高纯四光子GHZ态的实验制备及非定域性研究

采用Ⅱ型参量下转换（SPDC）和后选择技术,实验上制备了保真度达0．831的高纯四光子Greenberger—Home—Zeilinger（GHZ）态．通过理论分析和实验优化,提出一种有效提高四光子对比度的实验方法．同时利用制备的纠缠态,研究了四光子GHZ态的非定域性,得到了对四光子Bell不等式28标准差的偏离,证明制备的态为真正四体纠缠态．本文的研究对实验上获得高对比度的多光子纠缠态及非定域性研究具有非常好的借鉴意义．     

通过三能级原子与双模场的共振相互作用生成纠缠GHZ态

在光场为Fock态的情况下，推导出A型三能级原子和V型三能级原子分别与双模场的共振相互作用体系状态波函数．在光场与原子的耦合常数选取适当情况下，通过改变相互作用时间，体系将演化为三体最大纠缠态——纠缠GHZ态，且光场与原子的耦合常数选取以及体系生成为三体纠缠GHZ态的时间随体系初态的不同而不同，这为制备三体纠缠GHZ态提供了一种有效方案．     

在单模真空场中利用纠缠GHZ态制备纠缠W态

提出在单模真空场中利用纠缠GHZ态制备纠缠W态方案.3个原子(1,2,3)之间初始为纠缠GHZ态,原子1远离单模真空场,而原子2和原子3进入单模真空场与场相互作用.在原子间耦合量适当时,通过控制单模真空场与原子2和原子3的相互作用时间,对原子1的状态进行测量,制备出理想纠缠W态.     

非马尔科夫环境中海森堡XYZ自旋链的隐形传态

利用量子隐形传态方案研究两比特各向异性海森堡XYZ模型隐形传态保真度随时间的演化过程.以最大纠缠态■作为信道，通过量子态扩散方法求解非马尔科夫主方程，从而获得系统的约化密度矩阵；再通过标准的隐形传态方案计算海森堡XYZ自旋链的平均保真度.经过研究发现：当环境关联系数γ较小时，平均保真度明显地呈现出上升的趋势，这表明非马尔科夫环境能够增加平均保真度；同时较大的自旋耦合系数J和余弦磁场强度B也能够增加平均保真度.     

非最大纠缠GHZ态的受控量子隐形传态

提出2种方案来完成非最大纠缠GHZ态的受控量子隐形传态,2种方案的信道分别为非最大纠缠EPR对和GHZ态的复合以及非最大纠缠EPR对和W态的复合.为提高隐形传态的安全性和可控性,2种方案中发送者和接收者之间都没有直接的量子纠缠信道,发送者需要向控制者提出申请,控制者同意后进行Bell基测量,从而使发送者和接收者之间建立量子纠缠信道.随后,发送者进行Bell基测量和von-Neumann测量.接收者根据控制者和发送者的测量结果进行幺正变换,完成隐形传态.     

三粒子GHZ态隐形传输令牌环网的保真度分析

通过对三粒子纠缠GHZ态隐形传输令牌环网通信过程的分析,得到了影响保真度的2个关节点,并分别对其保真度进行了计算.计算结果表明：当|a|=0、|b|=1或|a|=1、|b|=0时,保真度达最大值1;当|a|=|b|=2/2时,Bell基测量时的保真度为0.5,平均保真度为0.67.Von Neuman测量时的保真度与分析角θ有关,若θ=π/4±kπ （k=0,1,2,…）,保真度取最大值1;若θ=π/4±kπ/2 （k=0,1,3,5…）时,保真度取最小值0,平均保真度为0.5.     

3粒子GHZ态的量子远程克隆

提出了一个能实现3粒子GHZ态1→2的量子远程克隆方案:运用2个4粒子纠缠态作为量子信道,通过发送者的2次Bell测量、Hadamard变换、单粒子测量及经典通信;2个接收者进行相应的幺正变换、引入附加粒子和通过Toffoli门,就可以得到原未知态的近似拷贝,此方案的保真度与输入态有关.另外,还推广了一种实现N粒子GHZ态1→2的量子远程克隆方案.     

2种量子测量方案对非最大纠缠相干态隐形传态平均保真度的影响

在纠缠相干态的隐形传态过程中,使用双模光子数测量和奇偶态测量2种粒子数测量方案,得到的平均保真度存在较大差异.据此对使用非最大纠缠相干态作为量子信道时,2种测量方法得到的平均保真度进行了对比.结果显示:相较于双光子数测量,用奇偶态粒子数测量方案时,隐形传态的平均保重度有了较大提高.     

N粒子部分纠缠态的量子受控传递

由于制备与传输中的环境耦合,现实中的纠缠态大部分是非最大纠缠态．在研究现有量子受控传递方案的基础上,提出了一种利用非最大纠缠态作为量子通道来概率地传输N粒子部分纠缠态的量子控制方案,在该方案中,利用非最大（N＋2）-粒子GHZ态作为量子通道,在发送者进行一次Bell基测量、N-1次Hadamard门操作及｜0〉和｜1〉基测量,控制者实施一次Hadamard门操作及｜0〉和｜1〉基测量之后,接收者根据他们的测量结果,再进行一些适当的幺正变换以及一些必要的投影测量就实现了N粒子未知量子态的受控传递．该方案是一种基于非最大纠缠态的概率受控的隐形传态方案,并且成功的概率为2｜a｜^2。     

三粒子W纠缠态的概率量子隐形传态

提出一个对未知三粒子W纠缠态的量子隐形传态方案.该方案用一个非最大GHZ纠缠态和一个非最大EPR纠缠态作为量子信道实现对未知经典W纠缠态的概率量子隐形传态和对未知一般W纠缠态受控的概率量子隐形传态.     

关于量子超光速通信的一个理论设想

本文深入分析了拟议中的量子超光速通信理论,提出了实现量子超光速通信的原理与技术方案。此外,本文还讨论了适用于量子超光速通信的新的时空变换,并提出了两种检测特惠Lorentz参照系的实验方法。     

热库对原子-场耦合系统中量子非局域性的影响

讨论了光场初态和热库性质对原子场耦合系统中量子非局域性的作用．结果显示，如果腔损耗很弱，热库的平均光子数很小．系统会周期性地展现出量子非局域性．非局域性的消失速度依赖于初始压缩真空态的振幅、热库的平均光子数和腔的衰退系数．场越强、平均光子数和衰退系数越大，非局域性减小得越快。     

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.     

Realization of the Cirac-Zoller controlled-NOT quantum gate

Quantum computers have the potential to perform certain computational tasks more efficiently than their classical counterparts. The Cirac-Zoller proposal for a scalable quantum computer is based on a string of trapped ions whose electronic states represent the quantum bits of information (or qubits). In this scheme, quantum logical gates involving any subset of ions are realized by coupling the ions through their collective quantized motion. The main experimental step towards realizing the scheme is to implement the controlled-NOT (CNOT) gate operation between two individual ions. The CNOT quantum logical gate corresponds to the XOR gate operation of classical logic that flips the state of a target bit conditioned on the state of a control bit. Here we implement a CNOT quantum gate according to the Cirac-Zoller proposal. In our experiment, two 40Ca+ ions are held in a linear Paul trap and are individually addressed using focused laser beams; the qubits are represented by superpositions of two long-lived electronic states. Our work relies on recently developed precise control of atomic phases and the application of composite pulse sequences adapted from nuclear magnetic resonance techniques.     

二粒态的概率隐形传态

量子隐形传态是量子信息学的一个重要组成部分,本文提出一个利用两个三粒子纠缠态作为信道的量子隐形传态方案,方案中一个两粒子态将被概率的传输。     

给定不确定结果的量子比特的量子态区分

给出更简单的利用辅助测量比特和系统的幺正演化的方法用于在给定不确定结果时的量子比特的两个量子态区分。方案涵盖了当不确定结果的概率为零时的最小错误区分,以及当不确定结果的概率为某些数值时的最优确定性区分。文中给出最大全局正确概率和给定不确定概率关系的解析式。利用辅助测量比特,对被测的非正交量子态初态系统实施幺正演化,对输出态进行正交测量,就可以完成非正交态的量子态区分。方案提供了一种对于两个非正交量子态的正定算符值测量的物理实现方法。     

Real-time quantum feedback prepares and stabilizes photon number states

Feedback loops are central to most classical control procedures. A controller compares the signal measured by a sensor (system output) with the target value or set-point. It then adjusts an actuator (system input) to stabilize the signal around the target value. Generalizing this scheme to stabilize a micro-system's quantum state relies on quantum feedback, which must overcome a fundamental difficulty: the sensor measurements cause a random back-action on the system. An optimal compromise uses weak measurements, providing partial information with minimal perturbation. The controller should include the effect of this perturbation in the computation of the actuator's operation, which brings the incrementally perturbed state closer to the target. Although some aspects of this scenario have been experimentally demonstrated for the control of quantum or classical micro-system variables, continuous feedback loop operations that permanently stabilize quantum systems around a target state have not yet been realized. Here we have implemented such a real-time stabilizing quantum feedback scheme following a method inspired by ref. 13. It prepares on demand photon number states (Fock states) of a microwave field in a superconducting cavity, and subsequently reverses the effects of decoherence-induced field quantum jumps. The sensor is a beam of atoms crossing the cavity, which repeatedly performs weak quantum non-demolition measurements of the photon number. The controller is implemented in a real-time computer commanding the actuator, which injects adjusted small classical fields into the cavity between measurements. The microwave field is a quantum oscillator usable as a quantum memory or as a quantum bus swapping information between atoms. Our experiment demonstrates that active control can generate non-classical states of this oscillator and combat their decoherence, and is a significant step towards the implementation of complex quantum information operations.     

利用腔量子电动力学方法实现压缩叠加态的量子隐形传态

利用大失谐的原子-腔场相互作用和Bell态测量，提出了一个量子隐形传态方案。在这个方案中，两个具有相同振幅和相对位相的压缩真空态的任意相干叠加态被传送。为了实现这个方案，一个最大纠缠压缩真空态被用来作为量子信道。这个量子隐形传态方案成功的几率是0．5。     

非最大量子纠缠信道的量子态传送——在腔QED系统中利用非最大三粒子纠缠GHZ态传送未知原子态

讨论在腔QED中如何利用非最大三粒子纠缠GHZ态实现未知单原子态、两原子纠缠态的概率隐形传送.在量子态传送过程中需要引入一个辅助粒子以解决使用非最大纠缠量子信道导致的态畸变问题.本方案在两原子与腔相互作用的整个过程中,由于经典场同时对两原子进行驱动,量子态的演化不依赖于腔场的态,因而不受腔泄漏和热腔场的影响.