广义GHZ态的制备和非定域性的测量

我们用参量下转换的方法,合理设计实验方案,制备了广义GHZ（Greenberger-Horne-Zeilinger）三体偏振纠缠态,测量了所制备状态的密度矩阵,并通过密度矩阵计算出了状态相对于目标态的保真度.实验制备出了保真度达0.85左右的高保真三体纠缠态.测量了GGHZ（generalized GHZ）纠缠态的Bell-Mermin算符期望值,得到了和理论计算一致的测量结果.结果表明,量子态非定域特性与体系的纠缠程度密切相关,这说明制备GGHZ纠缠态的方案和非定域特性的测量方法是可行的.     

冷原子系综中窄线宽纠缠光子对的产生和调制

窄线宽纠缠光子对可以用来验证量子力学的一些基本概念,如非定域性.另外,窄线宽纠缠光子对应用于量子信息处理,可以极大增加量子通信的距离以及分布式量子计算的规模.论文系统介绍在冷原子系综中,利用四波混频和电磁诱导透明来产生窄线宽纠缠光子对以及对窄线宽纠缠光子对的波形进行调制的实验方法.     

一个基于腔衰减的四原子纠缠态制备方案

基于A型三能级原子与腔场及经典场的相互作用理论,利用单光子探测器对从光腔中泄漏出来的光子进行符合测量,提出了一个四原子纠缠态的制备方案,四个分别处于不同光腔中的原子将以一定的概率处于GHZ态。     

基于四粒子GHZ态的任意三粒子GHZ态量子隐形传送

量子隐形传态已成为量子信息学中的一个重要研究领域,笔者从节省量子纠缠资源的目的出发,通过极化分束器(PBS)将2个EPR对制备成单个四粒子GHZ纠缠态,并将此四粒子GHZ纠缠态作为量子信道,从而实现三粒子纠缠GHZ态的量子隐形传输.     

中国科大制备出八光子纠缠态刷新多光子纠缠制备与操作的世界纪录

本刊讯（记者）日前，中国科学技术大学郭光灿院士领导的中科院量子信息重点实验室李传锋、黄运锋研究组成功制备出八光子纠缠态，刷新了多光子纠缠制备与操作数目的世界纪录。11月22日，《自然·通信》在线发表了这一研究成果。量子纠缠态是实现各种量子信息过程的基础。由于光子具有抗干扰能力强、传播速度快等优点，多光子纠缠态的制备和操控一直是量子信息领域的研究重点和难点。目前世界上普遍利用晶体中的非线性过程来产生多光子纠缠态．由于此过程是概率性的，因此产生多光子纠缠态的难度会随着光子数目的增加而成倍增大。此前，国际学术界已报道的纪录是最多能制备出六光子纠缠态。     

阶梯型三能级原子与双模腔作用制备四光子GHZ态

通过阶梯型三能级原子与双模腔大失谐作用,提出制备四光子Greenberger-Horne-Zeilinger（GHZ）态的方案。光子作为量子信息载体,通过对原子的测量得到所需要的量子态。该方案具有实验可行性。     

量子时空中的面积量子

用不同于其他文献的方法推导出量子化空间中面积谱为16πhA（j），用该面积谱以便于推算出黑洞的熵．利用最小面积量子值为1／2探讨qubit的来源，同时对空间存在的EPR纠缠态、纠缠态的非定域性，以及面积量子具有的关联变更给出论证．     

量子理论与光的电磁起因

1935年爱因斯坦、波多尔斯基和罗森建议纠缠电子的非定域性。1941年费曼通过证明一个电子从A点经过所有可能的途径到达B点来证明自由电子的非定域性。1982年阿斯佩克等人提供了实验汪据。实验得出下列结论：两个纠缠粒子的行为说明——或者量子力学理论是不完整的，或者是发生在一个粒子上的事件影响另一个粒子而没有时间上的延迟并且独立于它们之间的物理间距。本书作者假设了本征态电子的非定域性，并且发现了由经典电磁场理论而产生的量子理论。     

在微腔中利用超导量子干涉仪实现GHZ纠缠态

在这里,我们提出一种实现多粒子的Greenberg-Horne-Zeilinger(GHZ)纠缠态方案.是将超导量子干涉仪植入单模微腔中,在大失谐的情况下完成纠缠态的制备.在这种方法中,GHZ能被制备而不需要任何测量.在整个制备过程中微腔和超导量子干涉仪是没有信息交换的,因为腔一直处于真空态,没有任何光子,这样对腔的品质因数要求就比较低.     

对四光子德布罗意波长测量实验的理论解释

采用对自发参量下转换过程产生的光子对的量子态和光场的非纠缠态描述,分析了由自发参量下转换过程产生的四光子的德布罗意波长测量的实验,得到了与实验观测相一致的结果。这一结果进一步验证了对自发参量下转换过程产生的光子对的量子态和光场的非纠缠态描述的正确性。     

Experimental demonstration of five-photon entanglement and open-destination teleportation

Quantum-mechanical entanglement of three or four particles has been achieved experimentally, and has been used to demonstrate the extreme contradiction between quantum mechanics and local realism. However, the realization of five-particle entanglement remains an experimental challenge. The ability to manipulate the entanglement of five or more particles is required for universal quantum error correction. Another key process in distributed quantum information processing, similar to encoding and decoding, is a teleportation protocol that we term 'open-destination' teleportation. An unknown quantum state of a single particle is teleported onto a superposition of N particles; at a later stage, this teleported state can be read out (for further applications) at any of the N particles, by a projection measurement on the remaining particles. Here we report a proof-of-principle demonstration of five-photon entanglement and open-destination teleportation (for N = 3). In the experiment, we use two entangled photon pairs to generate a four-photon entangled state, which is then combined with a single-photon state. Our experimental methods can be used for investigations of measurement-based quantum computation and multi-party quantum communication.     

两种情形下两原子的纠缠演化特性

通过negativity方法,研究了两个空间分离的纠缠二能级原子依次通过高Q粒子数场时,两个原子的纠缠演化特性,发现：初始原子状态和光场状态对两原子的纠缠有明显的影响．通过选择初始状态可以得到较长时间的最大纠缠．然后与两个纠缠原子同时在高Q粒子真空场时两原子纠缠的演化做了有意义的比较,结果表明：当反映原子初始状态的参数θ〈π/2时,两种情形的纠缠演化趋势大致相同．当θ〈π/2时,纠缠演化大不一样．     

任意多光子Dicke态的制备

量子Dicke态对于多粒子量子纠缠结构和性质的研究,以及量子网络的构建有着重要的意义.基于线性光学和交叉相位调制技术,利用三个基本量子门,给出了四光子Dicke态制备、任意系数W态制备以及任意系数和光子数的Dicke态制备方案.这些方案的确定性、任意性、高效性将为研究量子纠缠结构性质以及量子网络提供一定的便利.     

De Broglie wavelength of a non-local four-photon state

Superposition is one of the most distinctive features of quantum theory and has been demonstrated in numerous single-particle interference experiments. Quantum entanglement, the coherent superposition of states in multi-particle systems, yields more complex phenomena. One important type of multi-particle experiment uses path-entangled number states, which exhibit pure higher-order interference and the potential for applications in metrology and imaging; these include quantum interferometry and spectroscopy with phase sensitivity at the Heisenberg limit, or quantum lithography beyond the classical diffraction limit. It has been generally understood that in optical implementations of such schemes, lower-order interference effects always decrease the overall performance at higher particle numbers. Such experiments have therefore been limited to two photons. Here we overcome this limitation, demonstrating a four-photon interferometer based on linear optics. We observe interference fringes with a periodicity of one-quarter of the single-photon wavelength, confirming the presence of a four-particle mode-entangled state. We anticipate that this scheme should be extendable to arbitrary photon numbers, holding promise for realizable applications with entanglement-enhanced performance.     

光场迭加态的场平方振幅压缩

本文讨论了光场Fock态和真空态、相干态和真空态的迭加态中场平方振幅的压缩效应。结果表明只有两光子或四光子Fock态及平均光子数不太大的相干态与真空态的迭加态在一定条件下能存在场平方振幅的压缩。     

量子信道对纠缠鲁棒性的影响

考虑量子信道对多体纠缠鲁棒性的影响. 先根据可分态之集为一个凸闭集, 证明可以取到纠缠鲁棒性定义中的下确界, 再证明同一纠缠态ρ的两个最优态的凸组合仍是最优态, 纠缠鲁棒性作为定义态集合上函数是下半连续的; 最后, 分别给出一个量子信道不增加、 不减少[KG*8]及保持所有量子态的纠缠鲁棒性的充分必
要条件.     

Experimental determination of entanglement with a single measurement

Nearly all protocols requiring shared quantum information--such as quantum teleportation or key distribution--rely on entanglement between distant parties. However, entanglement is difficult to characterize experimentally. All existing techniques for doing so, including entanglement witnesses or Bell inequalities, disclose the entanglement of some quantum states but fail for other states; therefore, they cannot provide satisfactory results in general. Such methods are fundamentally different from entanglement measures that, by definition, quantify the amount of entanglement in any state. However, these measures suffer from the severe disadvantage that they typically are not directly accessible in laboratory experiments. Here we report a linear optics experiment in which we directly observe a pure-state entanglement measure, namely concurrence. Our measurement set-up includes two copies of a quantum state: these 'twin' states are prepared in the polarization and momentum degrees of freedom of two photons, and concurrence is measured with a single, local measurement on just one of the photons.     

The entanglement evolution of two coupling two level atoms interacting with a single mode vacuum field in multiphoton Tavis Cummings model

Using multipohton Tavis-Cummings model, the entanglement evolution of two coupling two-level atoms in Bell states interacting with a single-mode vacuum field is investigated by using Negativity. The influences of coupling constants between atoms, the atomic initial states and the photon number of transition on the entanglement evolution of two coupling two-level atoms are discussed. The results obtained using the numerical method show that the entanglement of two atoms is related with coupling constants between atoms, the atomic initial states and the photon number of transition. The two-atom entanglement state will forever stay in the maximum entanglement state when the initial state is .When the initial state of two atoms is , the entanglement of two atoms displays periodic oscillation behavior.and its oscillation period decreases with increasing of coupling constant between atoms or the photon number of transition. On the other hand, when the initial state is or , the entanglement of two atoms displays quasiperiodic oscillation behavior and its oscillation period decreases with increasing of coupling constant between atoms or the photon number of transition .     

两原子依次通过粒子场的纠缠突然死亡与突然产生

通过数值计算的方法,研究了两纠缠二能级原子依次通过粒子场时的纠缠演化特性,发现原子的初态和光场的初态对两原子的纠缠有影响.当光场的粒子数为0时,纠缠处于周期性变化;当粒子数不为0时,纠缠非周期性变化,并出现纠缠突然死亡与突然产生现象.通过改变原子的初态和光场的初态,纠缠突然死亡和产生现象会随之改变.