利用两能级原子与相干态腔场相互作用实现纠缠浓缩

根据大失谐条件下原子–腔场相互作用的特点,讨论了一个制备纠缠相干态的方法,提出了一个利用两能级原子与腔场相干态相互作用实现纠缠浓缩的方案。在这个方案中,2个具有相同振幅但有着π相位差的相干光|α〉和|-α〉构成的纠缠态光场被用来作为量子信道,通过利用两能级原子与腔场的相互作用以及两模正交态测量实现了这个纠缠浓缩的过程。结果表明:对于纠缠相干态,无论其初始的纠缠是多么微弱,利用这种方法总有一定的几率可以从部分纠缠态中提取出最大纠缠态。     

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.     

光场压缩态与纠缠态的增强及量子信息网络

光场压缩态和纠缠态是进行量子精密测量和量子信息研究的重要资源.文章简要介绍光场压缩态和纠缠态的增强及其在量子密钥分发、连续变量多组分纠缠态光场的制备、量子通信网络和量子计算中的应用.     

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 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.     

Measurement-induced entanglement for excitation stored in remote atomic ensembles

A critical requirement for diverse applications in quantum information science is the capability to disseminate quantum resources over complex quantum networks. For example, the coherent distribution of entangled quantum states together with quantum memory (for storing the states) can enable scalable architectures for quantum computation, communication and metrology. Here we report observations of entanglement between two atomic ensembles located in distinct, spatially separated set-ups. Quantum interference in the detection of a photon emitted by one of the samples projects the otherwise independent ensembles into an entangled state with one joint excitation stored remotely in 10(5) atoms at each site. After a programmable delay, we confirm entanglement by mapping the state of the atoms to optical fields and measuring mutual coherences and photon statistics for these fields. We thereby determine a quantitative lower bound for the entanglement of the joint state of the ensembles. Our observations represent significant progress in the ability to distribute and store entangled quantum states.     

Entanglement of linear cluster states in terms of averaged entropies

An entanglement measure,multiple entropy measures(MEMS) was proposed recently by using the geometric mean of partial entropies over all possible i-body combinations of the quantum system.In this work,we study the average subsystem von Neumann entropies of the linear cluster state and investigated the quantum entanglement of linear cluster states in terms of MEMS.Explicit results with specific particle numbers are calculated,and some analytical results are given for systems with arbitrary particle numbers.Compared with other example quantum states such as the GHZ states and W states,the linear cluster states are "more entangled" in terms of MEMS,namely their averaged entropies are larger than the GHZ states and W states.     

Experimental long-lived entanglement of two macroscopic objects

Entanglement is considered to be one of the most profound features of quantum mechanics. An entangled state of a system consisting of two subsystems cannot be described as a product of the quantum states of the two subsystems. In this sense, the entangled system is considered inseparable and non-local. It is generally believed that entanglement is usually manifest in systems consisting of a small number of microscopic particles. Here we demonstrate experimentally the entanglement of two macroscopic objects, each consisting of a caesium gas sample containing about 1012 atoms. Entanglement is generated via interaction of the samples with a pulse of light, which performs a non-local Bell measurement on the collective spins of the samples. The entangled spin-state can be maintained for 0.5 milliseconds. Besides being of fundamental interest, we expect the robust and long-lived entanglement of material objects demonstrated here to be useful in quantum information processing, including teleportation of quantum states of matter and quantum memory.     

Generation of three-qubit entangled states using superconducting phase qubits

Entanglement is one of the key resources required for quantum computation, so the experimental creation and measurement of entangled states is of crucial importance for various physical implementations of quantum computers. In superconducting devices, two-qubit entangled states have been demonstrated and used to show violations of Bell's inequality and to implement simple quantum algorithms. Unlike the two-qubit case, where all maximally entangled two-qubit states are equivalent up to local changes of basis, three qubits can be entangled in two fundamentally different ways. These are typified by the states |GHZ>= (|000+?|111>)/ sqrt [2] and |W>= (|001>?+?|010>?+?|100>)/ sqrt [3]. Here we demonstrate the operation of three coupled superconducting phase qubits and use them to create and measure |GHZ> and |W>states. The states are fully characterized using quantum state tomography and are shown to satisfy entanglement witnesses, confirming that they are indeed examples of three-qubit entanglement and are not separable into mixtures of two-qubit entanglement.     

用非最大纠缠态实现三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几率地传输三粒子部分纠缠态的量子控制方案．在该方案中,选择量子通道中的一个粒子作为控制粒子,发送者进行一次Bell基测量和2次Hadamard门测量;控制者实施一次Hadamard门测量,并将他们的测量结果利用经典信道发给接收者;接收者根据他们的测量结果进行适当的幺正变换以及一些必要的投影测量就能得到待传的未知量子态．该方案是一种基于非最大纠缠态的几率受控的隐形传态方案,可以应用于远程量子计算、远程量子克隆、量子远程控制等．     

Super-resolving phase measurements with a multiphoton entangled state

Interference phenomena are ubiquitous in physics, often forming the basis of demanding measurements. Examples include Ramsey interferometry in atomic spectroscopy, X-ray diffraction in crystallography and optical interferometry in gravitational-wave studies. It has been known for some time that the quantum property of entanglement can be exploited to perform super-sensitive measurements, for example in optical interferometry or atomic spectroscopy. The idea has been demonstrated for an entangled state of two photons, but for larger numbers of particles it is difficult to create the necessary multiparticle entangled states. Here we demonstrate experimentally a technique for producing a maximally entangled three-photon state from initially non-entangled photons. The method can in principle be applied to generate states of arbitrary photon number, giving arbitrarily large improvement in measurement resolution. The method of state construction requires non-unitary operations, which we perform using post-selected linear-optics techniques similar to those used for linear-optics quantum computing.     

Coherent dynamics of a flux qubit coupled to a harmonic oscillator

In the emerging field of quantum computation and quantum information, superconducting devices are promising candidates for the implementation of solid-state quantum bits (qubits). Single-qubit operations, direct coupling between two qubits and the realization of a quantum gate have been reported. However, complex manipulation of entangled states-such as the coupling of a two-level system to a quantum harmonic oscillator, as demonstrated in ion/atom-trap experiments and cavity quantum electrodynamics-has yet to be achieved for superconducting devices. Here we demonstrate entanglement between a superconducting flux qubit (a two-level system) and a superconducting quantum interference device (SQUID). The latter provides the measurement system for detecting the quantum states; it is also an effective inductance that, in parallel with an external shunt capacitance, acts as a harmonic oscillator. We achieve generation and control of the entangled state by performing microwave spectroscopy and detecting the resultant Rabi oscillations of the coupled system.     

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．     

Stimulated emission of polarization-entangled photons

Entangled photon pairs-discrete light quanta that exhibit non-classical correlations-play a crucial role in quantum information science (for example, in demonstrations of quantum non-locality, quantum teleportation and quantum cryptography). At the macroscopic optical-field level non-classical correlations can also be important, as in the case of squeezed light, entangled light beams and teleportation of continuous quantum variables. Here we use stimulated parametric down-conversion to study entangled states of light that bridge the gap between discrete and macroscopic optical quantum correlations. We demonstrate experimentally the onset of laser-like action for entangled photons, through the creation and amplification of the spin-1/2 and spin-1 singlet states consisting of two and four photons, respectively. This entanglement structure holds great promise in quantum information science where there is a strong demand for entangled states of increasing complexity.     

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.     

量子纠缠控制的核磁共振实验演示

量子纠缠在量子信息领域中有着极重要的应用,是当前人们研究的热门课题之一[1-2].量子纠缠控制是一种量子相干控制,即系统的量子力学进程在我们的控制之下,并使之产生期望的结果[3-4].我们在一个3量子位的核磁共振量子计算机实现了2个方面量子纠缠控制:纠缠互换与存储.我们使用赝纯态作为系统的初始状态.通过赝纯态,系统的微观性质可用宏观信号表述出来.     

Experimental entanglement distillation and 'hidden' non-locality

Entangled states are central to quantum information processing, including quantum teleportation, efficient quantum computation and quantum cryptography. In general, these applications work best with pure, maximally entangled quantum states. However, owing to dissipation and decoherence, practically available states are likely to be non-maximally entangled, partially mixed (that is, not pure), or both. To counter this problem, various schemes of entanglement distillation, state purification and concentration have been proposed. Here we demonstrate experimentally the distillation of maximally entangled states from non-maximally entangled inputs. Using partial polarizers, we perform a filtering process to maximize the entanglement of pure polarization-entangled photon pairs generated by spontaneous parametric down-conversion. We have also applied our methods to initial states that are partially mixed. After filtering, the distilled states demonstrate certain non-local correlations, as evidenced by their violation of a form of Bell's inequality. Because the initial states do not have this property, they can be said to possess 'hidden' non-locality.     

任意多体高维偏振纠缠态的有效制备

纠缠态与高维量子态是量子信息科学关注的焦点,在光学领域,如何有效制备高维纠缠态是值得探讨的问题．基于交又相位调制技术,利用单光子高维空间态作为辅助,通过纠缠光子对与单光子之间的相互作用,以及多光子的干涉,可以制备出任意系数的两体高维偏振纠缠态．其成功概率由高维偏振量子态,也就是多光子干涉决定．同时,这一方案可以很容易推广到任意多体高维偏振纠缠态的制备．所制备纠缠态的灵活性,将为研究高维纠缠态的性质、多体相互作用等问题提供一定的便利．     

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。     

Bell inequality,separability and entanglement distillation

In this review,we introduce well-known Bell inequalities,the relations between the Bell inequality and quantum separability,and the entanglement distillation of quantum states.It is shown that any pure entangled quantum state violates one of Bell-like inequalities.Moreover,quantum states that violate any one of these Bell-like inequalities are shown to be distillable.New Bell inequalities that detect more entangled mixed states are also introduced.