Sufficient and necessary conditions of separability for bipartite pure states in infinite-dimensional systems

The PHC criterion and the realignment criterion for pure states in infinite-dimensional bipartite quantum systems are given. Furthermore, several equivalent conditions for pure states to be separable are generalized to infinite-dimensional systems.     

An error-free protocol for quantum entanglement distribution in long-distance quantum communication

Quantum entanglement distribution is an essential part of quantum communication and computation protocols. Here, linear optic elements are employed for the distribution of quantum entanglement over a long distance. Polarization beam splitters and wave plates are used to realize an error-free protocol for broadcasting quantum entanglement in optical quantum communication. This protocol can determine the maximum distance of quantum communication without decoherence. Error detection and error correc-tion are performed in the proposed scheme. In other words, if there is a bit flip along the quantum channel, the end stations (Alice and Bob) can detect this state change and obtain the correct state (entangled photon) at another port. Existing general error detec-tion protocols are based on the quantum controlled-NOT (CNOT) or similar quantum logic operations, which are very difficult to implement experimentally. Here we present a feasible scheme for the implementation of entanglement distribution based on a linear optics element that does not need a quantum CNOT gate.     

量子纠缠的新理论

利用独立态矢特性,首次提出了一个纠缠态的定义及构造纠缠的理论,并进一步给出了多粒子系统的纠缠方式,进而给出基本纠缠方式的纠缠度的定义式,并推导出其公式,得出了三粒子系统的非纠缠判据.     

在分子自旋量子系统中的可控多体纠缠态

在由N＋1个相互作用的反铁磁分子环构成的量子自旋系统中,可以调控1种多体纠缠态。N个周边分子环的电子自旋和1个中心分子环的电子存在相互交换,从而在分子间形成可调的相互作用。通过整个系统的有效自旋哈密顿量解析得出系统的量子动力学行为。研究发现在量子涨落的条件下,1种高精度的形纠缠态可以被制备出来。通过控制分子间的相互作用,这种多体纠缠态也可以从一些分子环传输到其他分子环上。     

The generalized partial transposition criterion for infinite-dimensional quantum systems

The positive partial transposition(PPT)criterion and the realignment criterion constitute two of the most important criteria for detecting entanglement.The generalized partial transposition(GPT)criterion which contains the PPT criterion and the realignment criterion as special cases,provides a necessary condition for a multipartite state to be separable.Here we extend the GPT criterion to the infinitedimensional multipartite(bipartite)case and show that it includes multipartite(bipartite)PPT criterion and multipartite(bipartite)realignment criterion as special cases as well.     

量子纠缠与超光速量子通信

量子通信是经典通信和量子力学相结合的一门新兴交叉学科.在介绍量子纠缠特性的基础上,对量子隐形传态进行了探讨,提出了超光速量子通信的途径.     

Review on the study of entanglement in quantum computation speedup

The role the quantum entanglement plays in quantum computation speedup has been widely disputed. Some believe that quantum computation's speedup over classical computation is impossible if entan-glement is absent,while others claim that the presence of entanglement is not a necessary condition for some quantum algorithms. This paper discusses this problem systematically. Simulating quantum computation with classical resources is analyzed and entanglement in known algorithms is reviewed. It is concluded that the presence of entanglement is a necessary but not sufficient condition in the pure state or pseudo-pure state quantum computation speedup. The case with the mixed state remains open. Further work on quantum computation will benefit from the presented results.     

量子Dicke模型中纠缠与自旋压缩性质的研究

在经典极限条件下,量子化的Dicke模型表现为混沌动力学特征.通过研究量子纠缠和量子自旋压缩特性,得到以下结论:线性熵对于初态函数的选择,在相空间的不同区域有不同的表现,混沌区域的量子纠缠明显增大,规则区域则减弱;量子自旋压缩在相空间也表现出对混沌和规则区域的敏感性,混沌区域与规则区域相比较,存在自旋压缩的可能性较大;相同条件下,线性熵和压缩系数的变化趋势具有相似的表现行为.     

高阶多模系统的可分性判据

利用海森堡不确定关系和薛定谔-罗伯逊不确定关系推导高阶两粒子多模的分离判据条件.通过比较由两种不确定关系得到纠缠判据条件,发现利用薛定谔-罗伯逊不确定关系进行推导,能得到更强的分离判据条件.     

双模最小关联混态的纠缠及其量子起伏

研究了最小关联态的纠缠度与相关参数之间的关系。分析了不同参数情况下最小关联态发生纠缠时相关参数的取值范围以及全量子起伏与相关参数的关系;另外进一步分析了最小关联态的Wehrl熵,同时对标准EPR型纠缠态为的纠缠度作了进一步的计算。     

An efficient scheme for multi-party quantum state sharing via non-maximally entangled states

We present a new scheme for investigating the usefulness of non-maximally entangled states for multi-party quantum state shar-ing in a simple and elegant manner.In our scheme,the sender,Alice shares n various probabilistic channels composed of non-maximally entangled states with n agents in a network.Our protocol involves only Bell-basis measurements,single qubit measurements,and a two-qubit unitary transformation operated by free optional agents.Our scheme is a more convenient realiza-tion because no other multipartite joint measurements are needed.Furthermore,in our scheme various probabilistic channels lessen the requirement for quantum channels,which makes it more practical for physical implementation.     

量子统计系统极限判据的讨论

目的对量子统计系统的极限判据进行讨论。方法对不同量子极限进行比较分析。结果推导得到了量子统计系统的极限判据,并进行了讨论。结论对近独立粒子体系的统计分布,小Planck常数极限与高温、低密度极限具有相同的极限效应。     

违犯Bell不等式的正交态及非正交态的量子缠绕

给出正交态及非正交态的量子缠绕，通过分析两个实例．发现能够产生不遵守Bell不等式最大限度的缠绕态．     

Tavis-Cummings模型中的量子态平均保真度

运用全量子理论,研究了Tavis-Cummings模型中量子态平均保真度的演化特性,对原子和光场的初态以及两原子的关联程度对平均保真度的影响进行了研究.结果表明,原子、光场和系统的平均保真度依赖于初态时2个原子处在基态的几率以及光场处于真空态的几率;初态两原子在基态无关联,光场处于真空态,体系不失真.     

Amplifying stationary quantum discord and entanglement between a superconducting qubit and a data bus by time-dependent electromagnetic field

We study the dynamics of quantum discord and entanglement between a superconducting qubit and a data bus,which is driven by a controllable time-dependent electromagnetic field,in the presence of phase decoherence and find that the quantum discord and entanglement remain at a stationary non-zero value for long time evolution.It is shown that the amount of stationary quantum discord and entanglement can be enhanced by applying the time-dependent electromagnetic field.     

连续变量系统中准对称多模高斯态的纠缠

基于现今连续变量系统中高斯态纠缠的一些研究成果,利用负性对数为纠缠度量来讨论一个准对称的N模高斯态的纠缠问题.以一个准对称八模高斯态为例,寻找八模高斯态和与其对应的两模高斯态共变矩阵参数之间的解析关系.研究发现在多模高斯态对应两模高斯态的方案中由于省略了多模高斯态辛本征值的部分信息,导致这种方案虽然可以使多模高斯态对应为一个两模高斯态,但是这种对应关系不是唯一的,一个两模高斯态可以对应很多不同的多模高斯态,而且这些高斯态拥有着相同的纠缠信息.通过寻找八模高斯态与两模高斯态参数间的解析关系,研究发现只要确定辛本征值就能得到八模高斯态和两模高斯态参数之间的解析关系,按照这个思路可以得到N模高斯态与两模高斯态参数间的解析关系.     

探讨量子态纠缠的判据

围绕判断一个两体量子态是否纠缠的问题(量子态的可分离判据问题),寻求能够解决该问题的全新论据或重要结论.利用矩阵分析和线性代数的纯数学分析,得到了可分离态的一些性质和一些特殊情况下的纠缠判据,并重点探讨了2×N和N×2维量子态这一特殊情况.     

含能流的各向同性XY链中的基态纠缠

分析了含能流的横磁场三比特各向同性XY自旋链的基态纠缠.发现随着外磁场的变化,系统会发生量子相变.以三比特系统为例给出有效哈密顿量的能谱,讨论了体系基态纠缠随λ的变化.当λ＜3/3时,在相变点处基态由W态跳跃到非纠缠态,此时自旋链中没有能量流动.当3/3≤λ≤3时,在能级交错点处发生量子相变,体系由非能流相进入能流相,同时基态由W态ф1跳跃到另一W态ф5;此时,由于能流的影响,使得基态保持在W态,而不再处于非纠缠态;并且随着λ的增大,系统将在更大范围内处于能流相.当λ＞3时,基态波函数表示为ф5,系统完全处于能流相.     

无限维两体量子系统态的一些纠缠判据

探讨了无限维两体复合量子系统态的可分性问题,利用局域不确定性关系提出了无限维两体量子态可分的一些必要条件,得到了可分态所满足的一些不等式,推广了作者Otfried Guhne（PhysRev Lett,2004）和Zhang Chengjie（Phys Rev A,2010）的有限维的相关结果,对于识别无限维量子系统态的可分性是一个补充。     

Control problems in quantum systems

The past decade or two has witnessed tremendous progress in theory and practice of quantum control technologies.Bridging different scientific disciplines ranging from fundamental particle physics to nanotechnology,the goal of quantum control has been to develop effective and efficient tools for common analysis and design,but more importantly would pave the way for future technological applications.This article briefly reviews basic quantum control theory from the perspective of modeling,analysis and design,as well as considers future research directions.