group, SPEC (CNRS URA 2464), CEA-Saclay, 91191 Gif-sur-Yvette (France); Diniz, Igor; Auffeves, Alexia [Institut Neel, CNRS, BP 166, 38042 Grenoble (France); Isoya, Jun-ichi [Research Center for Knowledge Communities, University of Tsukuba, 305-8550 Tsukuba (Japan); Jacques, Vincent; Dreau, Anais; Roch, Jean-Francois [LPQM (CNRS, UMR 8537), Ecole Normale Superieure de Cachan, 94235 Cachan (France)

2013-07-01

We report the experimental realization of a hybrid quantum circuit combining a superconducting qubit and an ensemble of electronic spins. The qubit, of the transmon type, is coherently coupled to the spin ensemble consisting of nitrogen-vacancy (NV) centers in a diamond crystal via a frequency-tunable superconducting resonator acting as a quantum bus. Using this circuit, we prepare arbitrary superpositions of the qubit states that we store into collective excitations of the spin ensemble and retrieve back into the qubit. We also report a new method for detecting the magnetic resonance of electronic spins at low temperature with a qubit using the hybrid quantum circuit, as well as our recent progress on spin echo experiments.

Simulating a topological transition in a superconducting phase qubit by fast adiabatic trajectories

Science.gov (United States)

Wang, Tenghui; Zhang, Zhenxing; Xiang, Liang; Gong, Zhihao; Wu, Jianlan; Yin, Yi

2018-04-01

The significance of topological phases has been widely recognized in the community of condensed matter physics. The well controllable quantum systems provide an artificial platform to probe and engineer various topological phases. The adiabatic trajectory of a quantum state describes the change of the bulk Bloch eigenstates with the momentum, and this adiabatic simulation method is however practically limited due to quantum dissipation. Here we apply the "shortcut to adiabaticity" (STA) protocol to realize fast adiabatic evolutions in the system of a superconducting phase qubit. The resulting fast adiabatic trajectories illustrate the change of the bulk Bloch eigenstates in the Su-Schrieffer-Heeger (SSH) model. A sharp transition is experimentally determined for the topological invariant of a winding number. Our experiment helps identify the topological Chern number of a two-dimensional toy model, suggesting the applicability of the fast adiabatic simulation method for topological systems.

Universal quantum computation in a semiconductor quantum wire network

International Nuclear Information System (INIS)

Sau, Jay D.; Das Sarma, S.; Tewari, Sumanta

2010-01-01

Universal quantum computation (UQC) using Majorana fermions on a two-dimensional topological superconducting (TS) medium remains an outstanding open problem. This is because the quantum gate set that can be generated by braiding of the Majorana fermions does not include any two-qubit gate and also no single-qubit π/8 phase gate. In principle, it is possible to create these crucial extra gates using quantum interference of Majorana fermion currents. However, it is not clear if the motion of the various order parameter defects (vortices, domain walls, etc.), to which the Majorana fermions are bound in a TS medium, can be quantum coherent. We show that these obstacles can be overcome using a semiconductor quantum wire network in the vicinity of an s-wave superconductor, by constructing topologically protected two-qubit gates and any arbitrary single-qubit phase gate in a topologically unprotected manner, which can be error corrected using magic-state distillation. Thus our strategy, using a judicious combination of topologically protected and unprotected gate operations, realizes UQC on a quantum wire network with a remarkably high error threshold of 0.14 as compared to 10 -3 to 10 -4 in ordinary unprotected quantum computation.

Two dimensional kicked quantum Ising model: dynamical phase transitions

International Nuclear Information System (INIS)

Pineda, C; Prosen, T; Villaseñor, E

2014-01-01

Using an efficient one and two qubit gate simulator operating on graphical processing units, we investigate ergodic properties of a quantum Ising spin 1/2 model on a two-dimensional lattice, which is periodically driven by a δ-pulsed transverse magnetic field. We consider three different dynamical properties: (i) level density, (ii) level spacing distribution of the Floquet quasienergy spectrum, and (iii) time-averaged autocorrelation function of magnetization components. Varying the parameters of the model, we found transitions between ordered (non-ergodic) and quantum chaotic (ergodic) phases, but the transitions between flat and non-flat spectral density do not correspond to transitions between ergodic and non-ergodic local observables. Even more surprisingly, we found good agreement of level spacing distribution with the Wigner surmise of random matrix theory for almost all values of parameters except where the model is essentially non-interacting, even in regions where local observables are not ergodic or where spectral density is non-flat. These findings question the versatility of the interpretation of level spacing distribution in many-body systems and stress the importance of the concept of locality. (paper)

Using qubits to reveal quantum signatures of an oscillator

Science.gov (United States)

Agarwal, Shantanu

In this thesis, we seek to study the qubit-oscillator system with the aim to identify and quantify inherent quantum features of the oscillator. We show that the quantum signatures of the oscillator get imprinted on the dynamics of the joint system. The two key features which we explore are the quantized energy spectrum of the oscillator and the non-classicality of the oscillator's wave function. To investigate the consequences of the oscillator's discrete energy spectrum, we consider the qubit to be coupled to the oscillator through the Rabi Hamiltonian. Recent developments in fabrication technology have opened up the possibility to explore parameter regimes which were conventionally inaccessible. Motivated by these advancements, we investigate in this thesis a parameter space where the qubit frequency is much smaller than the oscillator frequency and the Rabi frequency is allowed to be an appreciable fraction of the bare frequency of the oscillator. We use the adiabatic approximation to understand the dynamics in this quasi-degenerate qubit regime. By deriving a dressed master equation, we systematically investigate the effects of the environment on the system dynamics. We develop a spectroscopic technique, using which one can probe the steady state response of the driven and damped system. The spectroscopic signal clearly reveals the quantized nature of the oscillator's energy spectrum. We extend the adiabatic approximation, earlier developed only for the single qubit case, to a scenario where multiple qubits interact with the oscillator. Using the extended adiabatic approximation, we study the collapse and revival of multi-qubit observables. We develop analytic expressions for the revival signals which are in good agreement with the numerically evaluated results. Within the quantum restriction imposed by Heisenberg's uncertainty principle, the uncertainty in the position and momentum of an oscillator is minimum and shared equally when the oscillator is prepared

Three qubit entanglement within graphical Z/X-calculus

Directory of Open Access Journals (Sweden)

Bob Coecke

2011-03-01

Full Text Available The compositional techniques of categorical quantum mechanics are applied to analyse 3-qubit quantum entanglement. In particular the graphical calculus of complementary observables and corresponding phases due to Duncan and one of the authors is used to construct representative members of the two genuinely tripartite SLOCC classes of 3-qubit entangled states, GHZ and W. This nicely illustrates the respectively pairwise and global tripartite entanglement found in the W- and GHZ-class states. A new concept of supplementarity allows us to characterise inhabitants of the W class within the abstract diagrammatic calculus; these method extends to more general multipartite qubit states.

An elementary quantum network using robust nuclear spin qubits in diamond

Science.gov (United States)

Kalb, Norbert; Reiserer, Andreas; Humphreys, Peter; Blok, Machiel; van Bemmelen, Koen; Twitchen, Daniel; Markham, Matthew; Taminiau, Tim; Hanson, Ronald

Quantum registers containing multiple robust qubits can form the nodes of future quantum networks for computation and communication. Information storage within such nodes must be resilient to any type of local operation. Here we demonstrate multiple robust memories by employing five nuclear spins adjacent to a nitrogen-vacancy defect centre in diamond. We characterize the storage of quantum superpositions and their resilience to entangling attempts with the electron spin of the defect centre. The storage fidelity is found to be limited by the probabilistic electron spin reset after failed entangling attempts. Control over multiple memories is then utilized to encode states in decoherence protected subspaces with increased robustness. Furthermore we demonstrate memory control in two optically linked network nodes and characterize the storage capabilities of both memories in terms of the process fidelity with the identity. These results pave the way towards multi-qubit quantum algorithms in a remote network setting.

Flux qubit to a transmission line

Energy Technology Data Exchange (ETDEWEB)

Haeberlein, Max; Baust, Alexander; Zhong, Ling; Gross, Rudolf [Walther-Meissner-Institut, Bayerische Akademie der Wissenschaften, Garching (Germany); Physik-Department, TU Muenchen, Garching (Germany); Nanosystems Initiative Munich (NIM), Muenchen (Germany); Anderson, Gustav; Wang, Lujun; Eder, Peter; Fischer, Michael; Goetz, Jan; Xie, Edwar; Schwarz, Manuel; Wulschner, Karl Friedrich; Deppe, Frank; Fedorov, Kirill; Huebl, Hans; Menzel, Edwin [Walther-Meissner-Institut, Bayerische Akademie der Wissenschaften, Garching (Germany); Physik-Department, TU Muenchen, Garching (Germany); Marx, Achim [Walther-Meissner-Institut, Bayerische Akademie der Wissenschaften, Garching (Germany)

2015-07-01

Within the last decade, superconducting qubits coupled to microwave resonators have been extensively studied within the framework of quantum electrodynamics. Ultimately, quantum computing seems within reach in such architectures. However, error correction schemes are necessary to achieve the required fidelity in multi-qubit operations, drastically increasing the number of qubits involved. In this work, we couple a flux qubit to a transmission line where it interacts with itinerant microwave photons granting access to all-optical quantum computing. In this approach, travelling photons generate entanglement between two waveguides, containing the qubit information. In this presentation, we show experimental data on flux qubits coupled to transmission lines. Furthermore, we will discuss entanglement generation between two separate paths.

Generating stationary entangled states in superconducting qubits

International Nuclear Information System (INIS)

Zhang Jing; Liu Yuxi; Li Chunwen; Tarn, T.-J.; Nori, Franco

2009-01-01

When a two-qubit system is initially maximally entangled, two independent decoherence channels, one per qubit, would greatly reduce the entanglement of the two-qubit system when it reaches its stationary state. We propose a method on how to minimize such a loss of entanglement in open quantum systems. We find that the quantum entanglement of general two-qubit systems with controllable parameters can be controlled by tuning both the single-qubit parameters and the two-qubit coupling strengths. Indeed, the maximum fidelity F max between the stationary entangled state, ρ ∞ , and the maximally entangled state, ρ m , can be about 2/3≅max(tr(ρ ∞ ρ m ))=F max , corresponding to a maximum stationary concurrence, C max , of about 1/3≅C(ρ ∞ )=C max . This is significant because the quantum entanglement of the two-qubit system can be produced and kept, even for a long time. We apply our proposal to several types of two-qubit superconducting circuits and show how the entanglement of these two-qubit circuits can be optimized by varying experimentally controllable parameters.

Fisher information of a single qubit interacts with a spin-qubit in the presence of a magnetic field

Science.gov (United States)

Metwally, N.

2018-06-01

In this contribution, quantum Fisher information is utilized to estimate the parameters of a central qubit interacting with a single-spin qubit. The effect of the longitudinal, transverse and the rotating strengths of the magnetic field on the estimation degree is discussed. It is shown that, in the resonance case, the number of peaks and consequently the size of the estimation regions increase as the rotating magnetic field strength increases. The precision estimation of the central qubit parameters depends on the initial state settings of the central and the spin-qubit, either encode classical or quantum information. It is displayed that, the upper bounds of the estimation degree are large if the two qubits encode classical information. In the non-resonance case, the estimation degree depends on which of the longitudinal/transverse strength is larger. The coupling constant between the central qubit and the spin-qubit has a different effect on the estimation degree of the weight and the phase parameters, where the possibility of estimating the weight parameter decreases as the coupling constant increases, while it increases for the phase parameter. For large number of spin-particles, namely, we have a spin-bath particles, the upper bounds of the Fisher information with respect to the weight parameter of the central qubit decreases as the number of the spin particle increases. As the interaction time increases, the upper bounds appear at different initial values of the weight parameter.

Quantum Fisher information for a qubit system placed inside a dissipative cavity

International Nuclear Information System (INIS)

Berrada, K.; Abdel-Khalek, S.; Obada, A.-S.F.

2012-01-01

We study the time evolution of the quantum Fisher information of a system whose the dynamics is described by the phase-damped model. We discuss the correlation between the Fisher information and entanglement dynamics of a qubit and single-mode quantized field in a coherent state inside phase-damped cavity. Analytic results under certain parametric conditions are obtained, by means of which we analyze the influence of dissipation on the negativity and quantum Fisher information for different values of the estimator parameter. An interesting monotonic relation between the Fisher information and nonlocal correlation behavior is observed during the time evolution. -- Highlights: ► Relation between the Fisher information and nonlocal correlation dynamics. ► Definition of quantum Fisher information for the atomic density operator. ► Investigation of Fisher information and negativity for the phase-damped model. ► Analytic solution of the master equation for the atom-field system in cavity field. ► Quantum Fisher information may be helpful in quantum information tasks.

Tunable Hybrid Qubit in a Triple Quantum Dot

Science.gov (United States)

Wang, Bao-Chuan; Cao, Gang; Li, Hai-Ou; Xiao, Ming; Guo, Guang-Can; Hu, Xuedong; Jiang, Hong-Wen; Guo, Guo-Ping

2017-12-01

We experimentally demonstrate quantum-coherent dynamics of a triple-dot-based multielectron hybrid qubit. Pulsed experiments show that this system can be conveniently initialized, controlled, measured electrically, and has a good ratio Q ˜29 between the coherence time and gate time. Furthermore, the current multielectron hybrid qubit has an operation frequency that is tunable in a wide range, from 2 to about 15 GHz. We also provide a qualitative understanding of the experimental observations by mapping them onto a three-electron system. The demonstration of the high tunability in a triple dot system could be potentially useful for future quantum control.

Experimental comparison of two quantum computing architectures.

Science.gov (United States)

Linke, Norbert M; Maslov, Dmitri; Roetteler, Martin; Debnath, Shantanu; Figgatt, Caroline; Landsman, Kevin A; Wright, Kenneth; Monroe, Christopher

2017-03-28

We run a selection of algorithms on two state-of-the-art 5-qubit quantum computers that are based on different technology platforms. One is a publicly accessible superconducting transmon device (www. ibm.com/ibm-q) with limited connectivity, and the other is a fully connected trapped-ion system. Even though the two systems have different native quantum interactions, both can be programed in a way that is blind to the underlying hardware, thus allowing a comparison of identical quantum algorithms between different physical systems. We show that quantum algorithms and circuits that use more connectivity clearly benefit from a better-connected system of qubits. Although the quantum systems here are not yet large enough to eclipse classical computers, this experiment exposes critical factors of scaling quantum computers, such as qubit connectivity and gate expressivity. In addition, the results suggest that codesigning particular quantum applications with the hardware itself will be paramount in successfully using quantum computers in the future.

Quantum teleportation and information splitting via four-qubit cluster state and a Bell state

Science.gov (United States)

Ramírez, Marlon David González; Falaye, Babatunde James; Sun, Guo-Hua; Cruz-Irisson, M.; Dong, Shi-Hai

2017-10-01

Quantum teleportation provides a "bodiless" way of transmitting the quantum state from one object to another, at a distant location, using a classical communication channel and a previously shared entangled state. In this paper, we present a tripartite scheme for probabilistic teleportation of an arbitrary single qubit state, without losing the information of the state being teleported, via a fourqubit cluster state of the form | ϕ>1234 = α|0000>+ β|1010>+ γ|0101>- η|1111>, as the quantum channel, where the nonzero real numbers α, β, γ, and η satisfy the relation j αj2 + | β|2 + | γ|2 + | η|2 = 1. With the introduction of an auxiliary qubit with state |0>, using a suitable unitary transformation and a positive-operator valued measure (POVM), the receiver can recreate the state of the original qubit. An important advantage of the teleportation scheme demonstrated here is that, if the teleportation fails, it can be repeated without teleporting copies of the unknown quantum state, if the concerned parties share another pair of entangled qubit. We also present a protocol for quantum information splitting of an arbitrary two-particle system via the aforementioned cluster state and a Bell-state as the quantum channel. Problems related to security attacks were examined for both the cases and it was found that this protocol is secure. This protocol is highly efficient and easy to implement.

Optimal attacks on qubit-based Quantum Key Recycling

Science.gov (United States)

Leermakers, Daan; Škorić, Boris

2018-03-01

Quantum Key Recycling (QKR) is a quantum cryptographic primitive that allows one to reuse keys in an unconditionally secure way. By removing the need to repeatedly generate new keys, it improves communication efficiency. Škorić and de Vries recently proposed a QKR scheme based on 8-state encoding (four bases). It does not require quantum computers for encryption/decryption but only single-qubit operations. We provide a missing ingredient in the security analysis of this scheme in the case of noisy channels: accurate upper bounds on the required amount of privacy amplification. We determine optimal attacks against the message and against the key, for 8-state encoding as well as 4-state and 6-state conjugate coding. We provide results in terms of min-entropy loss as well as accessible (Shannon) information. We show that the Shannon entropy analysis for 8-state encoding reduces to the analysis of quantum key distribution, whereas 4-state and 6-state suffer from additional leaks that make them less effective. From the optimal attacks we compute the required amount of privacy amplification and hence the achievable communication rate (useful information per qubit) of qubit-based QKR. Overall, 8-state encoding yields the highest communication rates.

Unconditional polarization qubit quantum memory at room temperature

Science.gov (United States)

Namazi, Mehdi; Kupchak, Connor; Jordaan, Bertus; Shahrokhshahi, Reihaneh; Figueroa, Eden

2016-05-01

The creation of global quantum key distribution and quantum communication networks requires multiple operational quantum memories. Achieving a considerable reduction in experimental and cost overhead in these implementations is thus a major challenge. Here we present a polarization qubit quantum memory fully-operational at 330K, an unheard frontier in the development of useful qubit quantum technology. This result is achieved through extensive study of how optical response of cold atomic medium is transformed by the motion of atoms at room temperature leading to an optimal characterization of room temperature quantum light-matter interfaces. Our quantum memory shows an average fidelity of 86.6 +/- 0.6% for optical pulses containing on average 1 photon per pulse, thereby defeating any classical strategy exploiting the non-unitary character of the memory efficiency. Our system significantly decreases the technological overhead required to achieve quantum memory operation and will serve as a building block for scalable and technologically simpler many-memory quantum machines. The work was supported by the US-Navy Office of Naval Research, Grant Number N00141410801 and the Simons Foundation, Grant Number SBF241180. B. J. acknowledges financial assistance of the National Research Foundation (NRF) of South Africa.

Photonic quantum simulator for unbiased phase covariant cloning

Science.gov (United States)

Knoll, Laura T.; López Grande, Ignacio H.; Larotonda, Miguel A.

2018-01-01

We present the results of a linear optics photonic implementation of a quantum circuit that simulates a phase covariant cloner, using two different degrees of freedom of a single photon. We experimentally simulate the action of two mirrored 1→ 2 cloners, each of them biasing the cloned states into opposite regions of the Bloch sphere. We show that by applying a random sequence of these two cloners, an eavesdropper can mitigate the amount of noise added to the original input state and therefore, prepare clones with no bias, but with the same individual fidelity, masking its presence in a quantum key distribution protocol. Input polarization qubit states are cloned into path qubit states of the same photon, which is identified as a potential eavesdropper in a quantum key distribution protocol. The device has the flexibility to produce mirrored versions that optimally clone states on either the northern or southern hemispheres of the Bloch sphere, as well as to simulate optimal and non-optimal cloning machines by tuning the asymmetry on each of the cloning machines.

Application of Blind Quantum Computation to Two-Party Quantum Computation

Science.gov (United States)

Sun, Zhiyuan; Li, Qin; Yu, Fang; Chan, Wai Hong

2018-03-01

Blind quantum computation (BQC) allows a client who has only limited quantum power to achieve quantum computation with the help of a remote quantum server and still keep the client's input, output, and algorithm private. Recently, Kashefi and Wallden extended BQC to achieve two-party quantum computation which allows two parties Alice and Bob to perform a joint unitary transform upon their inputs. However, in their protocol Alice has to prepare rotated single qubits and perform Pauli operations, and Bob needs to have a powerful quantum computer. In this work, we also utilize the idea of BQC to put forward an improved two-party quantum computation protocol in which the operations of both Alice and Bob are simplified since Alice only needs to apply Pauli operations and Bob is just required to prepare and encrypt his input qubits.

Application of Blind Quantum Computation to Two-Party Quantum Computation

Science.gov (United States)

Sun, Zhiyuan; Li, Qin; Yu, Fang; Chan, Wai Hong

2018-06-01

Blind quantum computation (BQC) allows a client who has only limited quantum power to achieve quantum computation with the help of a remote quantum server and still keep the client's input, output, and algorithm private. Recently, Kashefi and Wallden extended BQC to achieve two-party quantum computation which allows two parties Alice and Bob to perform a joint unitary transform upon their inputs. However, in their protocol Alice has to prepare rotated single qubits and perform Pauli operations, and Bob needs to have a powerful quantum computer. In this work, we also utilize the idea of BQC to put forward an improved two-party quantum computation protocol in which the operations of both Alice and Bob are simplified since Alice only needs to apply Pauli operations and Bob is just required to prepare and encrypt his input qubits.

Maximal qubit violation of n-locality inequalities in a star-shaped quantum network

Science.gov (United States)

Andreoli, Francesco; Carvacho, Gonzalo; Santodonato, Luca; Chaves, Rafael; Sciarrino, Fabio

2017-11-01

Bell's theorem was a cornerstone for our understanding of quantum theory and the establishment of Bell non-locality played a crucial role in the development of quantum information. Recently, its extension to complex networks has been attracting growing attention, but a deep characterization of quantum behavior is still missing for this novel context. In this work we analyze quantum correlations arising in the bilocality scenario, that is a tripartite quantum network where the correlations between the parties are mediated by two independent sources of states. First, we prove that non-bilocal correlations witnessed through a Bell-state measurement in the central node of the network form a subset of those obtainable by means of a local projective measurement. This leads us to derive the maximal violation of the bilocality inequality that can be achieved by arbitrary two-qubit quantum states and arbitrary local projective measurements. We then analyze in details the relation between the violation of the bilocality inequality and the CHSH inequality. Finally, we show how our method can be extended to the n-locality scenario consisting of n two-qubit quantum states distributed among n+1 nodes of a star-shaped network.

Experiments on two-resonator circuit quantum electrodynamics. A superconducting quantum switch

Energy Technology Data Exchange (ETDEWEB)

Hoffmann, Elisabeth Christiane Maria

2013-05-29

The field of cavity quantum electrodynamics (QED) studies the interaction between light and matter on a fundamental level. In typical experiments individual natural atoms are interacting with individual photons trapped in three-dimensional cavities. Within the last decade the prospering new field of circuit QED has been developed. Here, the natural atoms are replaced by artificial solid state quantum circuits offering large dipole moments which are coupled to quasi-onedimensional cavities providing a small mode volume and hence a large vacuum field strength. In our experiments Josephson junction based superconducting quantum bits are coupled to superconducting microwave resonators. In circuit QED the number of parameters that can be varied is increased and regimes that are not accessible using natural atoms can be entered and investigated. Apart from design flexibility and tunability of system parameters a particular advantage of circuit QED is the scalability to larger system size enabled by well developed micro- and nanofabrication tools. When scaling up the resonator-qubit systems beyond a few coupled circuits, the rapidly increasing number of interacting subsystems requires an active control and directed transmission of quantum signals. This can, for example, be achieved by implementing switchable coupling between two microwave resonators. To this end, a superconducting flux qubit is used to realize a suitable coupling between two microwave resonators, all working in the Gigahertz regime. The resulting device is called quantum switch. The flux qubit mediates a second order tunable and switchable coupling between the resonators. Depending on the qubit state, this coupling can compensate for the direct geometric coupling of the two resonators. As the qubit may also be in a quantum superposition state, the switch itself can be ''quantum'': it can be a superposition of ''on'' and ''off''. This work

Quantum correlations dynamics of three-qubit states coupled to an XY spin chain: Role of coupling strengths

International Nuclear Information System (INIS)

Yin Shao-Ying; Song Jie; Xu Xue-Xin; Zhou Ke-Ya; Liu Shu-Tian; Liu Qing-Xin

2017-01-01

We investigate the prominent impacts of coupling strengths on the evolution of entanglement and quantum discord for a three-qubit system coupled to an XY spin-chain environment. In the case of a pure W state, more robust, even larger nonzero quantum correlations can be obtained by tailoring the coupling strengths between the qubits and the environment. For a mixed state consisting of the GHZ and W states, the dynamics of entanglement and quantum discord can characterize the critical point of quantum phase transition. Remarkably, a large nonzero quantum discord is generally retained, while the nonzero entanglement can only be obtained as the system-environment coupling satisfies certain conditions. We also find that the impact of each qubit’s coupling strength on the quantum correlation dynamics strongly depends on the variation schemes of the system-environment couplings. (paper)

Comment on 'Two-way protocols for quantum cryptography with a nonmaximally entangled qubit pair'

International Nuclear Information System (INIS)

Qin Sujuan; Gao Fei; Wen Qiaoyan; Guo Fenzhuo

2010-01-01

Three protocols of quantum cryptography with a nonmaximally entangled qubit pair [Phys. Rev. A 80, 022323 (2009)] were recently proposed by Shimizu, Tamaki, and Fukasaka. The security of these protocols is based on the quantum-mechanical constraint for a state transformation between nonmaximally entangled states. However, we find that the second protocol is vulnerable under the correlation-elicitation attack. An eavesdropper can obtain the encoded bit M although she has no knowledge about the random bit R.

Quantum teleportation and entanglement swapping of matter qubits with multiphoton signals

Energy Technology Data Exchange (ETDEWEB)

Torres, Juan Mauricio [Institut fuer Angewandte Physik, Technische Universitaet Darmstadt, D-64289 Germany (Germany); Departamento de Investigacion en Fisica, Universidad de Sonora, Hermosillo (Mexico); Bernad, Jozsef Zsolt; Alber, Gernot [Institut fuer Angewandte Physik, Technische Universitaet Darmstadt, D-64289 Germany (Germany)

2014-07-01

We introduce a probabilistic Bell measurement of atomic qubits based on two consecutive photonic field measurements of two single mode cavities with which the atoms interact in two separate stages. To this end, we solve the two-atoms Tavis-Cummings model and exploit the property that the antisymmetric Bell state is insensitive to the interaction with the field. We consider implementations for quantum teleportation and for entanglement swapping protocols both of which can be achieved with 25% success probability and with unit fidelity. We emphasize possible applications for hybrid quantum repeaters where the aforementioned quantum protocols play an essential role.

Trapped Ion Qubits

Energy Technology Data Exchange (ETDEWEB)

Maunz, Peter Lukas Wilhelm

2017-04-01

Qubits can be encoded in clock states of trapped ions. These states are well isolated from the environment resulting in long coherence times [1] while enabling efficient high-fidelity qubit interactions mediated by the Coulomb coupled motion of the ions in the trap. Quantum states can be prepared with high fidelity and measured efficiently using fluorescence detection. State preparation and detection with 99.93% fidelity have been realized in multiple systems [1,2]. Single qubit gates have been demonstrated below rigorous fault-tolerance thresholds [1,3]. Two qubit gates have been realized with more than 99.9% fidelity [4,5]. Quantum algorithms have been demonstrated on systems of 5 to 15 qubits [6–8].

Initialization of a spin qubit in a site-controlled nanowire quantum dot

International Nuclear Information System (INIS)

Lagoudakis, Konstantinos G; McMahon, Peter L; Fischer, Kevin A; Müller, Kai; Yamamoto, Yoshihisa; Vučković, Jelena; Puri, Shruti; Dan Dalacu; Poole, Philip J; Reimer, Michael E; Zwiller, Val

2016-01-01

A fault-tolerant quantum repeater or quantum computer using solid-state spin-based quantum bits will likely require a physical implementation with many spins arranged in a grid. Self-assembled quantum dots (QDs) have been established as attractive candidates for building spin-based quantum information processing devices, but such QDs are randomly positioned, which makes them unsuitable for constructing large-scale processors. Recent efforts have shown that QDs embedded in nanowires can be deterministically positioned in regular arrays, can store single charges, and have excellent optical properties, but so far there have been no demonstrations of spin qubit operations using nanowire QDs. Here we demonstrate optical pumping of individual spins trapped in site-controlled nanowire QDs, resulting in high-fidelity spin-qubit initialization. This represents the next step towards establishing spins in nanowire QDs as quantum memories suitable for use in a large-scale, fault-tolerant quantum computer or repeater based on all-optical control of the spin qubits. (paper)

Al transmon qubits on silicon-on-insulator for quantum device integration

Science.gov (United States)

Keller, Andrew J.; Dieterle, Paul B.; Fang, Michael; Berger, Brett; Fink, Johannes M.; Painter, Oskar

2017-07-01

We present the fabrication and characterization of an aluminum transmon qubit on a silicon-on-insulator substrate. Key to the qubit fabrication is the use of an anhydrous hydrofluoric vapor process which selectively removes the lossy silicon oxide buried underneath the silicon device layer. For a 5.6 GHz qubit measured dispersively by a 7.1 GHz resonator, we find T1 = 3.5 μs and T2* = 2.2 μs. This process in principle permits the co-fabrication of silicon photonic and mechanical elements, providing a route towards chip-scale integration of electro-opto-mechanical transducers for quantum networking of superconducting microwave quantum circuits. The additional processing steps are compatible with established fabrication techniques for aluminum transmon qubits on silicon.

Threshold quantum secret sharing based on single qubit

Science.gov (United States)

Lu, Changbin; Miao, Fuyou; Meng, Keju; Yu, Yue

2018-03-01

Based on unitary phase shift operation on single qubit in association with Shamir's ( t, n) secret sharing, a ( t, n) threshold quantum secret sharing scheme (or ( t, n)-QSS) is proposed to share both classical information and quantum states. The scheme uses decoy photons to prevent eavesdropping and employs the secret in Shamir's scheme as the private value to guarantee the correctness of secret reconstruction. Analyses show it is resistant to typical intercept-and-resend attack, entangle-and-measure attack and participant attacks such as entanglement swapping attack. Moreover, it is easier to realize in physic and more practical in applications when compared with related ones. By the method in our scheme, new ( t, n)-QSS schemes can be easily constructed using other classical ( t, n) secret sharing.

Complete quantum control of exciton qubits bound to isoelectronic centres.

Science.gov (United States)

Éthier-Majcher, G; St-Jean, P; Boso, G; Tosi, A; Klem, J F; Francoeur, S

2014-05-30

In recent years, impressive demonstrations related to quantum information processing have been realized. The scalability of quantum interactions between arbitrary qubits within an array remains however a significant hurdle to the practical realization of a quantum computer. Among the proposed ideas to achieve fully scalable quantum processing, the use of photons is appealing because they can mediate long-range quantum interactions and could serve as buses to build quantum networks. Quantum dots or nitrogen-vacancy centres in diamond can be coupled to light, but the former system lacks optical homogeneity while the latter suffers from a low dipole moment, rendering their large-scale interconnection challenging. Here, through the complete quantum control of exciton qubits, we demonstrate that nitrogen isoelectronic centres in GaAs combine both the uniformity and predictability of atomic defects and the dipole moment of semiconductor quantum dots. This establishes isoelectronic centres as a promising platform for quantum information processing.

Concurrence Measurement for the Two-Qubit Optical and Atomic States

Directory of Open Access Journals (Sweden)

Lan Zhou

2015-06-01

Full Text Available Concurrence provides us an effective approach to quantify entanglement, which is quite important in quantum information processing applications. In the paper, we mainly review some direct concurrence measurement protocols of the two-qubit optical or atomic system. We first introduce the concept of concurrence for a two-qubit system. Second, we explain the approaches of the concurrence measurement in both a linear and a nonlinear optical system. Third, we introduce some protocols for measuring the concurrence of the atomic entanglement system.

The top-transmon: a hybrid superconducting qubit for parity-protected quantum computation

International Nuclear Information System (INIS)

Hassler, F; Akhmerov, A R; Beenakker, C W J

2011-01-01

Qubits constructed from uncoupled Majorana fermions are protected from decoherence, but to perform a quantum computation this topological protection needs to be broken. Parity-protected quantum computation breaks the protection in a minimally invasive way, by coupling directly to the fermion parity of the system-irrespective of any quasiparticle excitations. Here, we propose to use a superconducting charge qubit in a transmission line resonator (the so-called transmon) to perform parity-protected rotations and read-out of a topological (top) qubit. The advantage over an earlier proposal using a flux qubit is that the coupling can be switched on and off with exponential accuracy, promising a reduced sensitivity to charge noise.

Nonlocality and entanglement in qubit systems

Energy Technology Data Exchange (ETDEWEB)

Batle, J [Departament de Fisica, Universitat de les Illes Balears, 07122 Palma de Mallorca (Spain); Casas, M, E-mail: vdfsjbv4@uib.es [Departament de Fisica and IFISC-CSIC, Universitat de les Illes Balears, 07122 Palma de Mallorca (Spain)

2011-11-04

Nonlocality and quantum entanglement constitute two special aspects of the quantum correlations existing in quantum systems, which are of paramount importance in quantum-information theory. Traditionally, they have been regarded as identical (equivalent, in fact, for pure two qubit states, that is, Gisin's Theorem), yet they constitute different resources. Describing nonlocality by means of the violation of several Bell inequalities, we obtain by direct optimization those states of two qubits that maximally violate a Bell inequality, in terms of their degree of mixture as measured by either their participation ratio R = 1/Tr({rho}{sup 2}) or their maximum eigenvalue {lambda}{sub max}. This optimum value is obtained as well, which coincides with previous results. Comparison with entanglement is performed too. An example of an application is given in the XY model. In this novel approximation, we also concentrate on the nonlocality for linear combinations of pure states of two qubits, providing a closed form for their maximal nonlocality measure. The case of Bell diagonal mixed states of two qubits is also extensively studied. Special attention concerning the connection between nonlocality and entanglement for mixed states of two qubits is paid to the so-called maximally entangled mixed states. Additional aspects for the case of two qubits are also described in detail. Since we deal with qubit systems, we will perform an analogous study for three qubits, employing similar tools. Relation between distillability and nonlocality is explored quantitatively for the whole space of states of three qubits. We finally extend our analysis to four-qubit systems, where nonlocality for generalized Greenberger-Horne-Zeilinger states of arbitrary number of parties is computed. (paper)

Micromachined integrated quantum circuit containing a superconducting qubit

Science.gov (United States)

Brecht, Teresa; Chu, Yiwen; Axline, Christopher; Pfaff, Wolfgang; Blumoff, Jacob; Chou, Kevin; Krayzman, Lev; Frunzio, Luigi; Schoelkopf, Robert

We demonstrate a functional multilayer microwave integrated quantum circuit (MMIQC). This novel hardware architecture combines the high coherence and isolation of three-dimensional structures with the advantages of integrated circuits made with lithographic techniques. We present fabrication and measurement of a two-cavity/one-qubit prototype, including a transmon coupled to a three-dimensional microwave cavity micromachined in a silicon wafer. It comprises a simple MMIQC with competitive lifetimes and the ability to perform circuit QED operations in the strong dispersive regime. Furthermore, the design and fabrication techniques that we have developed are extensible to more complex quantum information processing devices.

Steady-state entanglement and thermalization of coupled qubits in two common heat baths

Science.gov (United States)

Hu, Li-Zhen; Man, Zhong-Xiao; Xia, Yun-Jie

2018-03-01

In this work, we study the steady-state entanglement and thermalization of two coupled qubits embedded in two common baths with different temperatures. The common bath is relevant when the two qubits are difficult to be isolated to only contact with their local baths. With the quantum master equation constructed in the eigenstate representation of the coupled qubits, we have demonstrated the variations of steady-state entanglement with respect to various parameters of the qubits' system in both equilibrium and nonequilibrium cases of the baths. The coupling strength and energy detuning of the qubits as well as the temperature gradient of the baths are found to be beneficial to the enhancement of the entanglement. We note a dark state of the qubits that is free from time-evolution and its initial population can greatly influence the steady-state entanglement. By virtues of effective temperatures, we also study the thermalization of the coupled qubits and their variations with energy detuning.

Quantum Correlation Properties in Two Qubits One-axis Spin Squeezing Model

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Guo-Hui, Yang

2017-02-01

Using the concurrence (C) and quantum discord (QD) criterions, the quantum correlation properties in two qubits one-axis spin squeezing model with an external magnetic field are investigated. It is found that one obvious difference in the limit case T → 0 (ground state) is the sudden disappearance phenomenon (SDP) occured in the behavior of C, while not in QD. In order to further explain the SDP, we obtain the analytic expressions of ground state C and QD which reveal that the SDP is not really "entanglement sudden disappeared", it is decayed to zero very quickly. Proper tuning the parameters μ(the spin squeezing interaction in x direction) and Ω(the external magnetic field in z direction) not only can obviously broaden the scope of ground state C exists but also can enhance the value of ground state QD. For the finite temperature case, one evident difference is that the sudden birth phenomenon (SBP) is appeared in the evolution of C, while not in QD, and decreasing the coupling parameters μ or Ω can obviously prolong the time interval before entanglement sudden birth. The value of C and QD are both enhanced by increasing the parameters μ or Ω in finite temperature case. In addition, through investigating the effects of temperature T on the quantum correlation properties with the variation of Ω and μ, one can find that the temperature scope of C and QD exists are broadened with increasing the parameters μ or Ω, and one can obtain the quantum correlation at higher temperature through changing these parameters.

Influence of an anisotropic parabolic potential on the quantum dot qubit

International Nuclear Information System (INIS)

Zhao Cuilan; Cai Chunyu; Xiao Jingling

2013-01-01

To study the influence of an anisotropic parabolic potential (APP) on the properties of a quantum dot (QD) qubit, we obtain the eigenenergies and eigenfunctions of the ground and first excited state of an electron, which is strongly coupled to the bulk longitudinal optical (LO) phonons, in a QD under the influence of an APP by the celebrated Lee—Low—Pines (LLP) unitary transformation and the Pekar type variational (PTV) methods. Then, this kind of two-level quantum system can be excogitated to constitute a single qubit. When the electron locates at the superposition state of its related eigenfunctions, we get the time evolution of the electron's probability density. Finally, the influence of an APP on the QD qubit is investigated. The numerical calculations indicate that the probability density will oscillate periodically and it is a decreasing function of the effective confinement lengths of the APP in different directions. Whereas its oscillatory period is an increasing one and will diminish with enhancing the electron—phonon (EP) coupling strength. (semiconductor physics)

Proof-of-principle experimental realization of a qubit-like qudit-based quantum key distribution scheme

Science.gov (United States)

Wang, Shuang; Yin, Zhen-Qiang; Chau, H. F.; Chen, Wei; Wang, Chao; Guo, Guang-Can; Han, Zheng-Fu

2018-04-01

In comparison to qubit-based protocols, qudit-based quantum key distribution ones generally allow two cooperative parties to share unconditionally secure keys under a higher channel noise. However, it is very hard to prepare and measure the required quantum states in qudit-based protocols in general. One exception is the recently proposed highly error tolerant qudit-based protocol known as the Chau15 (Chau 2015 Phys. Rev. A 92 062324). Remarkably, the state preparation and measurement in this protocol can be done relatively easily since the required states are phase encoded almost like the diagonal basis states of a qubit. Here we report the first proof-of-principle demonstration of the Chau15 protocol. One highlight of our experiment is that its post-processing is based on practical one-way manner, while the original proposal in Chau (2015 Phys. Rev. A 92 062324) relies on complicated two-way post-processing, which is a great challenge in experiment. In addition, by manipulating time-bin qudit and measurement with a variable delay interferometer, our realization is extensible to qudit with high-dimensionality and confirms the experimental feasibility of the Chau15 protocol.

Quantum Logical Operations on Encoded Qubits

International Nuclear Information System (INIS)

Zurek, W.H.; Laflamme, R.

1996-01-01

We show how to carry out quantum logical operations (controlled-not and Toffoli gates) on encoded qubits for several encodings which protect against various 1-bit errors. This improves the reliability of these operations by allowing one to correct for 1-bit errors which either preexisted or occurred in the course of operation. The logical operations we consider allow one to carry out the vast majority of the steps in the quantum factoring algorithm. copyright 1996 The American Physical Society

Protecting unknown two-qubit entangled states by nesting Uhrig's dynamical decoupling sequences

International Nuclear Information System (INIS)

Mukhtar, Musawwadah; Soh, Wee Tee; Saw, Thuan Beng; Gong, Jiangbin

2010-01-01

Future quantum technologies rely heavily on good protection of quantum entanglement against environment-induced decoherence. A recent study showed that an extension of Uhrig's dynamical decoupling (UDD) sequence can (in theory) lock an arbitrary but known two-qubit entangled state to the Nth order using a sequence of N control pulses [Mukhtar et al., Phys. Rev. A 81, 012331 (2010)]. By nesting three layers of explicitly constructed UDD sequences, here we first consider the protection of unknown two-qubit states as superposition of two known basis states, without making assumptions of the system-environment coupling. It is found that the obtained decoherence suppression can be highly sensitive to the ordering of the three UDD layers and can be remarkably effective with the correct ordering. The detailed theoretical results are useful for general understanding of the nature of controlled quantum dynamics under nested UDD. As an extension of our three-layer UDD, it is finally pointed out that a completely unknown two-qubit state can be protected by nesting four layers of UDD sequences. This work indicates that when UDD is applicable (e.g., when the environment has a sharp frequency cutoff and when control pulses can be taken as instantaneous pulses), dynamical decoupling using nested UDD sequences is a powerful approach for entanglement protection.

Non-Poissonian quantum jumps of a fluxonium qubit due to quasiparticle excitations.

Science.gov (United States)

Vool, U; Pop, I M; Sliwa, K; Abdo, B; Wang, C; Brecht, T; Gao, Y Y; Shankar, S; Hatridge, M; Catelani, G; Mirrahimi, M; Frunzio, L; Schoelkopf, R J; Glazman, L I; Devoret, M H

2014-12-12

As the energy relaxation time of superconducting qubits steadily improves, nonequilibrium quasiparticle excitations above the superconducting gap emerge as an increasingly relevant limit for qubit coherence. We measure fluctuations in the number of quasiparticle excitations by continuously monitoring the spontaneous quantum jumps between the states of a fluxonium qubit, in conditions where relaxation is dominated by quasiparticle loss. Resolution on the scale of a single quasiparticle is obtained by performing quantum nondemolition projective measurements within a time interval much shorter than T₁, using a quantum-limited amplifier (Josephson parametric converter). The quantum jump statistics switches between the expected Poisson distribution and a non-Poissonian one, indicating large relative fluctuations in the quasiparticle population, on time scales varying from seconds to hours. This dynamics can be modified controllably by injecting quasiparticles or by seeding quasiparticle-trapping vortices by cooling down in a magnetic field.

Two-dimensional color-code quantum computation

International Nuclear Information System (INIS)

Fowler, Austin G.

2011-01-01

We describe in detail how to perform universal fault-tolerant quantum computation on a two-dimensional color code, making use of only nearest neighbor interactions. Three defects (holes) in the code are used to represent logical qubits. Triple-defect logical qubits are deformed into isolated triangular sections of color code to enable transversal implementation of all single logical qubit Clifford group gates. Controlled-NOT (CNOT) is implemented between pairs of triple-defect logical qubits via braiding.

Universal quantum gates on electron-spin qubits with quantum dots inside single-side optical microcavities.

Science.gov (United States)

Wei, Hai-Rui; Deng, Fu-Guo

2014-01-13

We present some compact quantum circuits for a deterministic quantum computing on electron-spin qubits assisted by quantum dots inside single-side optical microcavities, including the CNOT, Toffoli, and Fredkin gates. They are constructed by exploiting the giant optical Faraday rotation induced by a single-electron spin in a quantum dot inside a single-side optical microcavity as a result of cavity quantum electrodynamics. Our universal quantum gates have some advantages. First, all the gates are accomplished with a success probability of 100% in principle. Second, our schemes require no additional electron-spin qubits and they are achieved by some input-output processes of a single photon. Third, our circuits for these gates are simple and economic. Moreover, our devices for these gates work in both the weak coupling and the strong coupling regimes, and they are feasible in experiment.

High threshold distributed quantum computing with three-qubit nodes

International Nuclear Information System (INIS)

Li Ying; Benjamin, Simon C

2012-01-01

In the distributed quantum computing paradigm, well-controlled few-qubit ‘nodes’ are networked together by connections which are relatively noisy and failure prone. A practical scheme must offer high tolerance to errors while requiring only simple (i.e. few-qubit) nodes. Here we show that relatively modest, three-qubit nodes can support advanced purification techniques and so offer robust scalability: the infidelity in the entanglement channel may be permitted to approach 10% if the infidelity in local operations is of order 0.1%. Our tolerance of network noise is therefore an order of magnitude beyond prior schemes, and our architecture remains robust even in the presence of considerable decoherence rates (memory errors). We compare the performance with that of schemes involving nodes of lower and higher complexity. Ion traps, and NV-centres in diamond, are two highly relevant emerging technologies: they possess the requisite properties of good local control, rapid and reliable readout, and methods for entanglement-at-a-distance. (paper)

Influence of qubit displacements on quantum logic operations in a silicon-based quantum computer with constant interaction

International Nuclear Information System (INIS)

Kamenev, D. I.; Berman, G. P.; Tsifrinovich, V. I.

2006-01-01

The errors caused by qubit displacements from their prescribed locations in an ensemble of spin chains are estimated analytically and calculated numerically for a quantum computer based on phosphorus donors in silicon. We show that it is possible to polarize (initialize) the nuclear spins even with displaced qubits by using controlled-NOT gates between the electron and nuclear spins of the same phosphorus atom. However, a controlled-NOT gate between the displaced electron spins is implemented with large error because of the exponential dependence of exchange interaction constant on the distance between the qubits. If quantum computation is implemented on an ensemble of many spin chains, the errors can be small if the number of chains with displaced qubits is small

Atomic Evolution and Entanglement of Two Qubits in Photon Superfluid

Science.gov (United States)

Yin, Miao; Zhang, Xiongfeng; Deng, Yunlong; Deng, Huaqiu

2018-03-01

By using reservoir theory, we investigate the evolution of an atom placed in photon superfluid and study the entanglement properties of two qubits interacting with photon superfluid. It is found that the atomic decay rate in photon superfluid changes periodically with position of the atom and the decay rate can be inhibited compared to that in usual electromagnetic environment without photon superfluid. It is also found that when two atoms are separately immersed in their own local photon-superfluid reservoir, the entanglement sudden death or birth occurs or not only depends on the initial state of the qubits. What is more, we find a possible case that the concurrence between two qubits can remain a constant value by choosing proper values of parameters of the system, which may provide a new way to preserve quantum entanglement.

Exact synthesis of three-qubit quantum circuits from non-binary quantum gates

Science.gov (United States)

Yang, Guowu; Hung, William N. N.; Song, Xiaoyu; Perkowski, Marek A.

2010-04-01

Because of recent nano-technological advances, nano-structured systems have become highly ordered, making it quantum computing schemas possible. We propose an approach to optimally synthesise quantum circuits from non-permutative quantum gates such as controlled-square-root-of-not (i.e., controlled-V). Our approach reduces the synthesis problem to multiple-valued optimisation and uses group theory. We devise a novel technique that transforms the quantum logic synthesis problem from a multi-valued constrained optimisation problem to a permutable representation. The transformation enables us to use group theory to exploit the symmetric properties of the synthesis problem. Assuming a cost of one for each two-qubit gate, we found all reversible circuits with quantum costs of 4, 5, 6, etc., and give another algorithm to realise these reversible circuits with quantum gates. The approach can be used for both binary permutative deterministic circuits and probabilistic circuits such as controlled random-number generators and hidden Markov models.

Accurate Control of Josephson Phase Qubits

Science.gov (United States)

2016-04-14

for Bits and Atoms and Department of Physics, MIT, Cambridge , Massachusetts 02139, USA 2Solid State and Photonics Laboratory, Stanford University...computing to simulate tun- neling effects in Josephson junction qubits, illustrating how quantum computing is useful in modeling and simulating the...Computation and Quantum Information ~ Cambridge University Press, Cambridge , 2000!. 2 J. I. Cirac and P. Zoller, Phys. Rev. Lett. 74, 4091 ~1995!. 3 Y

Creation of Two-Particle Entanglement in Open Macroscopic Quantum Systems

Directory of Open Access Journals (Sweden)

M. Merkli

2012-01-01

Full Text Available We consider an open quantum system of N not directly interacting spins (qubits in contact with both local and collective thermal environments. The qubit-environment interactions are energy conserving. We trace out the variables of the thermal environments and N−2 qubits to obtain the time-dependent reduced density matrix for two arbitrary qubits. We numerically simulate the reduced dynamics and the creation of entanglement (concurrence as a function of the parameters of the thermal environments and the number of qubits, N. Our results demonstrate that the two-qubit entanglement generally decreases as N increases. We show analytically that, in the limit N→∞, no entanglement can be created. This indicates that collective thermal environments cannot create two-qubit entanglement when many qubits are located within a region of the size of the environment coherence length. We discuss possible relevance of our consideration to recent quantum information devices and biosystems.

Quantum Dense Coding About a Two-Qubit Heisenberg XYZ Model

Science.gov (United States)

Xu, Hui-Yun; Yang, Guo-Hui

2017-09-01

By taking into account the nonuniform magnetic field, the quantum dense coding with thermal entangled states of a two-qubit anisotropic Heisenberg XYZ chain are investigated in detail. We mainly show the different properties about the dense coding capacity ( χ) with the changes of different parameters. It is found that dense coding capacity χ can be enhanced by decreasing the magnetic field B, the degree of inhomogeneity b and temperature T, or increasing the coupling constant along z-axis J z . In addition, we also find χ remains the stable value as the change of the anisotropy of the XY plane Δ in a certain temperature condition. Through studying different parameters effect on χ, it presents that we can properly turn the values of B, b, J z , Δ or adjust the temperature T to obtain a valid dense coding capacity ( χ satisfies χ > 1). Moreover, the temperature plays a key role in adjusting the value of dense coding capacity χ. The valid dense coding capacity could be always obtained in the lower temperature-limit case.

Remote interactions on two distributed quantum systems: nonlocal unambiguous quantum-state discrimination

International Nuclear Information System (INIS)

Chen Libing; Jin Ruibo; Lu Hong

2008-01-01

Remote quantum-state discrimination is a critical step for the implementation of quantum communication network and distributed quantum computation. We present a protocol for remotely implementing the unambiguous discrimination between nonorthogonal states using quantum entanglements, local operations, and classical communications. This protocol consists of a remote generalized measurement described by a positive operator valued measurement (POVM). We explicitly construct the required remote POVM. The remote POVM can be realized by performing a nonlocal controlled-rotation operation on two spatially separated qubits, one is an ancillary qubit and the other is the qubit which is encoded by two nonorthogonal states to be distinguished, and a conventional local Von Neumann orthogonal measurement on the ancilla. The particular pair of states that can be remotely and unambiguously distinguished is specified by the state of the ancilla. The probability of successful discrimination is not optimal for all admissible pairs. However, for some subset it can be very close to an optimal value in an ordinary local POVM

Dynamics of interacting qubits coupled to a common bath: Non-Markovian quantum-state-diffusion approach

International Nuclear Information System (INIS)

Zhao Xinyu; Jing Jun; Corn, Brittany; Yu Ting

2011-01-01

Non-Markovian dynamics is studied for two interacting qubits strongly coupled to a dissipative bosonic environment. We derive a non-Markovian quantum-state-diffusion (QSD) equation for the coupled two-qubit system without any approximations, and in particular, without the Markov approximation. As an application and illustration of our derived time-local QSD equation, we investigate the temporal behavior of quantum coherence dynamics. In particular, we find a strongly non-Markovian regime where entanglement generation is significantly modulated by the environmental memory. Additionally, we study residual entanglement in the steady state by analyzing the steady-state solution of the QSD equation. Finally, we discuss an approximate QSD equation.

Environment-assisted error correction of single-qubit phase damping

International Nuclear Information System (INIS)

Trendelkamp-Schroer, Benjamin; Helm, Julius; Strunz, Walter T.

2011-01-01

Open quantum system dynamics of random unitary type may in principle be fully undone. Closely following the scheme of environment-assisted error correction proposed by Gregoratti and Werner [J. Mod. Opt. 50, 915 (2003)], we explicitly carry out all steps needed to invert a phase-damping error on a single qubit. Furthermore, we extend the scheme to a mixed-state environment. Surprisingly, we find cases for which the uncorrected state is closer to the desired state than any of the corrected ones.

Design and experimental realization of an optimal scheme for teleportation of an n-qubit quantum state

Science.gov (United States)

Sisodia, Mitali; Shukla, Abhishek; Thapliyal, Kishore; Pathak, Anirban

2017-12-01

An explicit scheme (quantum circuit) is designed for the teleportation of an n-qubit quantum state. It is established that the proposed scheme requires an optimal amount of quantum resources, whereas larger amount of quantum resources have been used in a large number of recently reported teleportation schemes for the quantum states which can be viewed as special cases of the general n-qubit state considered here. A trade-off between our knowledge about the quantum state to be teleported and the amount of quantum resources required for the same is observed. A proof-of-principle experimental realization of the proposed scheme (for a 2-qubit state) is also performed using 5-qubit superconductivity-based IBM quantum computer. The experimental results show that the state has been teleported with high fidelity. Relevance of the proposed teleportation scheme has also been discussed in the context of controlled, bidirectional, and bidirectional controlled state teleportation.

Multi-party quantum key agreement with five-qubit brown states

Science.gov (United States)

Cai, Tao; Jiang, Min; Cao, Gang

2018-05-01

In this paper, we propose a multi-party quantum key agreement protocol with five-qubit brown states and single-qubit measurements. Our multi-party protocol ensures each participant to contribute equally to the agreement key. Each party performs three single-qubit unitary operations on three qubits of each brown state. Finally, by measuring brown states and decoding the measurement results, all participants can negotiate a shared secret key without classical bits exchange between them. With the analysis of security, our protocol demonstrates that it can resist against both outsider and participant attacks. Compared with other schemes, it also possesses a higher information efficiency. In terms of physical operation, it requires single-qubit measurements only which weakens the hardware requirements of participant and has a better operating flexibility.

Stationary states of two-level open quantum systems

International Nuclear Information System (INIS)

Gardas, Bartlomiej; Puchala, Zbigniew

2011-01-01

A problem of finding stationary states of open quantum systems is addressed. We focus our attention on a generic type of open system: a qubit coupled to its environment. We apply the theory of block operator matrices and find stationary states of two-level open quantum systems under certain conditions applied on both the qubit and the surrounding.

Orbital hyperfine interaction and qubit dephasing in carbon nanotube quantum dots

Science.gov (United States)

Palyi, Andras; Csiszar, Gabor

2015-03-01

Hyperfine interaction (HF) is of key importance for the functionality of solid-state quantum information processing, as it affects qubit coherence and enables nuclear-spin quantum memories. In this work, we complete the theory of the basic hyperfine interaction mechanisms (Fermi contact, dipolar, orbital) in carbon nanotube quantum dots by providing a theoretical description of the orbital HF. We find that orbital HF induces an interaction between the nuclear spins of the nanotube lattice and the valley degree of freedom of the electrons confined in the quantum dot. We show that the resulting nuclear-spin-electron-valley interaction (i) is approximately of Ising type, (ii) is essentially local, in the sense that an effective atomic interaction strength can be defined, and (iii) has a strength that is comparable to the combined strength of Fermi contact and dipolar interactions. We argue that orbital HF provides a new decoherence mechanism for single-electron valley qubits and spin-valley qubits in a range of multi-valley materials. We explicitly evaluate the corresponding inhomogeneous dephasing time T2* for a nanotube-based valley qubit. We acknowledge funding from the EU Marie Curie CIG-293834, OTKA Grant PD 100373, and EU ERC Starting Grant CooPairEnt 258789. AP is supported by the Janos Bolyai Scholarship of the Hungarian Academy of Sciences.

Two-way and three-way negativities of three-qubit entangled states

International Nuclear Information System (INIS)

Sharma, S. Shelly; Sharma, N. K.

2007-01-01

We propose to quantify three-qubit entanglement using global negativity along with K-way negativities, where K=2 and 3. The principle underlying the definition of K-way negativity for pure and mixed states of N subsystems is a positive partial transpose sufficient condition. However, K-way partial transpose with respect to a subsystem is defined so as to shift the focus to K-way coherences instead of K subsystems of the composite system. A quantum state of a three-qubit system is characterized by the coherences measured by global, two-way, and three-way negativities. For a canonical state of three-qubit system, entanglement measures for genuine tripartite entanglement, W-like entanglement, and bipartite entanglement can be related to two-way and three-way negativities

Fast reconstruction of high-qubit-number quantum states via low-rate measurements

Science.gov (United States)

Li, K.; Zhang, J.; Cong, S.

2017-07-01

Due to the exponential complexity of the resources required by quantum state tomography (QST), people are interested in approaches towards identifying quantum states which require less effort and time. In this paper, we provide a tailored and efficient method for reconstructing mixed quantum states up to 12 (or even more) qubits from an incomplete set of observables subject to noises. Our method is applicable to any pure or nearly pure state ρ and can be extended to many states of interest in quantum information processing, such as a multiparticle entangled W state, Greenberger-Horne-Zeilinger states, and cluster states that are matrix product operators of low dimensions. The method applies the quantum density matrix constraints to a quantum compressive sensing optimization problem and exploits a modified quantum alternating direction multiplier method (quantum-ADMM) to accelerate the convergence. Our algorithm takes 8 ,35 , and 226 seconds, respectively, to reconstruct superposition state density matrices of 10 ,11 ,and12 qubits with acceptable fidelity using less than 1 % of measurements of expectation. To our knowledge it is the fastest realization that people can achieve using a normal desktop. We further discuss applications of this method using experimental data of mixed states obtained in an ion trap experiment of up to 8 qubits.

Controlled phase gate for solid-state charge-qubit architectures

International Nuclear Information System (INIS)

Schirmer, S.G.; Oi, D.K.L.; Greentree, Andrew D.

2005-01-01

We describe a mechanism for realizing a controlled phase gate for solid-state charge qubits. By augmenting the positionally defined qubit with an auxiliary state, and changing the charge distribution in the three-dot system, we are able to effectively switch the Coulombic interaction, effecting an entangling gate. We consider two architectures, and numerically investigate their robustness to gate noise

Ultrafast quantum computation in ultrastrongly coupled circuit QED systems

Science.gov (United States)

Wang, Yimin; Guo, Chu; Zhang, Guo-Qiang; Wang, Gangcheng; Wu, Chunfeng

2017-01-01

The latest technological progress of achieving the ultrastrong-coupling regime in circuit quantum electrodynamics (QED) systems has greatly promoted the developments of quantum physics, where novel quantum optics phenomena and potential computational benefits have been predicted. Here, we propose a scheme to accelerate the nontrivial two-qubit phase gate in a circuit QED system, where superconducting flux qubits are ultrastrongly coupled to a transmission line resonator (TLR), and two more TLRs are coupled to the ultrastrongly-coupled system for assistant. The nontrivial unconventional geometric phase gate between the two flux qubits is achieved based on close-loop displacements of the three-mode intracavity fields. Moreover, as there are three resonators contributing to the phase accumulation, the requirement of the coupling strength to realize the two-qubit gate can be reduced. Further reduction in the coupling strength to achieve a specific controlled-phase gate can be realized by adding more auxiliary resonators to the ultrastrongly-coupled system through superconducting quantum interference devices. We also present a study of our scheme with realistic parameters considering imperfect controls and noisy environment. Our scheme possesses the merits of ultrafastness and noise-tolerance due to the advantages of geometric phases. PMID:28281654

Spin-orbit mediated control of spin qubits

DEFF Research Database (Denmark)

Flindt, Christian; Sørensen, A.S; Flensberg, Karsten

2006-01-01

We propose to use the spin-orbit interaction as a means to control electron spins in quantum dots, enabling both single-qubit and two-qubit operations. Very fast single-qubit operations may be achieved by temporarily displacing the electrons. For two-qubit operations the coupling mechanism is bas...... on a combination of the spin-orbit coupling and the mutual long-ranged Coulomb interaction. Compared to existing schemes using the exchange coupling, the spin-orbit induced coupling is less sensitive to random electrical fluctuations in the electrodes defining the quantum dots....

Universal and Deterministic Manipulation of the Quantum State of Harmonic Oscillators: A Route to Unitary Gates for Fock State Qubits

International Nuclear Information System (INIS)

Santos, Marcelo Franca

2005-01-01

We present a simple quantum circuit that allows for the universal and deterministic manipulation of the quantum state of confined harmonic oscillators. The scheme is based on the selective interactions of the referred oscillator with an auxiliary three-level system and a classical external driving source, and enables any unitary operations on Fock states, two by two. One circuit is equivalent to a single qubit unitary logical gate on Fock states qubits. Sequences of similar protocols allow for complete, deterministic, and state-independent manipulation of the harmonic oscillator quantum state

Local correlations of mixed two-qubit states

International Nuclear Information System (INIS)

Zhang Fulin; Chen Jingling; Ren Changliang; Shi Mingjun

2010-01-01

The quantum probability distribution arising from single-copy von Neumann measurements on an arbitrary two-qubit state is decomposed into the local and nonlocal parts, in the approach of Elitzur, Popescu and Rohrlich [A. Elitzur, S. Popescu, D. Rohrlich, Phys. Lett. A 162 (1992) 25]. A lower bound of the local weight is proved being connected with the concurrence of the state p L max =1-C(ρ). The local probability distributions for two families of mixed states are constructed independently, which accord with the lower bound.

Enhancement of Quantum Correlations in Qubit-Qutrit Systems under the non-Markovian Environment

Institute of Scientific and Technical Information of China (English)

Abdul Basit; Hamad Ali; Fazal Badshah; Guo-Qin Ge

2017-01-01

We investigate the time evolution of quantum correlations of a hybrid qubit-qutrit system under the classical Ornstein-Uhlenbeck (OU) noise.Here we consider two different one-parameter families of qubit-qutrit states which independently interact with the non-Markovian reservoirs.A comparison with the Markovian dynamics reveals that for the same set of initial condition parameters,the non-Markovian behavior of the environment plays an important role in the enhancement of the survival time of quantum correlations.In addition,it is observed that the non-Markovian strength (γ/F) has a positive impact on the correlations time.For the initial separable states it is found that there is a finite time interval in which the geometric quantum discord is frozen despite the presence of a noisy environment and that interval can be further prolonged by using the non-Markovian property.Moreover,its decay can be significantly delayed.

Superconducting quantum circuits theory and application

Science.gov (United States)

Deng, Xiuhao

Superconducting quantum circuit models are widely used to understand superconducting devices. This thesis consists of four studies wherein the superconducting quantum circuit is used to illustrate challenges related to quantum information encoding and processing, quantum simulation, quantum signal detection and amplification. The existence of scalar Aharanov-Bohm phase has been a controversial topic for decades. Scalar AB phase, defined as time integral of electric potential, gives rises to an extra phase factor in wavefunction. We proposed a superconducting quantum Faraday cage to detect temporal interference effect as a consequence of scalar AB phase. Using the superconducting quantum circuit model, the physical system is solved and resulting AB effect is predicted. Further discussion in this chapter shows that treating the experimental apparatus quantum mechanically, spatial scalar AB effect, proposed by Aharanov-Bohm, can't be observed. Either a decoherent interference apparatus is used to observe spatial scalar AB effect, or a quantum Faraday cage is used to observe temporal scalar AB effect. The second study involves protecting a quantum system from losing coherence, which is crucial to any practical quantum computation scheme. We present a theory to encode any qubit, especially superconducting qubits, into a universal quantum degeneracy point (UQDP) where low frequency noise is suppressed significantly. Numerical simulations for superconducting charge qubit using experimental parameters show that its coherence time is prolong by two orders of magnitude using our universal degeneracy point approach. With this improvement, a set of universal quantum gates can be performed at high fidelity without losing too much quantum coherence. Starting in 2004, the use of circuit QED has enabled the manipulation of superconducting qubits with photons. We applied quantum optical approach to model coupled resonators and obtained a four-wave mixing toolbox to operate photons

Entanglement and Metrology with Singlet-Triplet Qubits

Science.gov (United States)

Shulman, Michael Dean

nuclear magnetic field control, as well as new techniques for calibrated measurement of the density matrix in a singlet-triplet qubit to entangle two adjacent single-triplet qubits. We fully characterize the generated entangled states and prove that they are, indeed, entangled. This work opens new opportunities to use qubits as sensors for improved metrological capabilities, as well as for improved quantum information processing. The singlet-triplet qubit is unique in that it can be used to probe two fundamentally different noise baths, which are important for a large variety of solid state qubits. More specifically, this work establishes the singlet-triplet qubit as a viable candidate for the building block of a scalable quantum information processor.

Endo-Fullerene and Doped Diamond Nanocrystallite Based Models of Qubits for Solid-State Quantum Computers

Science.gov (United States)

Park, Seongjun; Srivastava, Deepak; Cho, Kyeongjae; Biegel, Bryan (Technical Monitor)

2001-01-01

Models of encapsulated 1/2 nuclear spin H-1 and P-31 atoms in fullerene and diamond nanocrystallite, respectively, are proposed and examined with ab-initio local density functional method for possible applications as single quantum bits (qubits) in solid-state quantum computers. A H-1 atom encapsulated in a fully deuterated fullerene, C(sub 20)D(sub 20), forms the first model system and ab-initio calculation shows that H-1 atom is stable in atomic state at the center of the fullerene with a barrier of about 1 eV to escape. A P-31 atom positioned at the center of a diamond nanocrystallite is the second model system, and 3 1P atom is found to be stable at the substitutional site relative to interstitial sites by 15 eV, Vacancy formation energy is 6 eV in diamond so that substitutional P-31 atom will be stable against diffusion during the formation mechanisms within the nanocrystallite. The coupling between the nuclear spin and weakly bound (valance) donor electron coupling in both systems is found to be suitable for single qubit applications, where as the spatial distributions of (valance) donor electron wave functions are found to be preferentially spread along certain lattice directions facilitating two or more qubit applications. The feasibility of the fabrication pathways for both model solid-state qubit systems within practical quantum computers is discussed with in the context of our proposed solid-state qubits.

Qubit lattice coherence induced by electromagnetic pulses in superconducting metamaterials.

Science.gov (United States)

Ivić, Z; Lazarides, N; Tsironis, G P

2016-07-12

Quantum bits (qubits) are at the heart of quantum information processing schemes. Currently, solid-state qubits, and in particular the superconducting ones, seem to satisfy the requirements for being the building blocks of viable quantum computers, since they exhibit relatively long coherence times, extremely low dissipation, and scalability. The possibility of achieving quantum coherence in macroscopic circuits comprising Josephson junctions, envisioned by Legett in the 1980's, was demonstrated for the first time in a charge qubit; since then, the exploitation of macroscopic quantum effects in low-capacitance Josephson junction circuits allowed for the realization of several kinds of superconducting qubits. Furthermore, coupling between qubits has been successfully achieved that was followed by the construction of multiple-qubit logic gates and the implementation of several algorithms. Here it is demonstrated that induced qubit lattice coherence as well as two remarkable quantum coherent optical phenomena, i.e., self-induced transparency and Dicke-type superradiance, may occur during light-pulse propagation in quantum metamaterials comprising superconducting charge qubits. The generated qubit lattice pulse forms a compound "quantum breather" that propagates in synchrony with the electromagnetic pulse. The experimental confirmation of such effects in superconducting quantum metamaterials may open a new pathway to potentially powerful quantum computing.

A quick and easy test for deciding entanglement status of an N-qubit pure quantum state

International Nuclear Information System (INIS)

Mehendale, D.P.; Joag, P.S.

2018-01-01

We develop a simple criterion in terms of a necessary-sufficient condition (NS condition) for deciding separability of an arbitrary n-qubit pure quantum state. This NS condition provides a quick and easy test procedure to determine the entanglement status of a pure quantum state. We normalize the given quantum state and using this normalized state we can easily build a simplest system of equations containing trigonometric functions by making use of the well known Bloch Sphere representation for single qubit states and check whether or not this system of equations is consistent. According to proposed NS condition the given pure quantum state is separable (entangled) if and only if the above mentioned system of equations is consistent (inconsistent). We build this system of equations by equating the coefficients of computational basis states in the superposition representing the given pure quantum state with certain products of trigonometric functions obtained using standard Bloch Sphere representation for single qubit states. To establish separability of given state one requires to find a valid solution of the above mentioned system of equations but entanglement on the other hand follows when any two equations in this system of equations are mutually inconsistent. Thus, entanglement of the state can follow easily if one succeeds in finding any two mutually inconsistent equations in the above mentioned system of equations.

Dynamics and quantum Zeno effect for a qubit in either a low- or high-frequency bath beyond the rotating-wave approximation

International Nuclear Information System (INIS)

Cao Xiufeng; You, J. Q.; Zheng, H.; Kofman, A. G.; Nori, Franco

2010-01-01

We use a non-Markovian approach to study the decoherence dynamics of a qubit in either a low- or high-frequency bath modeling the qubit environment. This is done for two separate cases: either with measurements or without them. This approach is based on a unitary transformation and does not require the rotating-wave approximation. In the case without measurement, we show that, for low-frequency noise, the bath shifts the qubit energy toward higher energies (blue shift), while the ordinary high-frequency cutoff Ohmic bath shifts the qubit energy toward lower energies (red shift). In order to preserve the coherence of the qubit, we also investigate the dynamics of the qubit subject to measurements (quantum Zeno regime) in two cases: low- and high-frequency baths. For very frequent projective measurements, the low-frequency bath gives rise to the quantum anti-Zeno effect on the qubit. The quantum Zeno effect only occurs in the high-frequency-cutoff Ohmic bath, after counterrotating terms are considered. In the condition that the decay rate due to the two kinds of baths are equal under the Wigner-Weisskopf approximation, we find that without the approximation, for a high-frequency environment, the decay rate should be faster (without measurements) or slower (with frequent measurements, in the Zeno regime), compared to the low-frequency bath case. The experimental implementation of our results here could distinguish the type of bath (either a low- or high-frequency one) and protect the coherence of the qubit by modulating the dominant frequency of its environment.

Robust Concurrent Remote Entanglement Between Two Superconducting Qubits

Directory of Open Access Journals (Sweden)

A. Narla

2016-09-01

Full Text Available Entangling two remote quantum systems that never interact directly is an essential primitive in quantum information science and forms the basis for the modular architecture of quantum computing. When protocols to generate these remote entangled pairs rely on using traveling single-photon states as carriers of quantum information, they can be made robust to photon losses, unlike schemes that rely on continuous variable states. However, efficiently detecting single photons is challenging in the domain of superconducting quantum circuits because of the low energy of microwave quanta. Here, we report the realization of a robust form of concurrent remote entanglement based on a novel microwave photon detector implemented in the superconducting circuit quantum electrodynamics platform of quantum information. Remote entangled pairs with a fidelity of 0.57±0.01 are generated at 200 Hz. Our experiment opens the way for the implementation of the modular architecture of quantum computation with superconducting qubits.

Randomized benchmarking of single- and multi-qubit control in liquid-state NMR quantum information processing

International Nuclear Information System (INIS)

Ryan, C A; Laforest, M; Laflamme, R

2009-01-01

Being able to quantify the level of coherent control in a proposed device implementing a quantum information processor (QIP) is an important task for both comparing different devices and assessing a device's prospects with regards to achieving fault-tolerant quantum control. We implement in a liquid-state nuclear magnetic resonance QIP the randomized benchmarking protocol presented by Knill et al (2008 Phys. Rev. A 77 012307). We report an error per randomized π/2 pulse of 1.3±0.1x10 -4 with a single-qubit QIP and show an experimentally relevant error model where the randomized benchmarking gives a signature fidelity decay which is not possible to interpret as a single error per gate. We explore and experimentally investigate multi-qubit extensions of this protocol and report an average error rate for one- and two-qubit gates of 4.7±0.3x10 -3 for a three-qubit QIP. We estimate that these error rates are still not decoherence limited and thus can be improved with modifications to the control hardware and software.

Characterizing quantum phase transition by teleportation

Science.gov (United States)

Wu, Meng-He; Ling, Yi; Shu, Fu-Wen; Gan, Wen-Cong

2018-04-01

In this paper we provide a novel way to explore the relation between quantum teleportation and quantum phase transition. We construct a quantum channel with a mixed state which is made from one dimensional quantum Ising chain with infinite length, and then consider the teleportation with the use of entangled Werner states as input qubits. The fidelity as a figure of merit to measure how well the quantum state is transferred is studied numerically. Remarkably we find the first-order derivative of the fidelity with respect to the parameter in quantum Ising chain exhibits a logarithmic divergence at the quantum critical point. The implications of this phenomenon and possible applications are also briefly discussed.

An opto-magneto-mechanical quantum interface between distant superconducting qubits.

Science.gov (United States)

Xia, Keyu; Vanner, Michael R; Twamley, Jason

2014-07-04

A quantum internet, where widely separated quantum devices are coherently connected, is a fundamental vision for local and global quantum information networks and processing. Superconducting quantum devices can now perform sophisticated quantum engineering locally on chip and a detailed method to achieve coherent optical quantum interconnection between distant superconducting devices is a vital, but highly challenging, goal. We describe a concrete opto-magneto-mechanical system that can interconvert microwave-to-optical quantum information with high fidelity. In one such node we utilise the magnetic fields generated by the supercurrent of a flux qubit to coherently modulate a mechanical oscillator that is part of a high-Q optical cavity to achieve high fidelity microwave-to-optical quantum information exchange. We analyze the transfer between two spatially distant nodes connected by an optical fibre and using currently accessible parameters we predict that the fidelity of transfer could be as high as ~80%, even with significant loss.

Qubit Manipulations Techniques for Trapped-Ion Quantum Information Processing

Science.gov (United States)

Gaebler, John; Tan, Ting; Lin, Yiheng; Bowler, Ryan; Jost, John; Meier, Adam; Knill, Emanuel; Leibfried, Dietrich; Wineland, David; Ion Storage Team

2013-05-01

We report recent results on qubit manipulation techniques for trapped-ions towards scalable quantum information processing (QIP). We demonstrate a platform-independent benchmarking protocol for evaluating the performance of Clifford gates, which form a basis for fault-tolerant QIP. We report a demonstration of an entangling gate scheme proposed by Bermudez et al. [Phys. Rev. A. 85, 040302 (2012)] and achieve a fidelity of 0.974(4). This scheme takes advantage of dynamic decoupling which protects the qubit against dephasing errors. It can be applied directly on magnetic-field-insensitive states, and provides a number of simplifications in experimental implementation compared to some other entangling gates with trapped ions. We also report preliminary results on dissipative creation of entanglement with trapped-ions. Creation of an entangled pair does not require discrete logic gates and thus could reduce the level of quantum-coherent control needed for large-scale QIP. Supported by IARPA, ARO contract No. EAO139840, ONR, and the NIST Quantum Information Program.

Reduced randomness in quantum cryptography with sequences of qubits encoded in the same basis

International Nuclear Information System (INIS)

Lamoureux, L.-P.; Cerf, N. J.; Bechmann-Pasquinucci, H.; Gisin, N.; Macchiavello, C.

2006-01-01

We consider the cloning of sequences of qubits prepared in the states used in the BB84 or six-state quantum cryptography protocol, and show that the single-qubit fidelity is unaffected even if entire sequences of qubits are prepared in the same basis. This result is only valid provided that the sequences are much shorter than the total key. It is of great importance for practical quantum cryptosystems because it reduces the need for high-speed random number generation without impairing on the security against finite-size cloning attacks

Designing quantum-information-processing superconducting qubit circuits that exhibit lasing and other atomic-physics-like phenomena on a chip

Science.gov (United States)

Nori, Franco

2008-03-01

Superconducting (SC) circuits can behave like atoms making transitions between a few energy levels. Such circuits can test quantum mechanics at macroscopic scales and be used to conduct atomic-physics experiments on a silicon chip. This talk overviews a few of our theoretical studies on SC circuits and quantum information processing (QIP) including: SC qubits for single photon generation and for lasing; controllable couplings among qubits; how to increase the coherence time of qubits using a capacitor in parallel to one of the qubit junctions; hybrid circuits involving both charge and flux qubits; testing Bell's inequality in SC circuits; generation of GHZ states; quantum tomography in SC circuits; preparation of macroscopic quantum superposition states of a cavity field via coupling to a SC qubit; generation of nonclassical photon states using a SC qubit in a microcavity; scalable quantum computing with SC qubits; and information processing with SC qubits in a microwave field. Controllable couplings between qubits can be achieved either directly or indirectly. This can be done with and without coupler circuits, and with and without data-buses like EM fields in cavities (e.g., we will describe both the variable-frequency magnetic flux approach and also a generalized double-resonance approach that we introduced). It is also possible to ``turn a quantum bug into a feature'' by using microscopic defects as qubits, and the macroscopic junction as a controller of it. We have also studied ways to implement radically different approaches to QIP by using ``cluster states'' in SC circuits. For a general overview of this field, see, J.Q. You and F. Nori, Phys. Today 58 (11), 42 (2005)

Memory coherence of a sympathetically cooled trapped-ion qubit

International Nuclear Information System (INIS)

Home, J. P.; McDonnell, M. J.; Szwer, D. J.; Keitch, B. C.; Lucas, D. M.; Stacey, D. N.; Steane, A. M.

2009-01-01

We demonstrate sympathetic cooling of a 43 Ca + trapped-ion 'memory' qubit by a 40 Ca + 'coolant' ion sufficiently near the ground state of motion for fault-tolerant quantum logic, while maintaining coherence of the qubit. This is an essential ingredient in trapped-ion quantum computers. The isotope shifts are sufficient to suppress decoherence and phase shifts of the memory qubit due to the cooling light which illuminates both ions. We measure the qubit coherence during ten cycles of sideband cooling, finding a coherence loss of 3.3% per cooling cycle. The natural limit of the method is O(10 -4 ) infidelity per cooling cycle.

Superconducting Qubit with Integrated Single Flux Quantum Controller Part I: Theory and Fabrication

Science.gov (United States)

Beck, Matthew; Leonard, Edward, Jr.; Thorbeck, Ted; Zhu, Shaojiang; Howington, Caleb; Nelson, Jj; Plourde, Britton; McDermott, Robert

As the size of quantum processors grow, so do the classical control requirements. The single flux quantum (SFQ) Josephson digital logic family offers an attractive route to proximal classical control of multi-qubit processors. Here we describe coherent control of qubits via trains of SFQ pulses. We discuss the fabrication of an SFQ-based pulse generator and a superconducting transmon qubit on a single chip. Sources of excess microwave loss stemming from the complex multilayer fabrication of the SFQ circuit are discussed. We show how to mitigate this loss through judicious choice of process workflow and appropriate use of sacrificial protection layers. Present address: IBM T.J. Watson Research Center.

Implementability of two-qubit unitary operations over the butterfly network and the ladder network with free classical communication

Energy Technology Data Exchange (ETDEWEB)

Akibue, Seiseki [Department of Physics, Graduate School of Science, The University of Tokyo, Tokyo (Japan); Murao, Mio [Department of Physics, Graduate School of Science, The University of Tokyo, Tokyo, Japan and NanoQuine, The University of Tokyo, Tokyo (Japan)

2014-12-04

We investigate distributed implementation of two-qubit unitary operations over two primitive networks, the butterfly network and the ladder network, as a first step to apply network coding for quantum computation. By classifying two-qubit unitary operations in terms of the Kraus-Cirac number, the number of non-zero parameters describing the global part of two-qubit unitary operations, we analyze which class of two-qubit unitary operations is implementable over these networks with free classical communication. For the butterfly network, we show that two classes of two-qubit unitary operations, which contain all Clifford, controlled-unitary and matchgate operations, are implementable over the network. For the ladder network, we show that two-qubit unitary operations are implementable over the network if and only if their Kraus-Cirac number do not exceed the number of the bridges of the ladder.

Implementability of two-qubit unitary operations over the butterfly network and the ladder network with free classical communication

International Nuclear Information System (INIS)

Akibue, Seiseki; Murao, Mio

2014-01-01

We investigate distributed implementation of two-qubit unitary operations over two primitive networks, the butterfly network and the ladder network, as a first step to apply network coding for quantum computation. By classifying two-qubit unitary operations in terms of the Kraus-Cirac number, the number of non-zero parameters describing the global part of two-qubit unitary operations, we analyze which class of two-qubit unitary operations is implementable over these networks with free classical communication. For the butterfly network, we show that two classes of two-qubit unitary operations, which contain all Clifford, controlled-unitary and matchgate operations, are implementable over the network. For the ladder network, we show that two-qubit unitary operations are implementable over the network if and only if their Kraus-Cirac number do not exceed the number of the bridges of the ladder

Decoherence in qubits due to low-frequency noise

International Nuclear Information System (INIS)

Bergli, J; Galperin, Y M; Altshuler, B L

2009-01-01

The efficiency of the future devices for quantum information processing will be limited mostly by the finite decoherence rates of the qubits. Recently, substantial progress was achieved in enhancing the time within which a solid-state qubit demonstrates coherent dynamics. This progress is based mostly on a successful isolation of the qubits from external decoherence sources. Under these conditions, the material-inherent sources of noise start to play a crucial role. In most cases, the noise that the quantum device demonstrates has a 1/f spectrum. This suggests that the environment that destroys the phase coherence of the qubit can be thought of as a system of two-state fluctuators, which experience random hops between their states. In this short review, the current state of the theory of the decoherence due to the qubit interaction with the fluctuators is discussed. The effect of such an environment on two different protocols of the qubit manipulations, free induction and echo signal, is described. It turns out that in many important cases the noise produced by the fluctuators is non-Gaussian. Consequently, the results of the interaction of the qubit with the fluctuators are not determined by the pair correlation function alone. We describe the effect of the fluctuators using the so-called spin-fluctuator model. Being quite realistic, this model allows one to exactly evaluate the qubit dynamics in the presence of one fluctuator. This solution is found, and its features, including non-Gaussian effects, are analyzed in detail. We extend this consideration to systems of large numbers of fluctuators, which interact with the qubit and lead to the 1/f noise. We discuss existing experiments on the Josephson qubit manipulation and try to identify non-Gaussian behavior.

Control quantum evolution speed of a single dephasing qubit for arbitrary initial states via periodic dynamical decoupling pulses.

Science.gov (United States)

Song, Ya-Ju; Tan, Qing-Shou; Kuang, Le-Man

2017-03-08

We investigate the possibility to control quantum evolution speed of a single dephasing qubit for arbitrary initial states by the use of periodic dynamical decoupling (PDD) pulses. It is indicated that the quantum speed limit time (QSLT) is determined by initial and final quantum coherence of the qubit, as well as the non-Markovianity of the system under consideration during the evolution when the qubit is subjected to a zero-temperature Ohmic-like dephasing reservoir. It is shown that final quantum coherence of the qubit and the non-Markovianity of the system can be modulated by PDD pulses. Our results show that for arbitrary initial states of the dephasing qubit with non-vanishing quantum coherence, PDD pulses can be used to induce potential acceleration of the quantum evolution in the short-time regime, while PDD pulses can lead to potential speedup and slow down in the long-time regime. We demonstrate that the effect of PDD on the QSLT for the Ohmic or sub-Ohmic spectrum (Markovian reservoir) is much different from that for the super-Ohmic spectrum (non-Markovian reservoir).

Dynamical decoupling sequences for multi-qubit dephasing suppression and long-time quantum memory

International Nuclear Information System (INIS)

Paz-Silva, Gerardo A; Lee, Seung-Woo; Green, Todd J; Viola, Lorenza

2016-01-01

We consider a class of multi-qubit dephasing models that combine classical noise sources and linear coupling to a bosonic environment, and are controlled by arbitrary sequences of dynamical decoupling pulses. Building on a general transfer filter-function framework for open-loop control, we provide an exact representation of the controlled dynamics for arbitrary stationary non-Gaussian classical and quantum noise statistics, with analytical expressions emerging when all dephasing sources are Gaussian. This exact characterization is used to establish two main results. First, we construct multi-qubit sequences that ensure maximum high-order error suppression in both the time and frequency domain and that can be exponentially more efficient than existing ones in terms of total pulse number. Next, we show how long-time multi-qubit storage may be achieved by meeting appropriate conditions for the emergence of a fidelity plateau under sequence repetition, thereby generalizing recent results for single-qubit memory under Gaussian dephasing. In both scenarios, the key step is to endow multi-qubit sequences with a suitable displacement anti-symmetry property, which is of independent interest for applications ranging from environment-assisted entanglement generation to multi-qubit noise spectroscopy protocols. (paper)

Optimal cloning of qubits given by an arbitrary axisymmetric distribution on the Bloch sphere

International Nuclear Information System (INIS)

Bartkiewicz, Karol; Miranowicz, Adam

2010-01-01

We find an optimal quantum cloning machine, which clones qubits of arbitrary symmetrical distribution around the Bloch vector with the highest fidelity. The process is referred to as phase-independent cloning in contrast to the standard phase-covariant cloning for which an input qubit state is a priori better known. We assume that the information about the input state is encoded in an arbitrary axisymmetric distribution (phase function) on the Bloch sphere of the cloned qubits. We find analytical expressions describing the optimal cloning transformation and fidelity of the clones. As an illustration, we analyze cloning of qubit state described by the von Mises-Fisher and Brosseau distributions. Moreover, we show that the optimal phase-independent cloning machine can be implemented by modifying the mirror phase-covariant cloning machine for which quantum circuits are known.

Formation of multipartite entanglement using random quantum gates

International Nuclear Information System (INIS)

Most, Yonatan; Shimoni, Yishai; Biham, Ofer

2007-01-01

The formation of multipartite quantum entanglement by repeated operation of one- and two-qubit gates is examined. The resulting entanglement is evaluated using two measures: the average bipartite entanglement and the Groverian measure. A comparison is made between two geometries of the quantum register: a one-dimensional chain in which two-qubit gates apply only locally between nearest neighbors and a nonlocal geometry in which such gates may apply between any pair of qubits. More specifically, we use a combination of random single-qubit rotations and a fixed two-qubit gate such as the controlled-phase gate. It is found that in the nonlocal geometry the entanglement is generated at a higher rate. In both geometries, the Groverian measure converges to its asymptotic value more slowly than the average bipartite entanglement. These results are expected to have implications on different proposed geometries of future quantum computers with local and nonlocal interactions between the qubits

Silicon Carbide Defect Qubits/Quantum Memory with Field-Tuning: OSD Quantum Science and Engineering Program (QSEP)

Science.gov (United States)

2017-08-01

TECHNICAL REPORT 3073 August 2017 Silicon Carbide Defect Qubits/Quantum Memory with Field-tuning: OSD Quantum Science and Engineering Program...Quantum Science and Engineering Program) by the Advanced Concepts and Applied Research Branch (Code 71730), the Energy and Environmental Sustainability...the Secretary of Defense (OSD) Quantum Science and Engineering Program (QSEP). Their collaboration topic was to examine the effect of electric-field

Rotation gate for a three-level superconducting quantum interference device qubit with resonant interaction

International Nuclear Information System (INIS)

Yang, C.-P.; Han Siyuan

2006-01-01

We show a way to realize an arbitrary rotation gate in a three-level superconducting quantum interference device (SQUID) qubit using resonant interaction. In this approach, the two logical states of the qubit are represented by the two lowest levels of the SQUID and a higher-energy intermediate level is utilized for the gate manipulation. By considering spontaneous decay from the intermediate level during the gate operation, we present a formula for calculating average fidelity over all possible initial states. Finally, based on realistic system parameters, we show that an arbitrary rotation gate can be achieved with a high fidelity in a SQUID

Tunable coupling between fixed-frequency superconducting transmon qubits

Energy Technology Data Exchange (ETDEWEB)

Filipp, Stefan [IBM Research Zurich, 8803 Rueschlikon (Switzerland); McKay, David C.; Magesan, Easwar; Mezzacapo, Antonio; Chow, Jerry M.; Gambetta, Jay M. [IBM TJ Watson Research Center, Yorktown Heights, NY (United States)

2016-07-01

The controlled realization of qubit-qubit interactions is essential for both the physical implementation of quantum error-correction codes and for reliable quantum simulations. Ideally, the fidelity and speed of corresponding two-qubit gate operations is comparable to those of single qubit operations. In particular, in a scalable superconducting qubit architecture coherence must not be compromised by the presence of additional coupling elements mediating the interaction between qubits. Here we present a coupling method between fixed-frequency transmon qubits based on the frequency modulation of an auxiliary circuit coupling to the individual transmons. Since the coupler remains in its ground state at all times, its coherence does not significantly influence the fidelity of consequent entangling operations. Moreover, with the possibility to create interactions along different directions, our method is suited to engineer Hamiltonians with adjustable coupling terms. This property can be utilized for quantum simulations of spins or fermions in transmon arrays, in which pairwise couplings between adjacent qubits can be activated on demand.

Two-qubit Bell inequality for which positive operator-valued measurements are relevant

International Nuclear Information System (INIS)

Vertesi, T.; Bene, E.

2010-01-01

A bipartite Bell inequality is derived which is maximally violated on the two-qubit state space if measurements describable by positive operator valued measure (POVM) elements are allowed, rather than restricting the possible measurements to projective ones. In particular, the presented Bell inequality requires POVMs in order to be maximally violated by a maximally entangled two-qubit state. This answers a question raised by N. Gisin [in Quantum Reality, Relativistic Causality, and Closing the Epistemic Circle: Essays in Honour of Abner Shimony, edited by W. C. Myrvold and J. Christian (Springer, The Netherlands, 2009), pp. 125-138].

Qubit authentication

International Nuclear Information System (INIS)

Curty, Marcos; Santos, David J.; Perez, Esther; Garcia-Fernandez, Priscila

2002-01-01

Secure communication requires message authentication. In this paper we address the problem of how to authenticate quantum information sent through a quantum channel between two communicating parties with the minimum amount of resources. Specifically, our objective is to determine whether one elementary quantum message (a qubit) can be authenticated with a key of minimum length. We show that, unlike the case of classical-message quantum authentication, this is not possible

Rapid characterization of microscopic two-level systems using Landau-Zener transitions in a superconducting qubit

International Nuclear Information System (INIS)

Tan, Xinsheng; Yu, Haifeng; Yu, Yang; Han, Siyuan

2015-01-01

We demonstrate a fast method to detect microscopic two-level systems in a superconducting phase qubit. By monitoring the population leak after sweeping the qubit bias flux, we are able to measure the two-level systems that are coupled with the qubit. Compared with the traditional method that detects two-level systems by energy spectroscopy, our method is faster and more sensitive. This method supplies a useful tool to investigate two-level systems in solid-state qubits

Efficient experimental design of high-fidelity three-qubit quantum gates via genetic programming

Science.gov (United States)

Devra, Amit; Prabhu, Prithviraj; Singh, Harpreet; Arvind; Dorai, Kavita

2018-03-01

We have designed efficient quantum circuits for the three-qubit Toffoli (controlled-controlled-NOT) and the Fredkin (controlled-SWAP) gate, optimized via genetic programming methods. The gates thus obtained were experimentally implemented on a three-qubit NMR quantum information processor, with a high fidelity. Toffoli and Fredkin gates in conjunction with the single-qubit Hadamard gates form a universal gate set for quantum computing and are an essential component of several quantum algorithms. Genetic algorithms are stochastic search algorithms based on the logic of natural selection and biological genetics and have been widely used for quantum information processing applications. We devised a new selection mechanism within the genetic algorithm framework to select individuals from a population. We call this mechanism the "Luck-Choose" mechanism and were able to achieve faster convergence to a solution using this mechanism, as compared to existing selection mechanisms. The optimization was performed under the constraint that the experimentally implemented pulses are of short duration and can be implemented with high fidelity. We demonstrate the advantage of our pulse sequences by comparing our results with existing experimental schemes and other numerical optimization methods.

Quantum information transfer between topological and conventional charge qubits

International Nuclear Information System (INIS)

Li Jun; Zou Yan

2016-01-01

We propose a scheme to realize coherent quantum information transfer between topological and conventional charge qubits. We first consider a hybrid system where a quantum dot (QD) is tunnel-coupled to a semiconductor Majorana-hosted nanowire (MNW) via using gated control as a switch, the information encoded in the superposition state of electron empty and occupied state can be transferred to each other through choosing the proper interaction time to make measurements. Then we consider another system including a double QDs and a pair of parallel MNWs, it is shown that the entanglement information transfer can be realized between the two kinds of systems. We also realize long distance quantum information transfer between two quantum dots separated by an MNW, by making use of the nonlocal fermionic level formed with the pared Majorana feimions (MFs) emerging at the two ends of the MNW. Furthermore, we analyze the teleportationlike electron transfer phenomenon predicted by Tewari et al. [Phys. Rev. Lett. 100, 027001 (2008)] in our considered system. Interestingly, we find that this phenomenon exactly corresponds to the case that the information encoded in one QD just returns back to its original place during the dynamical evolution of the combined system from the perspective of quantum state transfer. (paper)

Phonon-assisted relaxation and decoherence of singlet-triplet qubits in Si/SiGe quantum dots

Directory of Open Access Journals (Sweden)

Viktoriia Kornich

2018-05-01

Full Text Available We study theoretically the phonon-induced relaxation and decoherence of spin states of two electrons in a lateral double quantum dot in a SiGe/Si/SiGe heterostructure. We consider two types of singlet-triplet spin qubits and calculate their relaxation and decoherence times, in particular as a function of level hybridization, temperature, magnetic field, spin orbit interaction, and detuning between the quantum dots, using Bloch-Redfield theory. We show that the magnetic field gradient, which is usually applied to operate the spin qubit, may reduce the relaxation time by more than an order of magnitude. Using this insight, we identify an optimal regime where the magnetic field gradient does not affect the relaxation time significantly, and we propose regimes of longest decay times. We take into account the effects of one-phonon and two-phonon processes and suggest how our theory can be tested experimentally. The spin lifetimes we find here for Si-based quantum dots are significantly longer than the ones reported for their GaAs counterparts.

Analysis and synthesis of multi-qubit, multi-mode quantum devices

Energy Technology Data Exchange (ETDEWEB)

Solgun, Firat

2015-03-27

In this thesis we propose new methods in multi-qubit multi-mode circuit quantum electrodynamics (circuit-QED) architectures. First we describe a direct parity measurement method for three qubits, which can be realized in 2D circuit-QED with a possible extension to four qubits in a 3D circuit-QED setup for the implementation of the surface code. In Chapter 3 we show how to derive Hamiltonians and compute relaxation rates of the multi-mode superconducting microwave circuits consisting of single Josephson junctions using an exact impedance synthesis technique (the Brune synthesis) and applying previous formalisms for lumped element circuit quantization. In the rest of the thesis we extend our method to multi-junction (multi-qubit) multi-mode circuits through the use of state-space descriptions which allows us to quantize any multiport microwave superconducting circuit with a reciprocal lossy impedance response.

Quantum control on entangled bipartite qubits

International Nuclear Information System (INIS)

Delgado, Francisco

2010-01-01

Ising interactions between qubits can produce distortion on entangled pairs generated for engineering purposes (e.g., for quantum computation or quantum cryptography). The presence of parasite magnetic fields destroys or alters the expected behavior for which it was intended. In addition, these pairs are generated with some dispersion in their original configuration, so their discrimination is necessary for applications. Nevertheless, discrimination should be made after Ising distortion. Quantum control helps in both problems; making some projective measurements upon the pair to decide the original state to replace it, or just trying to reconstruct it using some procedures which do not alter their quantum nature. Results about the performance of these procedures are reported. First, we will work with pure systems studying restrictions and advantages. Then, we will extend these operations for mixed states generated with uncertainty in the time of distortion, correcting them by assuming the control prescriptions for the most probable one.

Realization of seven-qubit Deutsch-Jozsa algorithm on NMR quantum computer

International Nuclear Information System (INIS)

Wei Daxiu; Yang Xiaodong; Luo Jun; Sun Xianping; Zeng Xizhi; Liu Maili; Ding Shangwu

2002-01-01

Recent years, remarkable progresses in experimental realization of quantum information have been made, especially based on nuclear magnetic resonance (NMR) theory. In all quantum algorithms, Deutsch-Jozsa algorithm has been widely studied. It can be realized on NMR quantum computer and also can be simplified by using the Cirac's scheme. At first the principle of Deutsch-Jozsa quantum algorithm is analyzed, then the authors implement the seven-qubit Deutsch-Jozsa algorithm on NMR quantum computer

Magnetic qubits as hardware for quantum computers

International Nuclear Information System (INIS)

Tejada, J.; Chudnovsky, E.; Barco, E. del

2000-01-01

We propose two potential realisations for quantum bits based on nanometre scale magnetic particles of large spin S and high anisotropy molecular clusters. In case (1) the bit-value basis states vertical bar-0> and vertical bar-1> are the ground and first excited spin states S z = S and S-1, separated by an energy gap given by the ferromagnetic resonance (FMR) frequency. In case (2), when there is significant tunnelling through the anisotropy barrier, the qubit states correspond to the symmetric, vertical bar-0>, and antisymmetric, vertical bar-1>, combinations of the two-fold degenerate ground state S z = ± S. In each case the temperature of operation must be low compared to the energy gap, Δ, between the states vertical bar-0> and vertical bar-1>. The gap Δ in case (2) can be controlled with an external magnetic field perpendicular to the easy axis of the molecular cluster. The states of different molecular clusters and magnetic particles may be entangled by connecting them by superconducting lines with Josephson switches, leading to the potential for quantum computing hardware. (author)

Magnetic qubits as hardware for quantum computers

Energy Technology Data Exchange (ETDEWEB)

Tejada, J.; Chudnovsky, E.; Barco, E. del [and others

2000-07-01

We propose two potential realisations for quantum bits based on nanometre scale magnetic particles of large spin S and high anisotropy molecular clusters. In case (1) the bit-value basis states vertical bar-0> and vertical bar-1> are the ground and first excited spin states S{sub z} = S and S-1, separated by an energy gap given by the ferromagnetic resonance (FMR) frequency. In case (2), when there is significant tunnelling through the anisotropy barrier, the qubit states correspond to the symmetric, vertical bar-0>, and antisymmetric, vertical bar-1>, combinations of the two-fold degenerate ground state S{sub z} = {+-} S. In each case the temperature of operation must be low compared to the energy gap, {delta}, between the states vertical bar-0> and vertical bar-1>. The gap {delta} in case (2) can be controlled with an external magnetic field perpendicular to the easy axis of the molecular cluster. The states of different molecular clusters and magnetic particles may be entangled by connecting them by superconducting lines with Josephson switches, leading to the potential for quantum computing hardware. (author)

Probing the interaction of microscopic material defects with quasiparticles using a superconducting qubit

Energy Technology Data Exchange (ETDEWEB)

Bilmes, Alexander; Lisenfeld, Juergen; Weiss, Georg; Ustinov, Alexey V. [PI, Fakultaet fuer Physik, KIT, Wolfgang-Gaede-Strasse 1, 76131 Karlsruhe (Germany); Heimes, Andreas; Zanker, Sebastian; Schoen, Gerd [TFP, Fakultaet fuer Physik, KIT, Wolfgang-Gaede-Strasse 1, 76131 Karlsruhe (Germany)

2015-07-01

Two-Level-Systems (TLS) are one of the main sources of decoherence in superconducting nano-scale devices such as SQUIDs, photon detectors, resonators and quantum bits (qubits), although the TLS' microscopic nature remains unclear. We use a superconducting phase qubit to detect TLS contained within the tunnel barrier of the qubit's Josephson junction. We coherently operate individual TLS by resonant microwave pulses and access their quantum state by utilizing their strong coupling to the qubit. Our previous measurements of TLS coherence in dependence of the temperature indicate that quasiparticles may give rise to TLS energy loss and dephasing. Here, we probe the TLS-quasiparticle interaction using a reliable method of in-situ quasiparticle injection via an on-chip dc-SQUID that is pulse-biased beyond its critical current. The quasiparticle density is calibrated by measuring associated characteristic changes to the qubit's resonance frequency and energy relaxation rate. We will present experimental data that clearly show the influence of quasiparticles on TLS coherence.

Valley qubit in a gated MoS2 monolayer quantum dot

Science.gov (United States)

Pawłowski, J.; Żebrowski, D.; Bednarek, S.

2018-04-01

The aim of the presented research is to design a nanodevice, based on a MoS2 monolayer, performing operations on a well-defined valley qubit. We show how to confine an electron in a gate-induced quantum dot within the monolayer, and to perform the not operation on its valley degree of freedom. The operations are carried out all electrically via modulation of the confinement potential by oscillating voltages applied to the local gates. Such quantum dot structure is modeled realistically. Through these simulations we investigate the possibility of realization of a valley qubit in analogy with a realization of the spin qubit. We accurately model the potential inside the nanodevice accounting for proper boundary conditions on the gates and space-dependent materials permittivity by solving the generalized Poisson's equation. The time evolution of the system is supported by realistic self-consistent Poisson-Schrödinger tight-binding calculations. The tight-binding calculations are further confirmed by simulations within the effective continuum model.

Subspace confinement: how good is your qubit?

International Nuclear Information System (INIS)

Devitt, Simon J; Schirmer, Sonia G; Oi, Daniel K L; Cole, Jared H; Hollenberg, Lloyd C L

2007-01-01

The basic operating element of standard quantum computation is the qubit, an isolated two-level system that can be accurately controlled, initialized and measured. However, the majority of proposed physical architectures for quantum computation are built from systems that contain much more complicated Hilbert space structures. Hence, defining a qubit requires the identification of an appropriate controllable two-dimensional sub-system. This prompts the obvious question of how well a qubit, thus defined, is confined to this subspace, and whether we can experimentally quantify the potential leakage into states outside the qubit subspace. We demonstrate how subspace leakage can be characterized using minimal theoretical assumptions by examining the Fourier spectrum of the oscillation experiment

Protecting entanglement by adjusting the velocities of moving qubits inside non-Markovian environments

Science.gov (United States)

Mortezapour, Ali; Ahmadi Borji, Mahdi; Lo Franco, Rosario

2017-05-01

Efficient entanglement preservation in open quantum systems is a crucial scope towards a reliable exploitation of quantum resources. We address this issue by studying how two-qubit entanglement dynamically behaves when two atom qubits move inside two separated identical cavities. The moving qubits independently interact with their respective cavity. As a main general result, we find that under resonant qubit-cavity interaction the initial entanglement between two moving qubits remains closer to its initial value as time passes compared to the case of stationary qubits. In particular, we show that the initial entanglement can be strongly protected from decay by suitably adjusting the velocities of the qubits according to the non-Markovian features of the cavities. Our results supply a further way of preserving quantum correlations against noise with a natural implementation in cavity-QED scenarios and are straightforwardly extendable to many qubits for scalability.

Entanglement loss in molecular quantum-dot qubits due to interaction with the environment

Science.gov (United States)

Blair, Enrique P.; Tóth, Géza; Lent, Craig S.

2018-05-01

We study quantum entanglement loss due to environmental interaction in a condensed matter system with a complex geometry relevant to recent proposals for computing with single electrons at the nanoscale. We consider a system consisting of two qubits, each realized by an electron in a double quantum dot, which are initially in an entangled Bell state. The qubits are widely separated and each interacts with its own environment. The environment for each is modeled by surrounding double quantum dots placed at random positions with random orientations. We calculate the unitary evolution of the joint system and environment. The global state remains pure throughout. We examine the time dependence of the expectation value of the bipartite Clauser–Horne–Shimony–Holt (CHSH) and Brukner–Paunković–Rudolph–Vedral (BPRV) Bell operators and explore the emergence of correlations consistent with local realism. Though the details of this transition depend on the specific environmental geometry, we show how the results can be mapped on to a universal behavior with appropriate scaling. We determine the relevant disentanglement times based on realistic physical parameters for molecular double-dots.

Electromagnetically induced interference in a superconducting flux qubit

International Nuclear Information System (INIS)

Du lingjie; Yu Yang; Lan Dong

2013-01-01

Interaction between quantum two-level systems (qubits) and electromagnetic fields can provide additional coupling channels to qubit states. In particular, the interwell relaxation or Rabi oscillations, resulting, respectively, from the multi- or single-mode interaction, can produce effective crossovers, leading to electromagnetically induced interference in microwave driven qubits. The environment is modeled by a multimode thermal bath, generating the interwell relaxation. Relaxation induced interference, independent of the tunnel coupling, provides deeper understanding to the interaction between the qubits and their environment. It also supplies a useful tool to characterize the relaxation strength as well as the characteristic frequency of the bath. In addition, we demonstrate the relaxation can generate population inversion in a strongly driving two-level system. On the other hand, different from Rabi oscillations, Rabi-oscillation-induced interference involves more complicated and modulated photon exchange thus offers an alternative means to manipulate the qubit, with more controllable parameters including the strength and position of the tunnel coupling. It also provides a testing ground for exploring nonlinear quantum phenomena and quantum state manipulation in qubits either with or without crossover structure.

Insufficiency of avoided crossings for witnessing large-scale quantum coherence in flux qubits

Science.gov (United States)

Fröwis, Florian; Yadin, Benjamin; Gisin, Nicolas

2018-04-01

Do experiments based on superconducting loops segmented with Josephson junctions (e.g., flux qubits) show macroscopic quantum behavior in the sense of Schrödinger's cat example? Various arguments based on microscopic and phenomenological models were recently adduced in this debate. We approach this problem by adapting (to flux qubits) the framework of large-scale quantum coherence, which was already successfully applied to spin ensembles and photonic systems. We show that contemporary experiments might show quantum coherence more than 100 times larger than experiments in the classical regime. However, we argue that the often-used demonstration of an avoided crossing in the energy spectrum is not sufficient to make a conclusion about the presence of large-scale quantum coherence. Alternative, rigorous witnesses are proposed.

Characterizing a four-qubit planar lattice for arbitrary error detection

Science.gov (United States)

Chow, Jerry M.; Srinivasan, Srikanth J.; Magesan, Easwar; Córcoles, A. D.; Abraham, David W.; Gambetta, Jay M.; Steffen, Matthias

2015-05-01

Quantum error correction will be a necessary component towards realizing scalable quantum computers with physical qubits. Theoretically, it is possible to perform arbitrarily long computations if the error rate is below a threshold value. The two-dimensional surface code permits relatively high fault-tolerant thresholds at the ~1% level, and only requires a latticed network of qubits with nearest-neighbor interactions. Superconducting qubits have continued to steadily improve in coherence, gate, and readout fidelities, to become a leading candidate for implementation into larger quantum networks. Here we describe characterization experiments and calibration of a system of four superconducting qubits arranged in a planar lattice, amenable to the surface code. Insights into the particular qubit design and comparison between simulated parameters and experimentally determined parameters are given. Single- and two-qubit gate tune-up procedures are described and results for simultaneously benchmarking pairs of two-qubit gates are given. All controls are eventually used for an arbitrary error detection protocol described in separate work [Corcoles et al., Nature Communications, 6, 2015].

Superconducting Qubit with Integrated Single Flux Quantum Controller Part II: Experimental Characterization

Science.gov (United States)

Leonard, Edward, Jr.; Beck, Matthew; Thorbeck, Ted; Zhu, Shaojiang; Howington, Caleb; Nelson, Jj; Plourde, Britton; McDermott, Robert

We describe the characterization of a single flux quantum (SFQ) pulse generator cofabricated with a superconducting quantum circuit on a single chip. Resonant trains of SFQ pulses are used to induce coherent qubit rotations on the Bloch sphere. We describe the SFQ drive characteristics of the qubit at the fundamental transition frequency and at subharmonics (ω01 / n , n = 2 , 3 , 4 , ⋯). We address the issue of quasiparticle poisoning due to the proximal SFQ pulse generator, and we characterize the fidelity of SFQ-based rotations using randomized benchmarking. Present address: IBM T.J. Watson Research Center.

Quantum memory on a charge qubit in an optical microresonator

Science.gov (United States)

Tsukanov, A. V.

2017-10-01

A quantum-memory unit scheme on the base of a semiconductor structure with quantum dots is proposed. The unit includes a microresonator with single and double quantum dots performing frequencyconverter and charge-qubit functions, respectively. The writing process is carried out in several stages and it is controlled by optical fields of the resonator and laser. It is shown that, to achieve high writing probability, it is necessary to use high-Q resonators and to be able to suppress relaxation processes in quantum dots.

Ultracoherent operation of spin qubits with superexchange coupling

Science.gov (United States)

Rančić, Marko J.; Burkard, Guido

2017-11-01

With the use of nuclear-spin-free materials such as silicon and germanium, spin-based quantum bits (qubits) have evolved to become among the most coherent systems for quantum information processing. The new frontier for spin qubits has therefore shifted to the ubiquitous charge noise and spin-orbit interaction, which are limiting the coherence times and gate fidelities of solid-state qubits. In this paper we investigate superexchange, as a means of indirect exchange interaction between two single electron spin qubits, each embedded in a single semiconductor quantum dot (QD), mediated by an intermediate, empty QD. Our results suggest the existence of "supersweet spots", in which the qubit operations implemented by superexchange interaction are simultaneously first-order-insensitive to charge noise and to errors due to spin-orbit interaction. The proposed spin-qubit architecture is scalable and within the manufacturing capabilities of semiconductor industry.

Entanglement dynamics of two electron-spin qubits in a strongly detuned and dissipative quantum-dot-cavity system

International Nuclear Information System (INIS)

Xiang Shaohua; Deng Xiaopeng; Song Kehui; Wen Wei; Shi Zhengang

2011-01-01

We investigate the entanglement dynamics of two electron-spin qubits in the quantum-dot (QD)-microcavity system in the large-detuning limit and subjected to two different noise sources: electron-spin dephasing and relaxation. We show that when one of the two dots is prepared initially in the excited state, the created entanglement exhibits oscillatory behavior at the beginning of evolution and then completely disappears over time. For two QDs that are initially in either the Einstein-Podolsky-Rosen-Bell states or the Werner states, their entanglement evolution exhibits the same behavior in the presence of pure dephasing, but is completely different under the relaxation process. We also show that the interdot interaction induced by a single-mode cavity field does not contribute to the dynamics of entanglement for these Bell states and Werner states.

Controllable gaussian-qubit interface for extremal quantum state engineering.

Science.gov (United States)

Adesso, Gerardo; Campbell, Steve; Illuminati, Fabrizio; Paternostro, Mauro

2010-06-18

We study state engineering through bilinear interactions between two remote qubits and two-mode gaussian light fields. The attainable two-qubit states span the entire physically allowed region in the entanglement-versus-global-purity plane. Two-mode gaussian states with maximal entanglement at fixed global and marginal entropies produce maximally entangled two-qubit states in the corresponding entropic diagram. We show that a small set of parameters characterizing extremally entangled two-mode gaussian states is sufficient to control the engineering of extremally entangled two-qubit states, which can be realized in realistic matter-light scenarios.

Weak Measurement and Quantum Smoothing of a Superconducting Qubit

Science.gov (United States)

Tan, Dian

In quantum mechanics, the measurement outcome of an observable in a quantum system is intrinsically random, yielding a probability distribution. The state of the quantum system can be described by a density matrix rho(t), which depends on the information accumulated until time t, and represents our knowledge about the system. The density matrix rho(t) gives probabilities for the outcomes of measurements at time t. Further probing of the quantum system allows us to refine our prediction in hindsight. In this thesis, we experimentally examine a quantum smoothing theory in a superconducting qubit by introducing an auxiliary matrix E(t) which is conditioned on information obtained from time t to a final time T. With the complete information before and after time t, the pair of matrices [rho(t), E(t)] can be used to make smoothed predictions for the measurement outcome at time t. We apply the quantum smoothing theory in the case of continuous weak measurement unveiling the retrodicted quantum trajectories and weak values. In the case of strong projective measurement, while the density matrix rho(t) with only diagonal elements in a given basis |n〉 may be treated as a classical mixture, we demonstrate a failure of this classical mixture description in determining the smoothed probabilities for the measurement outcome at time t with both diagonal rho(t) and diagonal E(t). We study the correlations between quantum states and weak measurement signals and examine aspects of the time symmetry of continuous quantum measurement. We also extend our study of quantum smoothing theory to the case of resonance fluorescence of a superconducting qubit with homodyne measurement and observe some interesting effects such as the modification of the excited state probabilities, weak values, and evolution of the predicted and retrodicted trajectories.

Pairwise correlations via quantum discord and its geometric measure in a four-qubit spin chain

Directory of Open Access Journals (Sweden)

Abdel-Baset A. Mohamed

2013-04-01

Full Text Available The dynamic of pairwise correlations, including quantum entanglement (QE and discord (QD with geometric measure of quantum discord (GMQD, are shown in the four-qubit Heisenberg XX spin chain. The results show that the effect of the entanglement degree of the initial state on the pairwise correlations is stronger for alternate qubits than it is for nearest-neighbor qubits. This parameter results in sudden death for QE, but it cannot do so for QD and GMQD. With different values for this entanglement parameter of the initial state, QD and GMQD differ and are sensitive for any change in this parameter. It is found that GMQD is more robust than both QD and QE to describe correlations with nonzero values, which offers a valuable resource for quantum computation.

Implementation of a three-qubit refined Deutsch-Jozsa algorithm using SFG quantum logic gates

International Nuclear Information System (INIS)

Duce, A Del; Savory, S; Bayvel, P

2006-01-01

In this paper we present a quantum logic circuit which can be used for the experimental demonstration of a three-qubit solid state quantum computer based on a recent proposal of optically driven quantum logic gates. In these gates, the entanglement of randomly placed electron spin qubits is manipulated by optical excitation of control electrons. The circuit we describe solves the Deutsch problem with an improved algorithm called the refined Deutsch-Jozsa algorithm. We show that it is possible to select optical pulses that solve the Deutsch problem correctly, and do so without losing quantum information to the control electrons, even though the gate parameters vary substantially from one gate to another

Implementation of a three-qubit refined Deutsch-Jozsa algorithm using SFG quantum logic gates

Energy Technology Data Exchange (ETDEWEB)

Duce, A Del; Savory, S; Bayvel, P [Department of Electronic and Electrical Engineering, University College London, Torrington Place, London WC1E 7JE (United Kingdom)

2006-05-31

In this paper we present a quantum logic circuit which can be used for the experimental demonstration of a three-qubit solid state quantum computer based on a recent proposal of optically driven quantum logic gates. In these gates, the entanglement of randomly placed electron spin qubits is manipulated by optical excitation of control electrons. The circuit we describe solves the Deutsch problem with an improved algorithm called the refined Deutsch-Jozsa algorithm. We show that it is possible to select optical pulses that solve the Deutsch problem correctly, and do so without losing quantum information to the control electrons, even though the gate parameters vary substantially from one gate to another.

Implementation of a three-qubit refined Deutsch Jozsa algorithm using SFG quantum logic gates

Science.gov (United States)

DelDuce, A.; Savory, S.; Bayvel, P.

2006-05-01

In this paper we present a quantum logic circuit which can be used for the experimental demonstration of a three-qubit solid state quantum computer based on a recent proposal of optically driven quantum logic gates. In these gates, the entanglement of randomly placed electron spin qubits is manipulated by optical excitation of control electrons. The circuit we describe solves the Deutsch problem with an improved algorithm called the refined Deutsch-Jozsa algorithm. We show that it is possible to select optical pulses that solve the Deutsch problem correctly, and do so without losing quantum information to the control electrons, even though the gate parameters vary substantially from one gate to another.

Searching for highly entangled multi-qubit states

International Nuclear Information System (INIS)

Brown, Iain D K; Stepney, Susan; Sudbery, Anthony; Braunstein, Samuel L

2005-01-01

We present a simple numerical optimization procedure to search for highly entangled states of 2, 3, 4 and 5 qubits. We develop a computationally tractable entanglement measure based on the negative partial transpose criterion, which can be applied to quantum systems of an arbitrary number of qubits. The search algorithm attempts to optimize this entanglement cost function to find the maximal entanglement in a quantum system. We present highly entangled 4-qubit and 5-qubit states discovered by this search. We show that the 4-qubit state is not quite as entangled, according to two separate measures, as the conjectured maximally entangled Higuchi-Sudbery state. Using this measure, these states are more highly entangled than the 4-qubit and 5-qubit GHZ states. We also present a conjecture about the NPT measure, inspired by some of our numerical results, that the single-qubit reduced states of maximally entangled states are all totally mixed

A New Quantum Proxy Multi-signature Scheme Using Maximally Entangled Seven-Qubit States

Science.gov (United States)

Cao, Hai-Jing; Zhang, Jia-Fu; Liu, Jian; Li, Zeng-You

2016-02-01

In this paper, we propose a new secure quantum proxy multi-signature scheme using seven-qubit entangled quantum state as quantum channels, which may have applications in e-payment system, e-government, e-business, etc. This scheme is based on controlled quantum teleportation. The scheme uses the physical characteristics of quantum mechanics to guarantee its anonymity, verifiability, traceability, unforgetability and undeniability.

Discrete Wigner formalism for qubits and noncontextuality of Clifford gates on qubit stabilizer states

Science.gov (United States)

Kocia, Lucas; Love, Peter

2017-12-01

We show that qubit stabilizer states can be represented by non-negative quasiprobability distributions associated with a Wigner-Weyl-Moyal formalism where Clifford gates are positive state-independent maps. This is accomplished by generalizing the Wigner-Weyl-Moyal formalism to three generators instead of two—producing an exterior, or Grassmann, algebra—which results in Clifford group gates for qubits that act as a permutation on the finite Weyl phase space points naturally associated with stabilizer states. As a result, a non-negative probability distribution can be associated with each stabilizer state's three-generator Wigner function, and these distributions evolve deterministically to one another under Clifford gates. This corresponds to a hidden variable theory that is noncontextual and local for qubit Clifford gates while Clifford (Pauli) measurements have a context-dependent representation. Equivalently, we show that qubit Clifford gates can be expressed as propagators within the three-generator Wigner-Weyl-Moyal formalism whose semiclassical expansion is truncated at order ℏ0 with a finite number of terms. The T gate, which extends the Clifford gate set to one capable of universal quantum computation, requires a semiclassical expansion of the propagator to order ℏ1. We compare this approach to previous quasiprobability descriptions of qubits that relied on the two-generator Wigner-Weyl-Moyal formalism and find that the two-generator Weyl symbols of stabilizer states result in a description of evolution under Clifford gates that is state-dependent, in contrast to the three-generator formalism. We have thus extended Wigner non-negative quasiprobability distributions from the odd d -dimensional case to d =2 qubits, which describe the noncontextuality of Clifford gates and contextuality of Pauli measurements on qubit stabilizer states.

D-Wave's Approach to Quantum Computing: 1000-qubits and Counting!

CERN Multimedia

CERN. Geneva

2017-01-01

In this talk I will describe D-Wave's approach to quantum computing, including the system architecture of our 1000-qubit D-Wave 2X, its programming model, and performance benchmarks. Furthermore, I will describe how the native optimization and sampling capabilities of the quantum processor can be exploited to tackle problems in a variety of fields including medicine, machine learning, physics, and computational finance.

Hybrid quantum logic and a test of Bell's inequality using two different atomic isotopes.

Science.gov (United States)

Ballance, C J; Schäfer, V M; Home, J P; Szwer, D J; Webster, S C; Allcock, D T C; Linke, N M; Harty, T P; Aude Craik, D P L; Stacey, D N; Steane, A M; Lucas, D M

2015-12-17

Entanglement is one of the most fundamental properties of quantum mechanics, and is the key resource for quantum information processing (QIP). Bipartite entangled states of identical particles have been generated and studied in several experiments, and post-selected or heralded entangled states involving pairs of photons, single photons and single atoms, or different nuclei in the solid state, have also been produced. Here we use a deterministic quantum logic gate to generate a 'hybrid' entangled state of two trapped-ion qubits held in different isotopes of calcium, perform full tomography of the state produced, and make a test of Bell's inequality with non-identical atoms. We use a laser-driven two-qubit gate, whose mechanism is insensitive to the qubits' energy splittings, to produce a maximally entangled state of one (40)Ca(+) qubit and one (43)Ca(+) qubit, held 3.5 micrometres apart in the same ion trap, with 99.8 ± 0.6 per cent fidelity. We test the CHSH (Clauser-Horne-Shimony-Holt) version of Bell's inequality for this novel entangled state and find that it is violated by 15 standard deviations; in this test, we close the detection loophole but not the locality loophole. Mixed-species quantum logic is a powerful technique for the construction of a quantum computer based on trapped ions, as it allows protection of memory qubits while other qubits undergo logic operations or are used as photonic interfaces to other processing units. The entangling gate mechanism used here can also be applied to qubits stored in different atomic elements; this would allow both memory and logic gate errors caused by photon scattering to be reduced below the levels required for fault-tolerant quantum error correction, which is an essential prerequisite for general-purpose quantum computing.

Circuit QED with transmon qubits

Energy Technology Data Exchange (ETDEWEB)

Wulschner, Karl Friedrich; Puertas, Javier; Baust, Alexander; Eder, Peter; Fischer, Michael; Goetz, Jan; Haeberlein, Max; Schwarz, Manuel; Xie, Edwar; Zhong, Ling; Deppe, Frank; Fedorov, Kirill; Marx, Achim; Menzel, Edwin; Gross, Rudolf [Walther-Meissner-Institut, Bayerische Akademie der Wissenschaften, Garching (Germany); Physik-Department, TU Muenchen, Garching (Germany); Nanosystems Initiative Munich (NIM), Muenchen (Germany); Huebl, Hans [Walther-Meissner-Institut, Bayerische Akademie der Wissenschaften, Garching (Germany); Nanosystems Initiative Munich (NIM), Muenchen (Germany); Weides, Martin [Karlsruhe Institute of Technology (KIT), Karlsruhe (Germany)

2015-07-01

Superconducting quantum bits are basic building blocks for circuit QED systems. Applications in the fields of quantum computation and quantum simulation require long coherence times. We have fabricated and characterized superconducting transmon qubits which are designed to operate at a high ratio of Josephson energy and charging energy. Due to their low sensitivity to charge noise transmon qubits show good coherence properties. We couple transmon qubits to coplanar waveguide resonators and coplanar slotline resonators and characterize the devices at mK-temperatures. From the experimental data we derive the qubit-resonator coupling strength, the qubit relaxation time and calibrate the photon number in the resonator via Stark shifts.

Formulas for Rational-Valued Separability Probabilities of Random Induced Generalized Two-Qubit States

Directory of Open Access Journals (Sweden)

Paul B. Slater

2015-01-01

Full Text Available Previously, a formula, incorporating a 5F4 hypergeometric function, for the Hilbert-Schmidt-averaged determinantal moments ρPTnρk/ρk of 4×4 density-matrices (ρ and their partial transposes (|ρPT|, was applied with k=0 to the generalized two-qubit separability probability question. The formula can, furthermore, be viewed, as we note here, as an averaging over “induced measures in the space of mixed quantum states.” The associated induced-measure separability probabilities (k=1,2,… are found—via a high-precision density approximation procedure—to assume interesting, relatively simple rational values in the two-re[al]bit (α=1/2, (standard two-qubit (α=1, and two-quater[nionic]bit (α=2 cases. We deduce rather simple companion (rebit, qubit, quaterbit, … formulas that successfully reproduce the rational values assumed for general  k. These formulas are observed to share certain features, possibly allowing them to be incorporated into a single master formula.

Coupled quantum electrodynamics in photonic crystal cavities towards controlled phase gate operations

International Nuclear Information System (INIS)

Xiao, Y-F; Gao, J; McMillan, J F; Yang, X; Wong, C W; Zou, X-B; Chen, Y-L; Han, Z-F; Guo, G-C

2008-01-01

In this paper, a scalable photonic crystal cavity array, in which single embedded quantum dots (QDs) are coherently interacting, is studied theoretically. Firstly, we examine the spectral character and optical delay brought about by the coupled cavities interacting with single QDs, in an optical analogue to electromagnetically induced transparency. Secondly, we then examine the usability of this coupled QD-cavity system for quantum phase gate operation and our numerical examples suggest that a two-qubit system with fidelity above 0.99 and photon loss below 0.04 is possible.

Protecting quantum coherence of two-level atoms from vacuum fluctuations of electromagnetic field

International Nuclear Information System (INIS)

Liu, Xiaobao; Tian, Zehua; Wang, Jieci; Jing, Jiliang

2016-01-01

In the framework of open quantum systems, we study the dynamics of a static polarizable two-level atom interacting with a bath of fluctuating vacuum electromagnetic field and explore under which conditions the coherence of the open quantum system is unaffected by the environment. For both a single-qubit and two-qubit systems, we find that the quantum coherence cannot be protected from noise when the atom interacts with a non-boundary electromagnetic field. However, with the presence of a boundary, the dynamical conditions for the insusceptible of quantum coherence are fulfilled only when the atom is close to the boundary and is transversely polarizable. Otherwise, the quantum coherence can only be protected in some degree in other polarizable direction. -- Highlights: •We study the dynamics of a two-level atom interacting with a bath of fluctuating vacuum electromagnetic field. •For both a single and two-qubit systems, the quantum coherence cannot be protected from noise without a boundary. •The insusceptible of the quantum coherence can be fulfilled only when the atom is close to the boundary and is transversely polarizable. •Otherwise, the quantum coherence can only be protected in some degree in other polarizable direction.

Phase-controlled coherent population trapping in superconducting quantum circuits

International Nuclear Information System (INIS)

Cheng Guang-Ling; Wang Yi-Ping; Chen Ai-Xi

2015-01-01

We investigate the influences of the-applied-field phases and amplitudes on the coherent population trapping behavior in superconducting quantum circuits. Based on the interactions of the microwave fields with a single Δ-type three-level fluxonium qubit, the coherent population trapping could be obtainable and it is very sensitive to the relative phase and amplitudes of the applied fields. When the relative phase is tuned to 0 or π, the maximal atomic coherence is present and coherent population trapping occurs. While for the choice of π/2, the atomic coherence becomes weak. Meanwhile, for the fixed relative phase π/2, the value of coherence would decrease with the increase of Rabi frequency of the external field coupled with two lower levels. The responsible physical mechanism is quantum interference induced by the control fields, which is indicated in the dressed-state representation. The microwave coherent phenomenon is present in our scheme, which will have potential applications in optical communication and nonlinear optics in solid-state devices. (paper)

Modal and polarization qubits in Ti:LiNbO3 photonic circuits for a universal quantum logic gate.

Science.gov (United States)

Saleh, Mohammed F; Di Giuseppe, Giovanni; Saleh, Bahaa E A; Teich, Malvin Carl

2010-09-13

Lithium niobate photonic circuits have the salutary property of permitting the generation, transmission, and processing of photons to be accommodated on a single chip. Compact photonic circuits such as these, with multiple components integrated on a single chip, are crucial for efficiently implementing quantum information processing schemes.We present a set of basic transformations that are useful for manipulating modal qubits in Ti:LiNbO(3) photonic quantum circuits. These include the mode analyzer, a device that separates the even and odd components of a state into two separate spatial paths; the mode rotator, which rotates the state by an angle in mode space; and modal Pauli spin operators that effect related operations. We also describe the design of a deterministic, two-qubit, single-photon, CNOT gate, a key element in certain sets of universal quantum logic gates. It is implemented as a Ti:LiNbO(3) photonic quantum circuit in which the polarization and mode number of a single photon serve as the control and target qubits, respectively. It is shown that the effects of dispersion in the CNOT circuit can be mitigated by augmenting it with an additional path. The performance of all of these components are confirmed by numerical simulations. The implementation of these transformations relies on selective and controllable power coupling among single- and two-mode waveguides, as well as the polarization sensitivity of the Pockels coefficients in LiNbO(3).

Temperature dependence of coherence in transmon qubits

Energy Technology Data Exchange (ETDEWEB)

Schloer, Steffen; Braumueller, Jochen; Lukashenko, Oleksandr; Rotzinger, Hannes; Weides, Martin; Ustinov, Alexey V. [Physikalisches Institut, KIT, Karlsruhe (Germany); Sandberg, Martin; Vissers, Michael R.; Pappas, David P. [NIST, Boulder, CO (United States)

2015-07-01

Superconducting qubits are a promising field of research, not only with respect to quantum computing but also as highly sensitive detectors and due to the possibility of using them to study fundamental implications of quantum mechanics. The requirements for qubits that can be used as building blocks in a potential quantum computer are challenging. Modern superconducting qubits like the transmon are strong candidates for achieving these goals. The main challenge here is to increase the coherence of prepared quantum states. Here, we experimentally investigate the influence of temperature variation on relaxation and dephasing of a transmon qubit. Our goal is to understand decoherence mechanisms in material optimized circuits. Aiming at longer coherence, in this case peaking over 50 μs for T{sub 1} and T{sub 2}, our samples are fabricated at NIST using two different materials. Low-loss TiN was used for the shunt capacitance as well as the resonator, combined with shadow evaporated ultra-small Al-AlO{sub x}-Al Josephson junctions. We will present temperature-dependent data on qubit relaxation and dephasing times as well as power spectra. Our data will be compared to previously obtained temperature dependent data for other types of qubits.

The Qubit as Key to Quantum Physics Part II: Physical Realizations and Applications

Science.gov (United States)

Dür, Wolfgang; Heusler, Stefan

2016-01-01

Using the simplest possible quantum system--the qubit--the fundamental concepts of quantum physics can be introduced. This highlights the common features of many different physical systems, and provides a unifying framework when teaching quantum physics at the high school or introductory level. In a previous "TPT" article and in a…

Inversion of Qubit Energy Levels in Qubit-Oscillator Circuits in the Deep-Strong-Coupling Regime

Science.gov (United States)

Yoshihara, F.; Fuse, T.; Ao, Z.; Ashhab, S.; Kakuyanagi, K.; Saito, S.; Aoki, T.; Koshino, K.; Semba, K.

2018-05-01

We report on experimentally measured light shifts of superconducting flux qubits deep-strongly coupled to L C oscillators, where the coupling constants are comparable to the qubit and oscillator resonance frequencies. By using two-tone spectroscopy, the energies of the six lowest levels of each circuit are determined. We find huge Lamb shifts that exceed 90% of the bare qubit frequencies and inversions of the qubits' ground and excited states when there are a finite number of photons in the oscillator. Our experimental results agree with theoretical predictions based on the quantum Rabi model.

An Improved Protocol for Controlled Deterministic Secure Quantum Communication Using Five-Qubit Entangled State

Science.gov (United States)

Kao, Shih-Hung; Lin, Jason; Tsai, Chia-Wei; Hwang, Tzonelih

2018-03-01

In early 2009, Xiu et al. (Opt. Commun. 282(2) 333-337 2009) presented a controlled deterministic secure quantum communication (CDSQC) protocol via a newly constructed five-qubit entangled quantum state. Later, Qin et al. (Opt. Commun. 282(13), 2656-2658 2009) pointed out two security loopholes in Xiu et al.'s protocol: (1) A correlation-elicitation (CE) attack can reveal the entire secret message; (2) A leakage of partial information for the receiver is noticed. Then, Xiu et al. (Opt. Commun. 283(2), 344-347 2010) presented a revised CDSQC protocol to remedy the CE attack problem. However, the information leakage problem still remains open. This work proposes a new CDSQC protocol using the same five-qubit entangled state which can work without the above mentioned security problems. Moreover, the Trojan Horse attacks can be automatically avoided without using detecting devices in the new CDSQC.

An Improved Protocol for Controlled Deterministic Secure Quantum Communication Using Five-Qubit Entangled State

Science.gov (United States)

Kao, Shih-Hung; Lin, Jason; Tsai, Chia-Wei; Hwang, Tzonelih

2018-06-01

In early 2009, Xiu et al. (Opt. Commun. 282(2) 333-337 2009) presented a controlled deterministic secure quantum communication (CDSQC) protocol via a newly constructed five-qubit entangled quantum state. Later, Qin et al. (Opt. Commun. 282(13), 2656-2658 2009) pointed out two security loopholes in Xiu et al.'s protocol: (1) A correlation-elicitation (CE) attack can reveal the entire secret message; (2) A leakage of partial information for the receiver is noticed. Then, Xiu et al. (Opt. Commun. 283(2), 344-347 2010) presented a revised CDSQC protocol to remedy the CE attack problem. However, the information leakage problem still remains open. This work proposes a new CDSQC protocol using the same five-qubit entangled state which can work without the above mentioned security problems. Moreover, the Trojan Horse attacks can be automatically avoided without using detecting devices in the new CDSQC.

Robust quantum secure direct communication and authentication protocol against decoherence noise based on six-qubit DF state

International Nuclear Information System (INIS)

Chang Yan; Zhang Shi-Bin; Yan Li-Li; Han Gui-Hua

2015-01-01

By using six-qubit decoherence-free (DF) states as quantum carriers and decoy states, a robust quantum secure direct communication and authentication (QSDCA) protocol against decoherence noise is proposed. Four six-qubit DF states are used in the process of secret transmission, however only the |0′〉 state is prepared. The other three six-qubit DF states can be obtained by permuting the outputs of the setup for |0′〉. By using the |0′〉 state as the decoy state, the detection rate and the qubit error rate reach 81.3%, and they will not change with the noise level. The stability and security are much higher than those of the ping–pong protocol both in an ideal scenario and a decoherence noise scenario. Even if the eavesdropper measures several qubits, exploiting the coherent relationship between these qubits, she can gain one bit of secret information with probability 0.042. (paper)

Leakage and sweet spots in triple-quantum-dot spin qubits: A molecular-orbital study

Science.gov (United States)

Zhang, Chengxian; Yang, Xu-Chen; Wang, Xin

2018-04-01

A triple-quantum-dot system can be operated as either an exchange-only qubit or a resonant-exchange qubit. While it is generally believed that the decisive advantage of the resonant-exchange qubit is the suppression of charge noise because it is operated at a sweet spot, we show that the leakage is also an important factor. Through molecular-orbital-theoretic calculations, we show that when the system is operated in the exchange-only scheme, the leakage to states with double electron occupancy in quantum dots is severe when rotations around the axis 120∘ from z ̂ is performed. While this leakage can be reduced by either shrinking the dots or separating them further, the exchange interactions are also suppressed at the same time, making the gate operations unfavorably slow. When the system is operated as a resonant-exchange qubit, the leakage is three to five orders of magnitude smaller. We have also calculated the optimal detuning point which minimizes the leakage for the resonant-exchange qubit, and have found that although it does not coincide with the double sweet spot for the charge noise, they are rather close. Our results suggest that the resonant-exchange qubit has another advantage, that leakage can be greatly suppressed compared to the exchange-only qubit, and operating at the double sweet spot point should be optimal both for reducing charge noise and suppressing leakage.

No-go theorem for one-way quantum computing on naturally occurring two-level systems

International Nuclear Information System (INIS)

Chen Jianxin; Chen Xie; Duan Runyao; Ji Zhengfeng; Zeng Bei

2011-01-01

The ground states of some many-body quantum systems can serve as resource states for the one-way quantum computing model, achieving the full power of quantum computation. Such resource states are found, for example, in spin-(5/2) and spin-(3/2) systems. It is, of course, desirable to have a natural resource state in a spin-(1/2), that is, qubit system. Here, we give a negative answer to this question for frustration-free systems with two-body interactions. In fact, it is shown to be impossible for any genuinely entangled qubit state to be a nondegenerate ground state of any two-body frustration-free Hamiltonian. What is more, we also prove that every spin-(1/2) frustration-free Hamiltonian with two-body interaction always has a ground state that is a product of single- or two-qubit states. In other words, there cannot be any interesting entanglement features in the ground state of such a qubit Hamiltonian.

Suppression of error in qubit rotations due to Bloch-Siegert oscillation via the use of off-resonant Raman excitation

International Nuclear Information System (INIS)

Pradhan, Prabhakar; Cardoso, George C; Shahriar, M S

2009-01-01

The rotation of a quantum bit (qubit) is an important step in quantum computation. The rotation is generally performed using a Rabi oscillation. In a direct two-level qubit system, if the Rabi frequency is comparable to its resonance frequency, the rotating wave approximation is not valid, and the Rabi oscillation is accompanied by the so-called Bloch-Siegert oscillation (BSO) that occurs at twice the frequency of the driving field. One implication of the BSO is that for a given interaction time and Rabi frequency, the degree of rotation experienced by the qubit depends explicitly on the initial phase of the driving field. If this effect is not controlled, it leads to an apparent fluctuation in the rotation of the qubit. Here we show that when an off-resonant lambda system is used to realize a two-level qubit, the BSO is inherently negligible, thus eliminating this source of potential error.

Digital Quantum Simulation of Spin Models with Circuit Quantum Electrodynamics

OpenAIRE

Salathé, Y.; Mondal, M.; Oppliger, M.; Heinsoo, J.; Kurpiers, P.; Potočnik, A.; Mezzacapo, Antonio; Las Heras García, Urtzi; Lamata Manuel, Lucas; Solano Villanueva, Enrique Leónidas; Filipp, S.; Wallraff, A.

2015-01-01

Systems of interacting quantum spins show a rich spectrum of quantum phases and display interesting many-body dynamics. Computing characteristics of even small systems on conventional computers poses significant challenges. A quantum simulator has the potential to outperform standard computers in calculating the evolution of complex quantum systems. Here, we perform a digital quantum simulation of the paradigmatic Heisenberg and Ising interacting spin models using a two transmon-qubit circuit...

Demonstration of Multisetting One-Way Einstein-Podolsky-Rosen Steering in Two-Qubit Systems

Science.gov (United States)

Xiao, Ya; Ye, Xiang-Jun; Sun, Kai; Xu, Jin-Shi; Li, Chuan-Feng; Guo, Guang-Can

2017-04-01

Einstein-Podolsky-Rosen (EPR) steering describes the ability of one party to remotely affect another's state through local measurements. One of the most distinguishable properties of EPR steering is its asymmetric aspect. Steering can work in one direction but fail in the opposite direction. This type of one-way steering, which is different from the symmetry concepts of entanglement and Bell nonlocality, has garnered much interest. However, an experimental demonstration of genuine one-way EPR steering in the simplest scenario, i.e., one that employs two-qubit systems, is still lacking. In this Letter, we experimentally demonstrate one-way EPR steering with multimeasurement settings for a class of two-qubit states, which are still one-way steerable even with infinite settings. The steerability is quantified by the steering radius, which represents a necessary and sufficient steering criterion. The demonstrated one-way steering in the simplest bipartite quantum system is of fundamental interest and may provide potential applications in one-way quantum information tasks.

Controlling bi-partite entanglement in multi-qubit systems

International Nuclear Information System (INIS)

Plesch, Martin; Novotny, Jaroslav; Dzurakova, Zuzana; Buzek, VladimIr

2004-01-01

Bi-partite entanglement in multi-qubit systems cannot be shared freely. The rules of quantum mechanics impose bounds on how multi-qubit systems can be correlated. In this paper, we utilize a concept of entangled graphs with weighted edges in order to analyse pure quantum states of multi-qubit systems. Here qubits are represented by vertexes of the graph, while the presence of bi-partite entanglement is represented by an edge between corresponding vertexes. The weight of each edge is defined to be the entanglement between the two qubits connected by the edge, as measured by the concurrence. We prove that each entangled graph with entanglement bounded by a specific value of the concurrence can be represented by a pure multi-qubit state. In addition, we present a logic network with O(N 2 ) elementary gates that can be used for preparation of the weighted entangled graphs of N qubits

Controlling bi-partite entanglement in multi-qubit systems

Science.gov (United States)

Plesch, Martin; Novotný, Jaroslav; Dzuráková, Zuzana; Buzek, Vladimír

2004-02-01

Bi-partite entanglement in multi-qubit systems cannot be shared freely. The rules of quantum mechanics impose bounds on how multi-qubit systems can be correlated. In this paper, we utilize a concept of entangled graphs with weighted edges in order to analyse pure quantum states of multi-qubit systems. Here qubits are represented by vertexes of the graph, while the presence of bi-partite entanglement is represented by an edge between corresponding vertexes. The weight of each edge is defined to be the entanglement between the two qubits connected by the edge, as measured by the concurrence. We prove that each entangled graph with entanglement bounded by a specific value of the concurrence can be represented by a pure multi-qubit state. In addition, we present a logic network with O(N2) elementary gates that can be used for preparation of the weighted entangled graphs of N qubits.

Induced bipartite entanglement from three qubit states and quantum teleportation

Energy Technology Data Exchange (ETDEWEB)

Park, Dae-Kil; Son, Jin-Woo; Cha, Seong-Keuck [Kyungnam University, Masan (Korea, Republic of)

2010-06-15

Only Greenberger-Horne-Zeilinger and W states are well known to have genuine tripartite entanglement in all three qubit states. The entanglement of quantum state is also well known to play an important role in various quantum information processes. Then, the following question naturally arises: which one is better between the Greenberger-Horne-Zeilinger and the W states in real quantum information processing? We try to give an answer to this question from two aspects. First, we compute the induced bipartite entanglement for a mixture consisting of Greenberger-Horne-Zeilinger and W states. If the entanglement is the only physical resource for information processing, the induced bipartite entanglement suggests that Greenberger-Horne-Zeilinger and W states are equally good. Second, we choose the bipartite teleportation scheme as an example of quantum information processing using the mixture as a quantum channel and compute the average fidelities. Our calculation shows that the W state is slightly more robust than the Greenberger-Horne-Zeilinger state when a small perturbation disturbs the teleportation process. This slight discrepancy seems to imply that entanglement is not the only resource for quantum information processing.

Induced bipartite entanglement from three qubit states and quantum teleportation

International Nuclear Information System (INIS)

Park, Dae-Kil; Son, Jin-Woo; Cha, Seong-Keuck

2010-01-01

Only Greenberger-Horne-Zeilinger and W states are well known to have genuine tripartite entanglement in all three qubit states. The entanglement of quantum state is also well known to play an important role in various quantum information processes. Then, the following question naturally arises: which one is better between the Greenberger-Horne-Zeilinger and the W states in real quantum information processing? We try to give an answer to this question from two aspects. First, we compute the induced bipartite entanglement for a mixture consisting of Greenberger-Horne-Zeilinger and W states. If the entanglement is the only physical resource for information processing, the induced bipartite entanglement suggests that Greenberger-Horne-Zeilinger and W states are equally good. Second, we choose the bipartite teleportation scheme as an example of quantum information processing using the mixture as a quantum channel and compute the average fidelities. Our calculation shows that the W state is slightly more robust than the Greenberger-Horne-Zeilinger state when a small perturbation disturbs the teleportation process. This slight discrepancy seems to imply that entanglement is not the only resource for quantum information processing.

Teleportation via thermally entangled states of a two-qubit Heisenberg XXZ chain

Institute of Scientific and Technical Information of China (English)

QIN Meng; TAO Ying-Juan; TIAN Dong-Ping

2008-01-01

We investigate quantum teleportation as a tool to study the thermally entangled state of a twoqubit Heisenberg XXZ chain.Our work is mainly to investigate the characteristics of a Heisenberg XXZ chain and get some analytical results of the fully entangled fraction.We also consider the entanglement teleportation via a two-qubit Heisenberg XXZ chain.

Quantum Secure Direct Communication with Five-Qubit Entangled State

International Nuclear Information System (INIS)

Lin Song; Liu Xiao-Fen; Gao Fei

2011-01-01

Recently, a genuine five-qubit entangled state has been achieved by Brown et al.[J. Phys. A 38 (2005) 1119]. Later it was indicated that this state can be used for quantum teleportation and quantum state sharing. Here we build a quantum secure direct communication protocol with this state, and prove that it is secure in ideal conditions. In the protocol, the sender performs unitary transformations to encode a secret message on his/her particles and sends them to the receiver. The receiver then performs projective determinate measurement to decode the secret message directly. Furthermore, this protocol utilizes superdense coding to achieve a high intrinsic efficiency and source capacity. (general)

Coupling spin qubits via superconductors

DEFF Research Database (Denmark)

Leijnse, Martin; Flensberg, Karsten

2013-01-01

We show how superconductors can be used to couple, initialize, and read out spatially separated spin qubits. When two single-electron quantum dots are tunnel coupled to the same superconductor, the singlet component of the two-electron state partially leaks into the superconductor via crossed...... Andreev reflection. This induces a gate-controlled singlet-triplet splitting which, with an appropriate superconductor geometry, remains large for dot separations within the superconducting coherence length. Furthermore, we show that when two double-dot singlet-triplet qubits are tunnel coupled...... to a superconductor with finite charging energy, crossed Andreev reflection enables a strong two-qubit coupling over distances much larger than the coherence length....

Electromagnetic-field dependence of the internal excited state of the polaron and the qubit in quantum dot with thickness

Science.gov (United States)

Bai, Xu-Fang; Xin, Wei; Yin, Hong-Wu; Eerdunchaolu

2017-06-01

The electromagnetic-field dependence of the ground and the first excited-state (GFES) energy eigenvalues and eigenfunctions of the strong-coupling polaron in a quantum dot (QD) was studied for various QD thicknesses by using the variational method of the Pekar type (VMPT). On this basis, we construct a qubit in the quantum dot (QQD) by taking a two-level structure of the polaron as the carrier. The results of numerical calculations indicate that the oscillation period of the qubit, {itT}{in0}, increases with increasing the thickness of the quantum dot (TQD) {itL}, but decreases with increasing the cyclotron frequency of the magnetic field (CFMF) ω{in{itc}}, electric-field strength {itF}, and electron-phonon coupling strength (EPCS) α. The probability density of the qubit |Ψ({itρ}, {itz}, {itt})|{su2} presents a normal distribution of the electronic transverse coordinate ρ, significantly influenced by the TQD and effective radius of the quantum dot (ERQD) {itR}{in0}, and shows a periodic oscillation with variations in the electronic longitudinal coordinate {itz}, polar angle φ and time {itt}. The decoherence time τ and the quality factor {itQ} of the free rotation increase with increasing the CFMF ω{in{itc}}, dispersion coefficient η, and EPCS α, but decrease with increasing the electric-field strength {itF}, TQD {itL}, and ERQD {itR}{in0}. The TQD is an important parameter of the qubit. Theoretically, the target, which is to regulate the oscillation period, decoherence time and quality factor of the free rotation of the qubit, can be achieved by designing different TQDs and regulating the strength of the electromagnetic field.

Three-electron spin qubits

Science.gov (United States)

Russ, Maximilian; Burkard, Guido

2017-10-01

The goal of this article is to review the progress of three-electron spin qubits from their inception to the state of the art. We direct the main focus towards the exchange-only qubit (Bacon et al 2000 Phys. Rev. Lett. 85 1758-61, DiVincenzo et al 2000 Nature 408 339) and its derived versions, e.g. the resonant exchange (RX) qubit, but we also discuss other qubit implementations using three electron spins. For each three-spin qubit we describe the qubit model, the envisioned physical realization, the implementations of single-qubit operations, as well as the read-out and initialization schemes. Two-qubit gates and decoherence properties are discussed for the RX qubit and the exchange-only qubit, thereby completing the list of requirements for quantum computation for a viable candidate qubit implementation. We start by describing the full system of three electrons in a triple quantum dot, then discuss the charge-stability diagram, restricting ourselves to the relevant subsystem, introduce the qubit states, and discuss important transitions to other charge states (Russ et al 2016 Phys. Rev. B 94 165411). Introducing the various qubit implementations, we begin with the exchange-only qubit (DiVincenzo et al 2000 Nature 408 339, Laird et al 2010 Phys. Rev. B 82 075403), followed by the RX qubit (Medford et al 2013 Phys. Rev. Lett. 111 050501, Taylor et al 2013 Phys. Rev. Lett. 111 050502), the spin-charge qubit (Kyriakidis and Burkard 2007 Phys. Rev. B 75 115324), and the hybrid qubit (Shi et al 2012 Phys. Rev. Lett. 108 140503, Koh et al 2012 Phys. Rev. Lett. 109 250503, Cao et al 2016 Phys. Rev. Lett. 116 086801, Thorgrimsson et al 2016 arXiv:1611.04945). The main focus will be on the exchange-only qubit and its modification, the RX qubit, whose single-qubit operations are realized by driving the qubit at its resonant frequency in the microwave range similar to electron spin resonance. Two different types of two-qubit operations are presented for the exchange

Efficient Atomic One-Qubit Phase Gate Realized by a Cavity QED and Identical Atoms System

International Nuclear Information System (INIS)

He Yong; Jiang Nianquan

2010-01-01

We present a scheme to implement a one-qubit phase gate with a two-level atom crossing an optical cavity in which some identical atoms are trapped. One can conveniently acquire an arbitrary phase shift of the gate by properly choosing the number of atoms trapped in the cavity and the velocity of the atom crossing the cavity. The present scheme provides a very simple and efficient way for implementing one-qubit phase gate. (general)

Optical generation of matter qubit graph states

International Nuclear Information System (INIS)

Benjamin, S C; Eisert, J; Stace, T M

2005-01-01

We present a scheme for rapidly entangling matter qubits in order to create graph states for one-way quantum computing. The qubits can be simple three-level systems in separate cavities. Coupling involves only local fields and a static (unswitched) linear optics network. Fusion of graph-state sections occurs with, in principle, zero probability of damaging the nascent graph state. We avoid the finite thresholds of other schemes by operating on two entangled pairs, so that each generates exactly one photon. We do not require the relatively slow single qubit local flips to be applied during the growth phase: growth of the graph state can then become a purely optical process. The scheme naturally generates graph states with vertices of high degree and so is easily able to construct minimal graph states, with consequent resource savings. The most efficient approach will be to create new graph-state edges even as qubits elsewhere are measured, in a 'just in time' approach. An error analysis indicates that the scheme is relatively robust against imperfections in the apparatus

Nonlinear optics quantum computing with circuit QED.

Science.gov (United States)

Adhikari, Prabin; Hafezi, Mohammad; Taylor, J M

2013-02-08

One approach to quantum information processing is to use photons as quantum bits and rely on linear optical elements for most operations. However, some optical nonlinearity is necessary to enable universal quantum computing. Here, we suggest a circuit-QED approach to nonlinear optics quantum computing in the microwave regime, including a deterministic two-photon phase gate. Our specific example uses a hybrid quantum system comprising a LC resonator coupled to a superconducting flux qubit to implement a nonlinear coupling. Compared to the self-Kerr nonlinearity, we find that our approach has improved tolerance to noise in the qubit while maintaining fast operation.

Linear optical quantum computing in a single spatial mode.

Science.gov (United States)

Humphreys, Peter C; Metcalf, Benjamin J; Spring, Justin B; Moore, Merritt; Jin, Xian-Min; Barbieri, Marco; Kolthammer, W Steven; Walmsley, Ian A

2013-10-11

We present a scheme for linear optical quantum computing using time-bin-encoded qubits in a single spatial mode. We show methods for single-qubit operations and heralded controlled-phase (cphase) gates, providing a sufficient set of operations for universal quantum computing with the Knill-Laflamme-Milburn [Nature (London) 409, 46 (2001)] scheme. Our protocol is suited to currently available photonic devices and ideally allows arbitrary numbers of qubits to be encoded in the same spatial mode, demonstrating the potential for time-frequency modes to dramatically increase the quantum information capacity of fixed spatial resources. As a test of our scheme, we demonstrate the first entirely single spatial mode implementation of a two-qubit quantum gate and show its operation with an average fidelity of 0.84±0.07.

Berry phase in superconducting charge qubits interacting with a cavity field

International Nuclear Information System (INIS)

Abdel-Aty, Mahmoud

2009-01-01

We propose a method for analyzing Berry phase for a multi-qubit system of superconducting charge qubits interacting with a microwave field. By suitably choosing the system parameters and precisely controlling the dynamics, novel connection found between the Berry phase and entanglement creations.

Quantum-information approach to the Ising model: Entanglement in chains of qubits

International Nuclear Information System (INIS)

Stelmachovic, Peter; Buzek, Vladimir

2004-01-01

Simple physical interactions between spin-1/2 particles may result in quantum states that exhibit exotic correlations that are difficult to find if one simply explores state spaces of multipartite systems. In particular, we present a detailed investigation of the well-known Ising model of a chain (ring) of spin-1/2 particles (qubits) in a transverse magnetic field. We present explicit expressions for eigenstates of the model Hamiltonian for arbitrary number of spin-1/2 particles in the chain in the standard (computer) basis, and we investigate quantum entanglement between individual qubits. We analyze bipartite as well as multipartite entanglement in the ground state of the model. In particular, we show that bipartite entanglement between pairs of qubits of the Ising chain (measured in terms of a concurrence) as a function of the parameter λ has a maximum around the point λ=1, and it monotonically decreases for large values of λ. We prove that in the limit λ→∞ this state is locally unitary equivalent to an N-partite Greenberger-Horn-Zeilinger state. We also analyze a very specific eigenstate of the Ising Hamiltonian with a zero eigenenergy (we denote this eigenstate as the X-state). This X-state exhibits the 'extreme' entanglement in a sense that an arbitrary subset A of k≤n qubits in the Ising chain composed of N=2n+1 qubits is maximally entangled with the remaining qubits (set B) in the chain. In addition, we prove that by performing a local operation just on the subset B, one can transform the X-state into a direct product of k singlets shared by the parties A and B. This property of the X-state can be utilized for new secure multipartite communication protocols