Properties on the distant distribution of entanglement for arbitrary two-qubit pure states via noisy quantum channels

This paper investigates the change of entanglement for transmitting an arbitrarily entangled two-qubit pure state via one of three typical kinds of noisy quantum channels: amplitude damping quantum channel, phase damping quantum channel and depolarizing quantum channel. It finds, in all these three cases, that the output distant entanglement (measured by concurrence) reduces proportionately with respect to its initial amount, and the decaying ratio is determined only by the noisy characteristics of quantum channels and independent of the form of initial input state.     

Robustness of two-qubit and three-qubit states in correlated quantum channels

We investigate how the correlated actions of quantum channels affect the robustness of entangled states. We consider the Bell-like state and random two-qubit pure states in the correlated depolarizing, bit flip, bit-phase flip, and phase flip channels. It is found that the robustness of two-qubit pure states can be noticeably enhanced due to the correlations between consecutive actions of these noisy channels, and the Bell-like state is always the most robust one. We also consider the robustness of three-qubit pure states in correlated noisy channels. For the correlated bit flip and phase flip channels, the result shows that although the most robust and most fragile states are locally unitary equivalent, they exhibit different robustness in different correlated channels, and the effect of channel correlations on them is also significantly different. However, for the correlated depolarizing and bit-phase flip channels, the robustness of two special three-qubit pure states is exactly the same. Moreover, compared with the random three-qubit pure states, they are neither the most robust states nor the most fragile states.     

Symmetric quantum discord for a two-qubit state

We discuss the symmetric quantum discord(SQD) for an arbitrary two-qubit state consisting of subsystems A and B and give the analysis formula of the symmetric quantum discord for the arbitrary two-qubit state. We also give the optimization process of the symmetric quantum discord for some states and obtain the symmetric quantum discord. We compare the quantum discord(QD) with the symmetric quantum discord, and find that the symmetric quantum discord is greater than the quantum discord. We also find that the symmetric quantum discord can be unequal to the quantum discord when the right quantum discord(measure on subsystem B) is equal to the left quantum discord(measure on subsystem A).     

Quantifying entanglement of two-qubit Werner states

In this article, we introduce a framework for entanglement quantification of photon pairs represented by two-qubit Werner states. The measurement scheme is based on the symmetric informationally complete POVM. To make the framework realistic, we impose the Poisson noise on the measured two-photon coincidences. For various settings, numerical simulations were performed to evaluate the efficiency of the framework.     

基于GHZ纠缠态实现纯EPR对的双向量子隐形传态

提出了一种基于Greenberger-Horne-Zeilinger(GHZ)纠缠态进行纯EPR对双向隐形传态的方案.通过使用纠缠交换技术,通信双方Alice和Bob共享两对三粒子GHZ纠缠态来构建量子信道.方案中通过使用受控非门操作,单量子位测量以及适当的幺正操作,通信双方可以同时发送一个纯EPR对给对方.故相比仅可以传送单一量子态的方案更经济.     

解说量子纠缠理论

量子纠缠态在量子信息处理,如量子隐形传态、量子密集码、量子纠错、量子保密通信、量子计算等过程中起了十分重要的作用.量子纠缠理论主要研究量子态的纠缠刻画、分类及其在量子信息处理中的应用.文章介绍了量子纠缠理论中的一些基本概念和结果,其中包括：量子力学的实在性、局域性的讨论与Bell不等式的联系,Bell不等式与量子态可分性间的关系;纯态和混合态可分性的定义及若干判别准则(包括矩阵正映照方法、部分转置判据、约化判据、重排判据、纠缠见证、协方差判据及局域测不准关系判据);部分纠缠度量的介绍(包括纠缠形成、并发度、相对熵、负度、缠结和纠缠帮助,以及纠缠度量的计算和上下界的估算).     

Probabilistic teleportation of an arbitrary GHZ-class state with a pure entangled two-particle quantum channel and its application in quantum state sharing

This paper presents a scheme for probabilistic teleportation of an arbitrary GHZ-class state with a pure entangled two-particle quantum channel. The sender Alice first teleports the coefficients of the unknown state to the receiver Bob, and then Bob reconstructs the state with an auxiliary particle and some unitary operations if the teleportation succeeds. This scheme has the advantage of transmitting much less particles for teleporting an arbitrary GHZ-class state than others. Moreover, it discusses the application of this scheme in quantum state sharing.     

Entanglement dynamics of two-qubit systems in different quantum noises

The entanglement dynamics of two-qubit systems in different quantum noises are investigated by means of the operator-sum representation method.We find that,except for the amplitude damping and phase damping quantum noise,the sudden death of entanglement is always observed in different two-qubit systems with generalized amplitude damping and depolarizing quantum noise.     

Direct measurement of the concurrence for two-qubit electron spin entangled pure state based on charge detection

We propose a protocol for directly measuring the concurrence of a two-qubit electronic pure entangled state. To complete this task, we first design a parity-check measurement(PCM) which is constructed by two polarization beam splitters(PBSs) and a charge detector. By using the PCM for three rounds, we can achieve the concurrence by calculating the total probability of picking up the odd parity states from the initial states. Since the conduction electron may be a good candidate for the realization of quantum computation, this protocol may be useful in future solid quantum computation.     

带有Dzyaloshinski-Mariya相互作用的两比特XXZ模型的纠缠量子热机

以带有Dzyaloshinski-Mariya 相互作用的两比特XXZ模型为工作物质构建纠缠量子热机. 在量子热力学平衡态下, 采用Kieu的形式描述了做功与热传递.对于不同的各向异性参数, 分析了热机循环中量子纠缠与热传递、做功以及机械效率等热力学量之间的关系. 结果表明: 在这个纠缠体系中, 热力学第二定律依然成立; 机械效率的等值线图是环状曲线; 当各向异性参数Δ较小时, 热机在C₁ > C₂ 和C₁ < C₂ 两区域运行, 当增大Δ值时, 热机只在C₁ > C₂ 区域运行. 关键词： 量子热机 并发度 机械效率     

负值量子条件熵与双量子系统一类混合态纠缠量度

运用负值量子条件熵研究了双量子系统一类混合态的纠缠量度.给出了负值量子条件作为条件熵纠缠度的定义，证明了条件熵纠缠满足作为2×2系统一类混合纠缠态量度的四个基本条件.当双量子系统处于纯态时，条件熵纠缠度即为部分熵纠缠度.应用条件熵纠缠度研究了真空腔场中两全同二能级原子之间纯态和一类混合态纠缠的时间演化，比较了相同条件下两全同原子系统concurrence纠缠度的时间演化.结果表明，两纠缠度演化规律完全一致，验证了负值量子条件熵可以作为双量子系统纯态和一类混合态的纠缠量度. 关键词： 双量子系统 负值量子条件熵 条件熵纠缠度 混合态纠缠度     

量子计算算法介绍

量子计算机利用量子力学原理进行计算,具有量子并行计算的优势,能够超越经典计算1990年中期,量子算法取得突破,舒尔(Shor)构造了大数质因子的量子算法,葛洛沃(Grover)构造了无序数据库的量子搜索算法,引起了人们对量子计算的重视,极大地推动了量子计算的研究.文章简单介绍了几个典型的量子算法以及量子算法研究的一些新进展.     

A thermal entangled quantum refrigerator based on a two-qubit Heisenberg model with Dzyaloshinskii-Moriya interaction in an external magnetic field

Based on an isotropic two spin-1/2 qubits Heisenberg model with the Dzyaloshinskii-Moriya interaction in an external magnetic field, we have constructed an entangled quantum refrigerator. Expressions for the basic thermodynamic quantities, i.e., the heat exchanged, the net work input, and the coefficient of performance, are derived. Some intriguing features and their qualitative explanations in zero and non zero magnetic fields are given. The influence of the thermal entanglement on the refrigerator is investigated. The results obtained here have general significance and will be helpful to understand the performance of an entangled quantum refrigerator.     

Quantum connectivity optimization algorithms for entanglement source deployment in a quantum multi-hop network

At first, the entanglement source deployment problem is studied in a quantum multi-hop network, which has a significant influence on quantum connectivity. Two optimization algorithms are introduced with limited entanglement sources in this paper. A deployment algorithm based on node position (DNP) improves connectivity by guaranteeing that all overlapping areas of the distribution ranges of the entanglement sources contain nodes. In addition, a deployment algorithm based on an improved genetic algorithm (DIGA) is implemented by dividing the region into grids. From the simulation results, DNP and DIGA improve quantum connectivity by 213.73% and 248.83% compared to random deployment, respectively, and the latter performs better in terms of connectivity. However, DNP is more flexible and adaptive to change, as it stops running when all nodes are covered.     

Thermal entanglement and teleportation of a thermally mixed entangled state of a Heisenberg chain through a Werner state

The thermal entanglement and teleportation of a thermally mixed entangled state of a two-qubit Heisenberg XXX chain under the Dzyaloshinski Moriya （DM） anisotropic antisymmetric interaction through a noisy quantum channel given by a Werner state is investigated. The dependences of the thermal entanglement of the teleported state on the DM coupling constant, the temperature and the entanglement of tbe noisy quantum channel are studied }n detail for both the ferromagnetic and the antiferromagnetic cases. The result shows that a minimum entanglement of the noisy quantum channel must be provided in order to realize the entanglement teleportation. The values of fidelity of the teleported state are also studied for these two cases. It is found that under certain conditions, we can transfer an initial state with a better fidelity than that for any classical communication protocol.     

双光子Tavis-Cummings模型中的量子纠缠

我们对两个全同二能级原子通过双光子跃迁与单模辐射场发生相互作用的Tavis-Cummings(T-C)模型中的量子纠缠特性进行了研究.通过数值计算与分析,结果表明:双光子T-C模型的量子纠缠比单光子T-C模型的量子纠缠要保持得好一些;另外分析了双光子T-C模型中原子间偶极-偶极相互作用强度系数、原子与场的耦合系数、光子数对两原子间纠缠特性与场熵的影响.     

Experimental realization of quantum controlled teleportation of arbitrary two-qubit state via a five-qubit entangled state

Quantum controlled teleportation is the transmission of the quantum state under the supervision of a third party. This paper presents the theoretical and experimental results of an arbitrary two-qubit quantum controlled teleportation scheme, in which the sender Alice only needs to perform two Bell state measurements and the receiver Bob can perform an appropriate unitary operation to reconstruct the arbitrary two-qubit states under the control of the supervisor Charlie. The operation process of the scheme is verified on the IBM quantum experience platform, and the accuracy of the transmitted quantum state is further checked by performing quantum state tomography. Meanwhile, a good fidelity is obtained by using the theoretical density matrix and the experimental density matrix. A sequence of photonic states is introduced to analyze the possible intercept-replace-resend, intercept-measure-resend, and entanglement-measure-resend attacks on this scheme. The results proved that our scheme is highly secure.     

Quantum correlation of a three-particle-class state under quantum decoherence

We investigate the quantum characteristics of a three-particle W-class state and reveal the relationship between quantum discord and quantum entanglement under decoherence.We can also identify the state for which discord takes a maximal value for a given decoherence factor,and present a strong bound on quantum entanglement–quantum discord.In contrast,a striking result will be obtained that the quantum discord is not always stronger than the entanglement of formation in the case of decoherence.Furthermore,we also theoretically study the variation trend of the monogamy of quantum correlations for the three-particle W-class state under the phase flip channel,and find that the three-particle W-class state could transform from polygamous into monogamous,owing to the decoherence.     

Avoiding the decay of entanglement for coupling two-qubit system interacting with a non-Markov environment

The entanglement evolution of the coupled qubits interacting with a non-Markov environment is investigated in terms of concurrence.The results show that the entanglement of the quantum systems depends not only on the initial state of the system but also on the coupling between the qubit and the environment.For the initial state （|00 ± |11） /2^1/2,the coupled qubits will always been in the maximum entangled state under an asymmetric coupling.For the initial state （|01 ± |10） /2^1/2,in contrast,the entangling degree of the coupled qubits is always equal to unity and does not depend on the evolving time under the symmetric coupling.We find that the stronger the interaction between the qubits is,the better the struggle against the entanglement sudden death is.     

两原子与数态场相互作用系统中纠缠的调控

通过计算并发度研究了两个处于初始激发态的两能级原子与数态场相互作用系统的纠缠动力学特性,并讨论了场的光子数、原子和场的失谐量以及原子操作对并发度的影响。结果表明当不存在原子操作时,两原子之间的纠缠出现突然产生现象,并且可以通过调节光子数和原子与场的失谐量来控制产生纠缠的阈值时间和纠缠的最大值。当存在原子操作时,两原子之间的纠缠随着时间的演化可以立即产生,而且通过对经典场的操作和控制可以实现两原子之间纠缠的调控。