基于轨道角动量的多自由度W态纠缠系统

提出了一种制备三光子纠缠W态的方案, 该方案利用携带轨道角动量为lħ的光子(其中l可取(-∞, +∞)的任意整数)可构成无穷维向量空间的特性, 采用两种类型的参量下转换, 产生轨道角动量-自旋角动量纠缠的两对光子和一对偏振纠缠光子, 通过纠缠交换制备三光子多自由度的W态, 实现三光子体系纠缠的高维度、大容量量子信息处理. 方案采用q-plate相位光学器件和单模光纤等器件制备两个不同自由度(轨道角动量与偏振)混合的W态, 并利用计算机全息相位图改进方案制备三个不同自由度(轨道角动量、线动量和偏振)混合的W态. 本方案可稳定产生两种等概率互为对称的W态, 具有高维度、强纠缠特性与抗比特丢失能力, 信息量达log₂^m+2比特(m为l的可取值个数), 有望实现可扩容量子比特的安全通信.     

一种基于纠缠态的量子中继通信系统

提出了一种基于纠缠态的量子中继通信系统,该系统应用纠缠为基本资源.纠缠为量子隐形传态和绝对安全的量子通信提供了保证.量子中继器用来延长高纠缠度的纠缠光子对的纠缠距离,利用纠缠交换和纠缠纯化在系统的发信者与受信者之间建立光子对的纠缠.应用量子隐形传态的原理传输量子信息.系统分析表明,量子通信系统的吞吐率随着通信双方成功建立纠缠的概率增大而显著增加,量子信号的传输距离取决于量子中继节点的级数.     

Demonstration of Quantum Entanglement Control Using Nuclear Magnetic Resonance

With the two forms of the quantum entanglement control, the quantum entanglement swapping and preservation are demonstrated in a three-qubit nuclear magnetic resonance quantum computer. The pseudopure state is prepared to represent the quantum entangled states through macroscopic signals. Entanglement swapping is directly realized by a swap operation. By controlling the interactions between the system and its environment,we can preserve an initial entangled state for a longer time. The experimental results are in agreement with the experiment.     

连续变量无条件纠缠交换 --纠缠态的量子离物传送

利用自行设计的两台非简并光学参量放大器，获得了一对具有经典相干性且量子起伏相互独立的连续变量纠缠态光场，并用它完成了连续变量的无条件纠缠交换，即纠缠态的量子离物传送．通过联合贝尔态探测与纠缠塌缩，使两个初始不纠缠而又从未发生过直接相互作用的光场产生了量子纠缠，其正交振幅和位相分量的量子起伏关联方差被直接测量，其测量值分别低于散离噪声极限1．23dB和1．12dB．理论计算与实验结果基本符合．     

How a single photon can mediate entanglement between two others

We describe a novel quantum information protocol, which probabilistically entangles two distant photons that have never interacted. Different from the entanglement swapping protocol, which requires two pairs of maximally entangled photons as the input states, as well as a Bell-state measurement (BSM), the present scheme only requires three photons: two to be entangled and another to mediate the correlation, and no BSM, in a process that we call “entanglement mediation”. Furthermore, in analyzing the paths of the photons in our arrangement, we conclude that one of them, the mediator, exchanges information with the two others simultaneously, which seems to be a new quantum-mechanical feature.     

Entanglement Swapping for Cluster-Class States and Its Applications in Quantum Information Processing

Entanglement swapping for four-qubit cluster-class states is studied. It is shown that a four-qubit cluster state (maximally entangled) can be obtained with a certain probability from two four-qubit cluster-class states by entanglement swapping. The probability is related to the smallest superposition coefficient of the cluster-class states (when all the moduli of amplitudes are equivalent, they are the usual cluster states and the probability hits to one). Two examples for the applications of the entanglement swapping are also presented. One is quantum teleportation of an arbitrary two-qubit state via a quantum repeater, in which the success probability can be improved by the entanglement swapping when the quantum channels are general cluster-class states (partially entangled). The other is quantum key distribution, in which a secret random sequence of bits (a “key”) can be efficiently established between two distant parties by the entanglement swapping of two groups of cluster states.     

具有经典相干性的两组EPR纠缠态光场的实验产生

纠缠交换，即纠缠态的量子离物传送，是实现远程量子通讯及量子信息网络的必要手段之一 . 要完成纠缠交换实验，首先必须具有两组相互独立的纠缠源. 对于连续变量系统，两独立 的纠缠源为具有经典相干但量子起伏互不关联的两组EPR纠缠态光场. 利用自行研制的瓦级 连续双波长输出Nd³⁺: YAP/KTP稳频激光器为抽运源，抽运两台结 构完全相同的非简 并光学参量放大器，获得了具有经典相干性的两组独立的EPR纠缠光束. 讨论了两组具有经 典相干性的EPR光束产生的实验方法，及不完善模匹配效率对关联测量的影响. 关键词： EPR纠缠态光场 经典相干 非简并光学参量放大器     

Storage and long-distance distribution of telecommunications-band polarization entanglement generated in an optical fiber

We demonstrate storage of polarization-entangled photons for 125 micros, a record storage time to date, in a 25-km-long fiber spool, using a telecommunications-band fiber-based source of entanglement. With this source we also demonstrate distribution of polarization entanglement over 50 km by separating the two photons of an entangled pair and transmitting them individually over separate 25-km fibers. The measured two-photon fringe visibilities were 82% in the storage experiment and 86% in the distribution experiment. Preservation of polarization entanglement over such long-distance transmission demonstrates the viability of all-fiber sources for use in quantum memories and quantum logic gates.     

纠缠Ξ-型三能级原子与纠缠腔场相互作用熵的纠缠演化

文章利用一对处于纠缠态的-型三能级原子与一对处于纠缠态的单模腔场,初始时刻原子与腔场之间互不纠缠,使其中一个原子与一个腔场发生相互作用,即纠缠交换,合适选择相互作用时间就可实现原子与腔场之间产生纠缠,并研究了系统原子熵的演化特性,运用量子熵理论,讨论了原子-腔场的耦合常数对原子熵的影响,结果表明：原子与光场跃迁耦合常数对系统熵的最大纠缠度有影响.当原子与光场两种跃迁耦合常数之比k值增大时,最大纠缠度在减小;当k增大到某一程度时,系统熵随时间周期性变化,并出现双峰现象。     

Projection of two biphoton qutrits onto a maximally entangled state

Bell state measurements, in which two quantum bits are projected onto a maximally entangled state, are an essential component of quantum information science. We propose and experimentally demonstrate the projection of two quantum systems with three states (qutrits) onto a generalized maximally entangled state. Each qutrit is represented by the polarization of a pair of indistinguishable photons-a biphoton. The projection is a joint measurement on both biphotons using standard linear optics elements. This demonstration enables the realization of quantum information protocols with qutrits, such as teleportation and entanglement swapping.     

Entanglement concentration and teleportation of multipartite entangled states in an ion trap

We propose an effective scheme for the entanglement concentration of a four-particle state via entanglement swapping in an ion trap. Taking the maximally entangled state after concentration as a quantum channel, we can faithfully and determinatively teleport quantum entangled states from Alice to Bob without the joint Bell-state measurement. In the process of constructing the quantum channel, we adopt entanglement swapping to avoid the decrease of entanglement during the distribution of particles. Thus our scheme provides a new prospect for quantum teleportation over a longer distance. Furthermore, the success probability of our scheme is 1.0.     

Entanglement Swapping for Distant Bose-Einstein Condensates

We propose protocols for the entanglement swapping of distant atomic Bose-Einstein condensates using the photon entanglement states as the quantum channel. Two protocols are introduced: one is a single-photon scheme in which an entangled single-photon state serves as the quantum channel, and the other is a multi-photon scheme where an entangled coherent state of the probe lasers is used as the quantum channel.     

Quantum Remote Control of Unitary Operations on a Qubit of Pure Entangled States

We consider the remote implementation of an arbitrary unitary operation on one qubit of a pure two-qubit entangled state with 100% efficiency via entanglement swapping in detail, then directly generalize this protocol from two-qubit to N-qubit entangled states. The overall classical information and distributed entanglement cost required for this quantum remote control protocol is less than the bi-directional quantum state teleportation method.     

量子信令交换机模型设计及性能分析

本文提出了量子信令交换机的模型, 该交换机由经典信息控制模块、交换控制模块和量子交换模块三部分组成. 经典控制模块负责将纠缠初态信息传送给纠缠测量及交换单元并更新路由信息. 交换控制模块实现通路选择, 为纠缠对的分发提供通路. 量子交换模块制备纠缠对, 进行Bell态的测量, 完成纠缠交换. 量子信令交换机可以实现多用户间的信令传输及局域网通信. 通过对交换机的性能分析与仿真, 结果表明该交换机结构简单、安全保密、便于扩展、时延小, 对于构建量子通信网有很好的支撑作用. 关键词： 量子通信 量子信令网 量子信令交换机 纠缠交换     

Experimental demonstration of unconditional entanglement swapping for continuous variables

The unconditional entanglement swapping for continuous variables is experimentally demonstrated. Two initial entangled states are produced from two nondegenerate optical parametric amplifiers operating at de-amplification. Through implementing the direct measurement of the Bell-state between two optical beams from each amplifier the remaining two optical beams, which have never directly interacted with each other, are entangled. The quantum correlation degrees of 1.23 and 1.12 dB below the shot noise limit for the amplitude and phase quadratures resulting from the entanglement swapping are measured straightly.     

变分法研究一维Bose-Fermi系统的稳定性

利用能量泛函变分法研究了一维Bose-Fermi系统稳定基态的存在条件.根据Bose-Fermi系统的Lagrange量可以得到三维Bose-Fermi体系所满足的非线性动力学方程组.当外势阱的横向囚禁频率远大于轴向囚禁频率时,体系可以当作一维模型来处理.从描述三维体系的动力学方程可以得到描述一维体系的动力学方程,选取适当的无量纲参数,可以对一维动力学方程组进行无量纲处理,得到数值计算和理论分析中常用到的无量纲方程.选择高斯型试探解(简单孤立子解),利用能量泛函变分法得到一维Bose-Fermi体系稳定的 关键词： Bose-Fermi 稳定性 基态 临界条件     

Entanglement formation and violation of Bell's inequality with a semiconductor single photon source

We report the generation of polarization-entangled photons, using a quantum dot single photon source, linear optics, and photodetectors. Two photons created independently are observed to violate Bell's inequality. The density matrix describing the polarization state of the postselected photon pairs is reconstructed and agrees well with a simple model predicting the quality of entanglement from the known parameters of the single photon source. Our scheme provides a method to create no more than one entangled photon pair per cycle after postselection, a feature useful to enhance quantum cryptography protocols based on shared entanglement.     

Quantum entanglement swapping of two arbitrary biqubit pure states

In this paper, the issue of swapping quantum entanglements in two arbitrary biqubit pure states via a local bipartite entangledstate projective measure in the middle node is studied in depth, especially with regard to quantitative aspects. Attention is mainly focused on the relation between the measure and the final entanglement obtained via swapping. During the study, the entanglement of formation (EoF) is employed as a quantifier to characterize and quantify the entanglements present in all involved states. All concerned EoFs are expressed analytically; thus, the relation between the final entanglement and the measuring state is established. Through concrete analyses, the measure demands for getting a certain amount of a final entanglement are revealed. It is found that a maximally entangled final state can be obtained from any two given initial entangled states via swapping with a certain probability; however, a peculiar measure should be performed. Moreover, some distinct properties are revealed and analyzed. Such a study will be useful in quantum information processes.     

Remote interactions between two d-dimensional distributed quantum systems： nonlocal generalized quantum control-NOT gate and entanglement swapping

We present a systematic simple method to implement a generalized quantum control-NOT （CNOT） gate on two d-dimensional distributed systems. First, we show how the nonlocal generalized quantum CNOT gate can be implemented with unity fidelity and unity probability by using a maximally entangled pair of qudits as a quantum channel. We also put forward a scheme for probabilistically implementing the nonlocal operation with unity fidelity by employing a partially entangled qudit pair as a quantum channel. Analysis of the scheme indicates that the use of partially entangled quantum channel for implementing the nonlocal generalized quantum CNOT gate leads to the problem of ＇the general optimal information extraction＇. We also point out that the nonlocal generalized quantum CNOT gate can be used in the entanglement swapping between particles belonging to distant users in a communication network and distributed quantum computer.[第一段]