Investigations on quantum correlation of coupled qubits in squeezed vacuum reservoir

In this paper, we investigate the quantum correlation of coupled qubits which are initially in maximally entangled mixed states in squeezed vacuum reservoir. We compare and analyse the effects of squeezed parameters on quantum discord and quantum concurrence. The results show that in squeezed vacuum reservoir, the quantum discord and quantum concurrence perform completely opposite behaviors to the change of squeezed parameters. Quantum discord survives longer with the increase of squeezed amplitude parameter, but entanglement death turns faster on the contrary. The results also indicate that the classical correlation of the system is smaller than quantum discord in vacuum reservoir, while it is bigger than quantum discord in squeezed vacuum reservoir. The quantum discord and classical correlation are more robust than quantum concurrence in the two reservoir environments, which indicates that the entanglement actually is easily affected by decoherence and quantum discord has stronger ability to avoid decoherence in squeezed vacuum reservoir.     

Investigations into quantum correlation of coupled qubits in a squeezed vacuum reservoir

In this paper,we investigate the quantum correlation of coupled qubits which are initially in maximally entangled mixed states in a squeezed vacuum reservoir.We compare and analyze the effects of squeezed parameters on quantum discord and quantum concurrence.The results show that in a squeezed vacuum reservoir,the quantum discord and quantum concurrence perform with completely opposite behaviors with the change of squeezed parameters.Quantum discord survives longer with the increase of squeezed amplitude parameter,but entanglement death is faster on the contrary.The results also indicate that the classical correlation of the system is smaller than quantum discord in a vacuum reservoir,while it is bigger than quantum discord in a squeezed vacuum reservoir.The quantum discord and classical correlation are more robust than quantum concurrence in the two reservoir environments,which indicates that the entanglement actually is easily affected by decoherence and quantum discord has a stronger ability to avoid decoherence in a squeezed vacuum reservoir.     

Oscillator model of qubits and its entanglement properties

The two-mode symmetric oscillator is used to construct the qubit model as the superposition of the first excited degenerate level states of the oscillator. The entanglement properties of the oscillator states are studied using the known criterion of separability. Application to the quantum computing model based on light modes propagating in optical waveguides is briefly discussed.     

Decoherence in solid-state qubits

The interaction of solid-state qubits with environmental degrees of freedom strongly affects the qubit dynamics, and leads to decoherence. In quantum information processing with solid-state qubits, decoherence significantly limits the performances of such devices. Therefore, it is necessary to fully understand the mechanisms that lead to decoherence. In this review, we discuss how decoherence affects two of the most successful realizations of solid-state qubits, namely, spin qubits and superconducting qubits. In the former, the qubit is encoded in the spin 1/2 of the electron, and it is implemented by confining the electron spin in a semiconductor quantum dot. Superconducting devices show quantum behaviour at low temperatures, and the qubit is encoded in the two lowest energy levels of a superconducting circuit. The electron spin in a quantum dot has two main decoherence channels, a (Markovian) phonon-assisted relaxation channel, due to the presence of a spin–orbit interaction, and a (non-Markovian) spin bath constituted by the spins of the nuclei in the quantum dot that interact with the electron spin via the hyperfine interaction. In a superconducting qubit, decoherence takes place as a result of fluctuations in the control parameters, such as bias currents, applied flux and bias voltages, and via losses in the dissipative circuit elements.     

Comparison and control of the robustness between quantum entanglement and quantum correlation in an open quantum system

We investigate the influence of environmental decoherence on the dynamics of a coupled qubit system and quantum correlation.We analyse the relationship between concurrence and the degree of initial entanglement or the purity of initial quantum state,and also their relationship with quantum discord.The results show that the decrease of the purity of an initial quantum state can induce the attenuation of concurrence or quantum discord,but the attenuation of quantum discord is obviously slower than the concurrence’s,correspondingly the survival time of quantum discord is longer.Further investigation reveals that the robustness of quantum discord and concurrence relies on the entanglement degree of the initial quantum state.The higher the degree of entanglement,the more robust the quantum discord is than concurrence.And the reverse is equally true.Birth and death happen to quantum discord periodically and a newborn quantum discord comes into being under a certain condition,so does the concurrence.     

Multipartite entanglement in the interaction system between a single-mode microwave cavity field and superconducting charge qubits

This paper proposes a method of generating multipartite entanglement through using d.c. superconducting quantum interference devices (SQUID) inside a standing wave cavity. In this scheme, the d.c. SQUID works in the charge region. It is shown that, a large number of important multipartite entangled states can be generated by a controllable interaction between a cavity field and qubits. It is even possible to produce entangled states involving different cavity modes based on the measurement of charge qubits states. After such superpositions states are created, the interaction can be switched off by the classical magnetic field through the SQUID, and there is no information transfer between the cavity field and the charge qubits.     

Control of purity and entanglement for two spatially two-separated qubits via phase damping

Control of purity and entanglement of two two-qubits dispersively coupled to a field with a reservoir are investigated.Initially the qubits are entangled,while the field is either in a coherent state or a statistical mixture of two coherent states.For an alternative entanglement measure we calculate the negativity of the eigenvalues of the partially transposed density matrix.A measure related to the mutual entropy,namely the index of entropy,is employed to measure the entanglement.Its results agree well with the negativity.It is found that the entanglement and purity have strong sensitivity to phase damping.The asymptotic behaviour of the states of the field,the two two-qubits,and the total system fall into mixed states.     

Entanglement and decoherence of coupled superconductor qubits in a non-Markovian environment

The sudden death of entanglement is investigated for the non-Markovian dynamic process of a pair of interacting flux qubits under a thermal bath. The results show that, for initially two-qubit entangled states, entanglement sudden death (ESD) always happens in the thermal reservoir, where its appearance strongly depends on the environment. In particular, ESD of the qubits occurs more easily for the non-Markovian process than for the Markovian one.     

Sudden death and revival qubits coupled collectively of entanglement of two to a thermal reservoir

In this paper, we investigate the entanglement of two qubits coupled collectively to a common thermal environment and find that the the collective decay can lead to a revival of the entanglement that has already been destroyed. We also show that the ability of the system to revival entanglement relies on the mean photon number of the thermal environment and the degree of entanglement of the initial state.     

Sudden death and revival of entanglement of two qubits coupled collectively to a thermal reservoir

In this paper, we investigate the entanglement of two qubits coupled collectively to a common thermal environment and find that the the collective decay can lead to a revival of the entanglement that has already been destroyed. We also show that the ability of the system to revival entanglement relies on the mean photon number of the thermal environment and the degree of entanglement of the initial state.     

用约瑟夫森电荷比特系统实现一种特殊量子态的传输

构造了一种约瑟夫森电荷量子比特电路系统,并研究了四量子位态在该系统中的传输特性.对均匀与非均匀传输通道两种情形分别讨论了怎样通过控制外加磁通来实现激发态 1〉从第一量子位到第四量子位的理想传输.此外还分析了量子态在该系统中传输时的平均保真度,结果表明该系统不能传输一个任意态. 关键词： 量子态传输 约瑟夫森电荷比特     

Transfer of quantum entangled states between superconducting qubits and microwave field qubits

Transferring entangled states between matter qubits and microwave-field (or optical-field) qubits is of fundamental interest in quantum mechanics and necessary in hybrid quantum information processing and quantum communication. We here propose a way for transferring entangled states between superconducting qubits (matter qubits) and microwave-field qubits. This proposal is realized by a system consisting of multiple superconducting qutrits and microwave cavities. Here, „qutrit” refers to a three-level quantum system with the two lowest levels encoding a qubit while the third level acting as an auxiliary state. In contrast, the microwave-field qubits are encoded with coherent states of microwave cavities. Because the third energy level of each qutrit is not populated during the operation, decoherence from the higher energy levels is greatly suppressed. The entangled states can be deterministically transferred because measurement on the states is not needed. The operation time is independent of the number of superconducting qubits or microwave-field qubits. In addition, the architecture of the circuit system is quite simple because only a coupler qutrit and an auxiliary cavity are required. As an example, our numerical simulations show that high-fidelity transfer of entangled states from two superconducting qubits to two microwave-field qubits is feasible with present circuit QED technology. This proposal is quite general and can be extended to transfer entangled states between other matter qubits (e.g., atoms, quantum dots, and NV centers) and microwave- or optical-field qubits encoded with coherent states.     

Three qubits in a symmetric environment: Dissipatively generated asymptotic entanglement

We study the asymptotic entanglement of three identical qubits under the action of a Markovian open system dynamics that does not distinguish them. We show that by adding a completely depolarized qubit to a special class of two-qubit states, by letting them reach the asymptotic state and by finally eliminating the added qubit, can provide more entanglement than by direct immersion of the two qubits within the same environment.     

Charge qubits and limitations of electrostatic quantum gates

We investigate quantum dot arrays and their application to quantum computation. The arrays analyzed contain a total of a few operating electrons with constant tunneling between the dots. We construct quantum two-level systems near the ground state with a large energy separation to the remainder of the states and with the electrostatic interaction modeled within the capacitance matrix formalism. A set of representative examples is investigated numerically.     

Coherent-state qubits: entanglement and decoherence

Arbitrary superpositions of any two optical coherent states are investigated as realizations of qubits for quantum information processing. Decoherence of these coherent-state qubits is described in detail, and visualized using a suitable adaptive Bloch-sphere. The entanglement that can be created by a beam splitter from these states is quantified, and its decoherence behavior is analyzed.Received: 13 May 2004, Published online: 10 August 2004PACS: 42.50.Dv Nonclassical states of the electromagnetic field, including entangled photon states; quantum state engineering and measurements - 03.65.Ud Entanglement and quantum nonlocality (e.g. EPR paradox, Bells inequalities, GHZ states, etc.) - 03.67.-a Quantum information     

Dynamics control of geometric quantum discord for two coupling qubits in a squeezed vacuum reservoir

We investigate the dynamics of geometric quantum discord of coupled qubits in a squeezed vacuum reservoir. The results show that there is distinct difference between the dynamics of geometric quantum discord and that of quantum entanglement near (or away from) the decoherence free subspace. We also find that the squeezed vacuum reservoir with high squeezed amplitude is more suitable for geometric quantum discord to survive. The robustness of geometric quantum discord is stronger than that of quantum entanglement.     

Geometric quantum discord and Berry phase between two charge qubits coupled by a quantum transmission line

Geometric quantum discord（GQD） and Berry phase between two charge qubits coupled by a quantum transmission line are investigated. We show how GQDs evolve and investigate their dependencies on the parameters of the system.We also calculate the energy and the Berry phase and compare them with GQD, finding that there are close connections between them.     

Faithful quantum entanglement sharing based on linear optics with additional qubits

This paper presents a scheme for faithfully distributing a pure entanglement between two parties over an arbitrary collective-noise channel with linear optics. The transmission is assisted by an additional qubit against collective noise. The receiver can take advantage of the time discrimination and the measurement results of the assistant qubit to reconstruct a pure entanglement with the sender. Although the scheme succeeds probabilistically, the resource used to get a pure entanglement state is finite, and so is easier to establish entanglement in practice than quantum entanglement purification.     

Entanglement dynamics of two qubits coupled by Heisenberg XY interaction under a nonuniform magnetic field in a spin bath

This paper investigates the entanglement dynamics of a Heisenberg XY model for a two-spin system in the presence of a nonuniform magnetic field.The master equations and the concurrence evolution equations for the initial α state are derived and analysed.It is shown that for the symmetric initial α state,only the nonuniform field can play a role in entanglement dynamics while the uniform field and the bath will not play such a role.For the asymmetric α state,the nonuniform field leads to the beat pattern oscillation of the concurrence evolution.The inhomogeneity of the field can enhance the entanglement by suppressing the decoherence effects of both the spin-orbit interaction and the spin bath.     

分子磁体中的量子隧穿及宏观量子效应

文章介绍了分子磁体中的量子隧穿和宏观量子效应理论和实验研究的新进展．分子磁体既有宏观磁体特性也呈现纯量子行为，例如磁化矢量的量子隧穿．文章作者解释了如何通过量子隧穿实现宏观量子相干(即薛定谔猫态的相干叠加)和量子态位相干涉．对隧穿率计算的瞬子方法，特别是有限温度隧穿理论及其在分子磁体量子隧穿中的应用也做了简要的阐述．