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.     

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.     

基于W态的代理型单服务器盲量子计算协议设计

三粒子W态与GHZ(Greenberger-Horne-Zeilinger)态相比具有较强的鲁棒性,根据这一特性并结合量子计算机未来的发展需求,提出了一种通过测量方式、利用三粒子W态的纠缠和坍缩性、并通过增加可信任中心的代理功能,来实现单服务器模式下盲量子计算的方案,协议主要是针对计算准备阶段来进行设计。协议执行过程中,可信任中心代理客户端完成与服务器之间的通信和数据发送任务,服务器无法获知客户端的属性信息。协议可以实现客户端完全经典,并且客户端可以不具有量子信道的访问能力,但与可信任中心之间需具有双向通信能力。协议具有结构简单、易于实现、执行步骤较少等特点。最后对提出的协议进行了安全性分析,协议具有无条件安全性。     

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.     

Scalable multiparticle entanglement of trapped ions

The generation, manipulation and fundamental understanding of entanglement lies at the very heart of quantum mechanics. Entangled particles are non-interacting but are described by a common wavefunction; consequently, individual particles are not independent of each other and their quantum properties are inextricably interwoven. The intriguing features of entanglement become particularly evident if the particles can be individually controlled and physically separated. However, both the experimental realization and characterization of entanglement become exceedingly difficult for systems with many particles. The main difficulty is to manipulate and detect the quantum state of individual particles as well as to control the interaction between them. So far, entanglement of four ions or five photons has been demonstrated experimentally. The creation of scalable multiparticle entanglement demands a non-exponential scaling of resources with particle number. Among the various kinds of entangled states, the 'W state' plays an important role as its entanglement is maximally persistent and robust even under particle loss. Such states are central as a resource in quantum information processing and multiparty quantum communication. Here we report the scalable and deterministic generation of four-, five-, six-, seven- and eight-particle entangled states of the W type with trapped ions. We obtain the maximum possible information on these states by performing full characterization via state tomography, using individual control and detection of the ions. A detailed analysis proves that the entanglement is genuine. The availability of such multiparticle entangled states, together with full information in the form of their density matrices, creates a test-bed for theoretical studies of multiparticle entanglement. Independently, 'Greenberger-Horne-Zeilinger' entangled states with up to six ions have been created and analysed in Boulder.     

Experimental test of quantum nonlocality in three-photon Greenberger-Horne-Zeilinger entanglement

Bell's theorem states that certain statistical correlations predicted by quantum physics for measurements on two-particle systems cannot be understood within a realistic picture based on local properties of each individual particle-even if the two particles are separated by large distances. Einstein, Podolsky and Rosen first recognized the fundamental significance of these quantum correlations (termed 'entanglement' by Schrodinger) and the two-particle quantum predictions have found ever-increasing experimental support. A more striking conflict between quantum mechanical and local realistic predictions (for perfect correlations) has been discovered; but experimental verification has been difficult, as it requires entanglement between at least three particles. Here we report experimental confirmation of this conflict, using our recently developed method to observe three-photon entanglement, or 'Greenberger-Horne-Zeilinger' (GHZ) states. The results of three specific experiments, involving measurements of polarization correlations between three photons, lead to predictions for a fourth experiment; quantum physical predictions are mutually contradictory with expectations based on local realism. We find the results of the fourth experiment to be in agreement with the quantum prediction and in striking conflict with local realism.     

Preparation and measurement of three-qubit entanglement in a superconducting circuit

Traditionally, quantum entanglement has been central to foundational discussions of quantum mechanics. The measurement of correlations between entangled particles can have results at odds with classical behaviour. These discrepancies grow exponentially with the number of entangled particles. With the ample experimental confirmation of quantum mechanical predictions, entanglement has evolved from a philosophical conundrum into a key resource for technologies such as quantum communication and computation. Although entanglement in superconducting circuits has been limited so far to two qubits, the extension of entanglement to three, eight and ten qubits has been achieved among spins, ions and photons, respectively. A key question for solid-state quantum information processing is whether an engineered system could display the multi-qubit entanglement necessary for quantum error correction, which starts with tripartite entanglement. Here, using a circuit quantum electrodynamics architecture, we demonstrate deterministic production of three-qubit Greenberger-Horne-Zeilinger (GHZ) states with fidelity of 88 per cent, measured with quantum state tomography. Several entanglement witnesses detect genuine three-qubit entanglement by violating biseparable bounds by 830?±?80 per cent. We demonstrate the first step of basic quantum error correction, namely the encoding of a logical qubit into a manifold of GHZ-like states using a repetition code. The integration of this encoding with decoding and error-correcting steps in a feedback loop will be the next step for quantum computing with integrated circuits.     

一类正线性映射的可分解性和最优性

非完全正的正线性映射在判定复合系统量子态的纠缠性中起关键作用.文章研究一类非完全正的正线性映射的性质,证明了此类正线性映射φ是可分解的,不是2-正的,并给出了由此类正线性映射φ生成的纠缠witnessesWφ成为最优的充分必要条件.     

利用2个EPR对实现3粒子GHZ态的控制隐形传态

为了实现经济的控制隐形传态,提出一种利用2个部分纠缠EPR对实现3粒子GHZ态的概率隐形传输方案.该方案首先需要发送者向控制者申请量子信道,若控制者同意,才能通过纠缠交换,使发送者和接收者之间建立量子信道.然后发送者进行一次Bell基联合测量、两次H变换和两次单粒子测量.接收者根据发送者和控制者的测量结果,引入辅助粒子,进行两次控制非门操作和相应的幺正变换,就可以得到原始未知信息态的信息,传输成功的概率为4|a|~2|c|~2.该方案可以推广到N粒子GHZ态的控制隐形传输.若增加到N个EPR对为量子信道,还可以推广到(N-1)个控制者参与的N粒子GHZ态的控制隐形传输.该方案可以很好的应对一般的窃听方式.     

一次量子通信量子密钥分发和认证协议的安全性分析

分析了一次量子通信量子密钥分发（QKD）和认证（QA）协议存在的安全性漏洞,协议能抵抗截取／重发攻击策略和纠缠攻击策略,但不能抵抗纠缠／截取攻击策略,严格意义上说,在有信道噪声情况下,仅通过一次量子通信不能实现QKD和QA,根据原协议的条件,提出了2个新协议：①直接测量共享的GHZ态;②通过在原协议中加入检测过程以抵抗纠缠／截取攻击策略。     

远程制备多粒子纠缠态优化方案

提出了一个较好的远程制备多粒子的纠缠态的方案,先讨论用(N+1)粒子的纠缠态作为量子通信信道制备2N粒子的纠缠态.然后在研究用一个(N+1)粒子的纠缠态和一个(N+2)粒子的纠缠态作为量子通信信道制备(2N+1)粒子的纠缠态.与现在已有人提出的制备多粒子的纠缠态的方案相比,这个方案的优点是仅花费了两位经典位和只有一次两粒子的投影测量.     

4粒子团簇态的可控隐形传态

为实现4粒子团簇态更加经济和安全的隐形传态,提出2种利用6粒子团簇态为量子信道的可控隐形传态方案,分别是3方参与的可控隐形传态方案和4方参与的可控隐形传态方案.在控制者同意信息传输的情况下,2种隐形传态方案成功的概率是一致的,为4|a|2,即选用最大纠缠的6粒子团簇态为量子信道时,传输成功率为100%.通过2种方案的对比发现,控制方的增多可以有效地提高信息传输的安全性,但同时会使接收方还原原始信息的操作更加复杂.     

Kerr介质中非线性Jaynes-Cummings模型多光子跃迁的纠缠特性

在线性和非线性Kerr介质Jaynes Cummings(J C)模型中, 考虑多光子跃迁时原子和光场纠缠随时间的演化特性, 给出原子和光场的纠缠度与跃迁光子数、 失谐量、 初始光子数、 初始态参数及非线性项系数之间的解析关系, 并分析其对原子和光场的纠缠度随时间演化的影响. 结果表明, 在非线性Kerr介质中, 当光场与原子初始态接近最大纠缠态, 且跃迁光子数较大时, 纠缠态可稳定接近最大纠缠态, 有利于量子纠缠态的远距离传输.     

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.     

腔QED中三粒子W态的隐形传递

首先,针对传递三粒子w态的困难,提出了腔QED中的两种方案来完成w态的隐形传态．在一种方案中,我们利用一个EPR对和GHZ态来作为量子通道,在真空腔中实现了对w态的传送．而第二种方案与第一种不同之处是,我们使用了两个EPR对来作为量子通道,并且弓l入了一个附加粒子来完成传态的．此外,这些方案的显著优点是不用Bell基测量,而且可忽略腔场热作用和腔延迟作用的影响．     

Entanglement of the orbital angular momentum states of photons.

Entangled quantum states are not separable, regardless of the spatial separation of their components. This is a manifestation of an aspect of quantum mechanics known as quantum non-locality. An important consequence of this is that the measurement of the state of one particle in a two-particle entangled state defines the state of the second particle instantaneously, whereas neither particle possesses its own well-defined state before the measurement. Experimental realizations of entanglement have hitherto been restricted to two-state quantum systems, involving, for example, the two orthogonal polarization states of photons. Here we demonstrate entanglement involving the spatial modes of the electromagnetic field carrying orbital angular momentum. As these modes can be used to define an infinitely dimensional discrete Hilbert space, this approach provides a practical route to entanglement that involves many orthogonal quantum states, rather than just two Multi-dimensional entangled states could be of considerable importance in the field of quantum information, enabling, for example, more efficient use of communication channels in quantum cryptography.     

基于GHZ态的四量子位秘密共享方案

利用GHZ态作为量子信道,再辅以经典信道传送经GHZ态测量后的信息,便可实现量子位的秘密共享．基于上述思想,充分利用六粒子GHZ纠缠态的相关性,通过1次Bell基测量、4次单粒子测量和相应的幺正变换,从而实现了4个量子位的秘密共享方案.     

基于法拉第旋转制备光子GHZ态和cluster态

制备GHZ态和cluster态是当前量子通讯及量子信息处理过程中一项重要课题,文章基于偏振光被囚禁原子光腔反射后所获得的法拉第旋转制备了子GHZ态和cluster态。该方案不需要腔场的强耦合条件,在低品质光腔中也能实现,从而大大降低了实验难度。     

无限维两体量子系统态的可分性鲁棒和纠缠鲁棒

研究了量子信息理论中无限维两体量子态的可分性鲁棒和纠缠鲁棒问题。利用无限维两体量子系统态的Concurrence纠缠度以及可分态的边缘态的概念,讨论了无限维两体量子态的可分性鲁棒和纠缠鲁棒的定义。得到了可分性鲁棒以及纠缠鲁棒的若干基本性质,推广了有限维两体量子态的相应结果,获得了无限维两体量子系统态的可分性鲁棒和纠缠鲁棒分别为零的等价条件。     

W纠缠辅助下的熵测不准关系

利用弱测量与反弱测量技术,研究了基于W纠缠辅助的熵测不准关系在振幅阻尼噪声下的演化行为.分别对单粒子和双粒子经历噪声的情况进行了模拟计算,发现弱测量与反弱测量技术对双粒子经历噪声的纠缠恢复优于单粒子噪声情况.在双粒子噪声情况下,2个不对易的力学量在同一个体系中的测不准量会明显减少,计算发现该测不准量与弱测量和反弱测量强度、阻尼衰减系数等因素有关.阻尼衰减系数在一定的范围内,弱测量与反弱测量技术有效地提高了W纠缠态抵御噪声的能力,本文给出了关于不确定量更紧的束缚.