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     

电荷量子比特与量子化光场之间的纠缠

研究了初态为混合态的电荷量子比特与量子化光场之间的纠缠.通过求解系统的concurrence下限, 研究初态的混合度λ和失谐量Δ对系统纠缠随时间演化的影响. 在弱场中, 电荷量子比特初始是激发态的系统, 其纠缠度远远大于电荷量子比特初始是基态的系统, 并且Δ对系统的纠缠有明显的抑制作用. 在强场中, 电荷量子比特初始分别为激发态和基态时系统的纠缠演化接近一致, 初态混合度最高时系统的纠缠度最小, 并且Δ对系统纠缠的影响变弱. 关键词： 约瑟夫森结 纠缠 混合态 concurrence下限     

C2(×)Cn量子系统的最大纠缠混合态

讨论了C2(×)Cn量子系统的最大纠缠混合态,得到了Negativity纠缠度下的最大纠缠混合态的解析结果,并计算了该态在非满秩情形下的量子相对熵纠缠度.     

C~2C~n量子系统的最大纠缠混合态

讨论了C~2C~n量子系统的最大纠缠混合态,得到了Negativity纠缠度下的最大纠缠混合态的解析结果,并计算了该态在非满秩情形下的量子相对熵纠缠度。     

从混合态构造相应的系统-热库纠缠纯态的新方法

在量子统计学和低温物理学中,常用密度算符(或混合态)来表示物理系综.本文旨在提出一个新方法能将混合态转化为系统与热库相纠缠的量子纯态,这样做不但可以彰显环境对系统的影响,探寻新的物理态,尤其是纠缠态,而且能将求系综平均的复杂计算转化为较为简洁的求纯态平均.我们的新方法是将相干态和有序算符内的积分理论结合起来,就可直接将混合态扩充为自由度加倍的纯态.     

级联环境中两比特纠缠态的量子非局域关联检验研究

量子纠缠态经级联环境中演化的量子非局域关联检验研究具有重要的现实意义。基于Hardy-type佯谬检验方案，本文分别以两比特量子纯态和混合态为研究对象，研究了其在级联环境中演化后的量子非局域关联检验情况。分析了纠缠态和腔的耦合强度、腔和库的耦合强度比值κ/γ以及马尔科夫环境和非马尔科夫环境对量子非局域关联检验的影响情况。结果表明，在马尔科夫环境中，且κ/γ越小，成功进行量子非局域关联检验的演化时间越长。进一步给出了量子混合态能够成功进行量子非局域关联检验的混合度参数m的范围，并给出了量子混合态经级联环境演化后，可成功进行量子非局域关联检验的演化时间范围。     

基于混合纠缠态的概率隐形传态

为了在实际中更好的利用隐形传态方案去传送信息,给出了以混合纠缠态为资源来传送未知混合态的概率隐形传态方案,分析了以混合纠缠态为资源的隐形传送 未知量子态成功概率的上界.研究发现,作为信道资源的混合纠缠态的最小特征值决定了隐形传态的成功概率上界,此结果可视作纯态的隐形转移方案到混合态情形的推广.     

基于双模压缩信道的双模高斯态量子隐形传态

研究了双模高斯态的两个模经由两个双模压缩量子信道的隐形传态.结果表明，当输入态有纠缠时，若要输出态的纠缠不为零，量子信道的纠缠必须大于一确定值，其大小在输入态为纯态时依赖于输入态的纠缠度；在输入态为混合态时不仅与输入态的纠缠有关，还依赖于输入态的整体纯度. 关键词： 量子隐形传态 量子纠缠 双模高斯态     

以磁场中海森堡链作为通道的纠缠隐形传递

在隐形传态理论中,利用的通道已经从最大纠缠纯态发展到非最大纠缠纯态.而混合纠缠态通道有可能更接近于实际情况.固体理论中海森堡链的热力学平衡态就是一类很重要的混合纠缠态,其中两个量子位的平衡态为:     

两比特纯态纠缠的直接测量

研究了如何在不需要量子态重构的前提下直接测量一个两比特任意纯态的纠缠concurrence。设计了一个concurrence的直接测量量子线路,即通过对待测纠缠粒子对的两个全同拷贝实施σz操作、单比特R旋转操作以及奇偶宇称校验检测,使得concurrence被编码在最后获得某些状态的概率中,从而实现对concurrence的直接测量。在给出concurrence直接测量的量子线路的基础上,基于光学系统设计了一个直接测量光子偏振纠缠concurrence的物理方案,实现两比特耦合的奇偶宇称校验检测操作只出现一次,大大降低了方案的复杂度。另外,该方案可实现远程纠缠态的纠缠直接测量。     

Mutual Restriction between Concurrence and Intrinsic Concurrence for Arbitrary Two-Qubit States

Concurrence is viewed as the most commonly approach for quantifying entanglement of two-qubit states, while intrinsic concurrence contains concurrence of four pure states consisting of a special pure state ensemble concerning an arbitrary two-qubit state. Thus, a natural question arises: Whether there is a specified relation between them.We firstly examine the relation between concurrence and intrinsic concurrence for the maximally nonlocal mixed state under a special unitary operation, which is not yet rigorously proved. In order to obtain a general result, we investigate the relation between concurrence and intrinsic concurrence using randomly generated two-qubit states,and derive an inequality relation between them. Finally, we take into account the relation between concurrence and intrinsic concurrence in open systems, and reveal the ratio of the two quantum resources, which is only correlated with the experiencing channels.     

Direct measurement of the concurrence for two-photon polarization entangled pure states by parity-check measurements

We present a protocol for directly measuring the concurrence of a two-photon polarization entangled pure or mixed state without prior quantum state tomography. By parity-check measurements and simple operations on two copies of the two-photon polarization entangled pure state, the concurrence is encoded in the total probability of picking up the odd parity states from the signal states. This protocol makes use of highly efficient homodyne detection, and it could be feasible in the near future with the help of the weak cross-Kerr nonlinearity. Moreover, our protocol can be used in a distributed fashion to directly determine the entanglement of remote states, which may find its important applications in quantum communication.     

Quantum Entanglement Swapping in a Non-Markovian Environment

We propose an entanglement swapping scheme for mixed states in a classical non-Markovian noises, which is modelled as the so-called Ornstein-Uhlenbeck processes. The two mixed states before entanglement swapping are two X-type mixed states, which are caused by the environment-induced decoherence on the initially Bell states. This is more practical than the pure state case in quantum information processing. The fidelity and concurrence of the post-swapping states are discussed.     

Entanglement in four qubit states: Polynomial invariant of degree 2, genuine multipartite concurrence and one-tangle

Characterization of the multipartite mixed state entanglement is still a challenging problem. This is due to the fact that the entanglement for the mixed states, in general, is defined by a convex-roof extension. That is the entanglement measure of a mixed state ρ of a quantum system can be defined as the minimum average entanglement of an ensemble of pure states. In this paper, we show that polynomial entanglement measures of degree 2 of even-N qubits X states is in the full agreement with the genuine multipartite (GM) concurrence. Then, we plot the hierarchy of entanglement classification for four qubit pure states and then using new invariants, we classify the four qubit pure states. We focus on the convex combination of the classes whose at most the one of the invariants is non-zero and find the relationship between entanglement measures consist of non-zero-invariant, GM concurrence and one-tangle. We show that in many entanglement classes of four qubit states, GM concurrence is equal to the square root of one-tangle.     

Comparative analysis of entanglement measures based on monogamy inequality

We evaluate the monogamy inequality for symmetric, non-symmetric pure states of importance in terms of squared concurrence, squared entanglement of formation, squared negativity of partial transpose and compare the corresponding tangles. We show that though concurrence and concurrence tangle are zero for two special classes of mixed entangled states, both negativity tangle and entanglement of formation(EOF) tangle turn out to be non-zero. A comparison of different tangles is carried out in each case and it is shown that while the concurrence tangle captures the genuine multiqubit entanglement in N-qubit pure states with N distinct spinors(containing GHZ and superposition of W-, obverse W states)either negativity tangle or EOF tangle is to be used as a better measure of entanglement in the W-class of states with two distinct spinors and in the special classes of mixed multiqubit states.     

Entanglement Transfer via Heisenberg Interaction in a Four-Qubit System

We investigate the entanglement transfer in a four-qubit system and calculate the concurrence between any two qubits in different initial states. We show that both the pure entangled state and mixed entangled state can be transferred. For some special coupling constants and some evolution time, entanglement can be completely transferred from one pair particles to another.     

Entanglement of Formation for Quantum States

We investigate the entanglement of formation for a class of high-dimensional quantum mixed states. We present a kind of generalized concurrence for a class of high-dimensional quantum pure states such that the entanglement of formation is a monotonically increasing convex function of the generallzed concurrence, from the monotonicity and convexity the entanglement of formafion for a class of high-dimensional mixed states has been calculated analytically,     

Teleportations of Mixed States and Multipartite Quantum States

In this paper, we propose a protocol to deterministically teleport an unknown mixed state of qubit by utilizing a maximally bipartite entangled state of qubits as quantum channel. Ira non-maximally entangled bipartite pure state is employed as quantum channel, the unknown mixed quantum state of qubit can be teleported with 1 -√ 1- C^2 probability, where C is the concurrence of the quantum channel. The protocol can also be generalized to teleport a mixed state of qudit or a multipartite mixed state. More important purpose is that, on the basis of the protocol, the teleportation of an arbitrary multipartite （pure or mixed） quantum state can be decomposed into the teleportation of each subsystem by employing separate entangled states as quantum channels. In the case of deterministic teleportation, Bob only needs to perform unitary transformations on his single particles in order to recover the initial teleported multipartite quantum state.     

Monogamous nature of symmetric N-qubit states of the W class: Concurrence and negativity tangle

Using Majorana representation of symmetric N-qubit pure states, we have examined the monogamous nature of the family of states with two-distinct spinors, the W class of states. We have evaluated the N-concurrence tangle and showed that all the states in this family have vanishing concurrence tangle. The negativity tangle for the W class of states is shown to be non-zero, illustrating the fact that the concurrence tangle is always lesser than or equal to the negativity tangle in a pure N-qubit state.     

Entanglement dynamics of two interacting qubits under the influence of local dissipation

We investigate the dynamics of entanglement given by the concurrence of a two-qubit system in the non-Markovian setting. A quantum master equation is derived, which is solved in the eigenbasis of the system Hamiltonian for X-type initial states. A closed formula for time evolution of concurrence is presented for a pure state. It is shown that under the influence of dissipation non-zero entanglement is created in unentangled two-qubit states which decay in the same way as pure entangled states. We also show that under real circumstances, the decay rate of concurrence is strongly modified by the non-Markovianity of the evolution.