概率性的量子态远程操作

提出一种理论方案,利用量子纠缠特性,实现对量子态的远程操作.采用两粒子非最大纠缠态作为资源,借助于辅助量子位,实现态算子的制备,并利用态算子特性,我们能够以一定的概率实现对量子态的远程操作.这种非局域的操作是量子世界特有的现象,这方面的研究有助于对量子力学基本问题的探讨.     

双模压缩态量子相干性演化的实验研究

量子相干性作为量子力学一个最显著的特征,被认为是量子信息过程中很重要的一种量子资源.单模压缩态和双模压缩态(Einstein-Podolsky-Rosen纠缠态)均具有量子相干性,在制备和传输过程中的量子相干性对于实际应用具有重要意义.利用平衡零拍探测重构量子态的协方差矩阵,本文定量分析了量子态制备过程中的不完美以及信道传输损耗对单模和双模压缩态量子相干性的影响.实验证明量子态的压缩和纠缠特性及量子相干性对损耗均是鲁棒的.特别地,压缩和纠缠特性会随着量子态制备过程中热光子数的增大而减小,直至消失,而当压缩和纠缠均已消失时,量子相干性依然存在.实验结果为压缩态、纠缠态光场的量子相干性作为量子资源在量子信息过程中的应用提供了参考.     

量子直接传态

把一个任意量子态在既有噪声又有窃听的信道下安全可靠地传输,是一个广泛而重要的问题.现在已有的方法是先传输大量的Einstein-Podolsky-Rosen (EPR)纠缠对,然后进行纠缠纯化,获得一对近似完美的纠缠对,再进行隐形传态或者远程态制备来传输量子态.本文给出一种直接安全传输量子态的方法,通过使用量子直接通信,安全地传输大量同样的任意量子态,然后利用单量子态的纯化方法,得到一个近于完美的量子态.这是一种不需要量子纠缠的量子态安全传输方法,避免使用纠缠资源.这种方案是量子隐形传态和远程态制备之外的又一途径.此外,这一方案将原来只是用来传输经典信息的量子安全直接通信扩展到传输任意量子态的新领域,扩大了量子直接通信的用途.这一方案将在未来量子互联网中有重要的应用.     

量子相干破坏信道的研究

量子相干态和量子纠缠态在量子计算中具有同等重要的地位,都是量子计算中重要的物理资源。量子相干态可用来制备量子纠缠态,一个量子相干态和一个辅助量子不相干态通过不相干操作可产生量子纠缠态。根据纠缠破坏信道原理提出输入分别是单体最大相干态和二体最大相干态的相干破坏信道,该信道将输入的相干态转化为不相干态,并简单验证了提出的相干破坏信道是否真正破坏了输入的量子相干态的相干性。同时提出量子相干破坏信道可作为检测量子态的相干性一种新的方法。     

用非最大纠缠信道对任意二粒子纠缠态的量子秘密分享

提出一个对任意二粒子纠缠态在N者之间的量子秘密分享方案,该方案利用非最大纠缠Einstein-Podolsky-Rosen(EPR)对作为量子信道,利用广义的贝尔基进行测量.接收者通过引入辅助粒子,并对其做选择性测量,就会概率性地得到最初的量子态. 关键词： 非最大纠缠的Einstein-Podolsky-Rosen(EPR)对 广义的贝尔测量     

三粒子纠缠Ｗ态的隐形传态

提出利用三个二粒子纠缠态作为量子信道，实现三粒子纠缠Ｗ态的隐形传态的方案.首先考察量子信道是最大纠缠态的情形，然后进一步考察量子信道是非最大纠缠态的情形.发现在量子信道为非最大纠缠态时，通过引进一个辅助粒子，并构造一个幺正变换矩阵，即可以一定的概率完成三粒子纠缠Ｗ态的隐形传态. 关键词： 隐形传态 三粒子纠缠 纠缠Ｗ态 非最大纠缠量子信道     

未知高维量子态的概率隐形传送

研究了未知的多维量子态的概率量子隐形传送问题.文中提出利用一个部分的三维两粒子纠缠态作为量子通道,概率传送未知的三维单粒子态的方案;然后通过利用两个部分的三维两粒子纠缠态作为量子通道,又提出了一种概率隐形传送未知的三维两粒子纠缠态的方案.并且对它们进行了推广:通过利用N个部分的d维两粒子纠缠态作为量子通道,可以把这种方案直接推广到未知的d维N粒子纠缠态的概率量子隐形传送.同样地,我们详细地列出了每个接收者所用的各种幺正变换.这样,我们就把未知的两维量子态的概率量子隐形传送问题研究推广到三维或多维量子态的概率量子隐形传送.与先前的研究方案相比,本文所提方案具有下列优点:处理方法不同,具体地说是在隐形传送的过程中所利用的幺正变换形式不同.在我们的方案中,接收者将引进一个具有初态为|0》A的d-维辅助粒子A,幺正变换U2采用d2×d2矩阵形式(对三维量子态来说d=3);而先前方案的接收者引进的是一个具有初态为|0》A的两维辅助粒子A,幺正变换U2采用2d×2d矩阵形式,即实际上就是过去方案的形式.也就是说,他们对其它粒子是进行多维处理,而对接收者引进的辅助粒子A只进行两维处理.     

量子隐形传态的新方案

提出一个量子隐形传态的新方案. 通过对一对压缩参数相同但相互独立的双模压缩真空态(1-2和3-4量子态系统)中的2-3系统施行粒子数-相位的联合测量,制备出另外两体系统(1-4系统)的纠缠态作为纠缠源,从而实现量子隐形传态. 关键词： 量子隐形传态 双模压缩真空态 纠缠态 粒子数-相位测量     

原子自发辐射中偶极矩的涨落与最大纠缠态的保持

研究了二能级原子与电磁场相互作用体系的自发辐射与量子纠缠态.在原子的自发辐射过程中,其偶极矩的期待值总是零,但偶极矩的涨落恒等于一个不为零的常量,因此原子的自发辐射是由真空起伏导致偶极矩的涨落引起的.Jaynes-Cummings模型是产生量子纠缠态的重要体系,研究发现,原子与场纠缠态的信息熵和纠缠度随时间作周期性的振荡,量子态在非纠缠与纠缠态之间变化.更为重要的是,在失谐量适当时,量子态将长时间停留在最大纠缠态.     

连续变量量子态的光学操控

量子态的光学操控是指在光场的传输、存储和频率变换等过程中用光学方法对光场的量子态进行操作与控制.量子态操控是量子通讯及量子态制备和应用的基础.在简要介绍连续变量多组份纠缠态光场制备的基础上,概述基于多组份纠缠态光场的连续变量量子通讯网络及在执行压缩态和纠缠态光场操控方面的实验研究进展.     

Witness for Non-Quasi Maximally Entangled States

Maximally entangled states, defined as those states that have the maximal entanglement of formation under some entanglement measure, are the ideal resource for many quantum missions. In this paper, we call a convex roof of maximally entangled pure states a quasi maximally entangled state. First, we present the concept of a witness for non-quasi maximally entangled states, which is an observable that can distinguish some non-quasi maximally entangled states from quasi maximally entangled ones. Then we prove that every non-quasi maximally entangled state can be witnessed by a witness and obtain some necessary and sufficient conditions for an observable to be a witness for non-quasi maximally entangled states. Lastly, we give some classes of Hermitian operators, which can become witnesses. Especially, we compute non-quasi maximally entangled states that can be detected by a specific product operator.     

Benchmarking a quantum teleportation protocol in superconducting circuits using tomography and an entanglement witness

Teleportation of a quantum state may be used for distributing entanglement between distant qubits in quantum communication and for quantum computation. Here we demonstrate the implementation of a teleportation protocol, up to the single-shot measurement step, with superconducting qubits coupled to a microwave resonator. Using full quantum state tomography and evaluating an entanglement witness, we show that the protocol generates a genuine tripartite entangled state of all three qubits. Calculating the projection of the measured density matrix onto the basis states of two qubits allows us to reconstruct the teleported state. Repeating this procedure for a complete set of input states we find an average output state fidelity of 86%.     

Preparation and Analysis of Two-Dimensional Four-Qubit Entangled States with Photon Polarization and Spatial Path

Entanglement states serve as the central resource for a number of important applications in quantum information science, including quantum key distribution, quantum precision measurement, and quantum computing. In pursuit of more promising applications, efforts have been made to generate entangled states with more qubits. However, the efficient creation of a high-fidelity multiparticle entanglement remains an outstanding challenge due to the difficulty that increases exponentially with the number of particles. We design an interferometer that is capable of coupling the polarization and spatial paths of photons and prepare 2-D four-qubit GHZ entanglement states. Using quantum state tomography, entanglement witness, and the violation of Ardehali inequality against local realism, the properties of the prepared 2-D four-qubit entangled state are analyzed. The experimental results show that the prepared four-photon system is an entangled state with high fidelity.     

Energy as Entanglement Witness in Bilinear-Biquadratic Spin-1 Chain

Energy is introduced as an entanglement witness to describe the entanglement property of a quantum system. The thermal equilibrium system is guaranteed to be entangled when system is cooled down below the entanglement temperature TE. By virtue of this concept we exploit the minimum separable state energy and entanglement temperature TE of the bilinear-biquadratic antiferromagnetic spin-1 chain model. We numerically calculate TE for arbitrary values of the strength of biquadratic exchange interaction Q up to N=7. We find TE decreases with Q for fixed N when Q is between -3 and 1/3 （J = 1）. In this regime TE also decreases with N for fixed Q and varies slowly for large N. While the thermal system is always entangled when Q is smaller than -3.     

通过纠缠交换实现N粒子任意态的概率传态

量子隐形传态的杰出安全特性使其在未来的通讯领域充满潜力.量子力学的不确定性原理和不可克隆定理禁止对量子态进行直接复制,因此,量子隐形传态将量子态划分为经典和量子两部分,信息分别经由经典和量子通道从发送者Alice传递给远方的接收者Bob,根据这两种信息,Bob实行相应操作就可以以一定的几率重建初始传送态.利用一般意义的隐形传态方案,提出一种简便的新方法实现了一个N粒子任意态的概率传态.方法采用N个非最大纠缠的三粒子GHZ态作为量子通道,避免了引入额外的辅助粒子.为了实现传态,Alice将所有粒子分成N份,对第i份的粒子对(i,x_i)实行Bell测量并将结果通过经典通道通知Bob,Bob对粒子(y_i,z_i)进行相应的操作就可以完成第i个粒子信息的传送.当完成N次相似的重复操作后,Bob就可以准确地重建初始传送态.文中以Bell态测量为基本手段,重复的操作同时也降低了实验难度,作为一个特例,文中给出了一个两粒子任意态的传态方案.     

Experimental detection of multipartite entanglement using witness operators

We present the experimental detection of genuine multipartite entanglement using entanglement witness operators. To this aim, we introduce a canonical way of constructing and decomposing witness operators so that they can be directly implemented with present technology. We apply this method to three- and four-qubit entangled states of polarized photons, giving experimental evidence that the considered states contain true multipartite entanglement.     

Measures of genuine multipartite entanglement for graph states

Graph states are special multipartite entangled states that have been proven useful in a variety of quantum information tasks. We address the issue of characterizing and quantifying the genuine multipartite entanglement of graph states up to eight qubits. The entanglement measures used are the geometric measure, the relative entropy of entanglement, and the logarithmic robustness, have been proved to be equal for the genuine entanglement of a graph state. We provide upper and lower bounds as well as an iterative algorithm to determine the genuine multipartite entanglement.     

Optimal multi-setting witness operators for two qudits

Quantum entanglement in bipartite systems is approached with the help of pseudo Bell operators, a relaxed form of Bell operators. Their connection to optimal witness operators is established. A method to construct pseudo Bell operators for pairs of qudits, based on multiple two-dimensional local measuring settings, is presented. Each of them provide two optimal (multi-setting) witness operators. They are rigorously proved to fulfill the conditions in the definition. We explain how to approximate the set of separable states and the set of entangled states. A class of Werner-like entangled states is identified. The spectral resolution of all multi-setting pseudo Bell operators is carried out in full.     

连续变量六组份和八组份星型Cluster纠缠态光场产生系统

Cluster纠缠态是执行单向量子计算的基本资源。星型Cluster态可用于构建量子信息网络。以非简并光学参变放大器产生的双模压缩态光场为基础,设计了产生连续变量六组份和八组份星型Cluster纠缠态光场的实验方案,并推导了相应的量子不可分判据。计算结果指出,当量子不可分判据中的增益因子取为1时,需要一定的初始压缩度才能制备星型Cluster纠缠态;但当选取最佳增益因子时,极低的压缩也能产生星型纠缠。计算为实验系统设计提供了直接参考。     

连续变量纠缠增强对实验参量的依赖关系

具有正交振幅和正交相位分量量子关联的连续变量量子纠缠态光场是进行量子信息和量子计算研究的最基本的资源。随着量子信息和量子计算研究的深入开展,为了实现高质量的信息传递和高效率的量子计算,必须尽可能提高所利用的纠缠态光场的纠缠度。基于光学参变过程量子纠缠增强是提高连续变量纠缠态光场纠缠度的一种有效方法,详细讨论了连续变量纠缠增强与非简并光学参变放大器各实验参量的关系,讨论了这些参量对纠缠增强的影响。计算结果将为优化利用非简并光学参变放大器构建的纠缠增强系统,进一步提高量子纠缠增强效率提供参考。