Experimental purification of two-atom entanglement

Entanglement is a necessary resource for quantum applications--entanglement established between quantum systems at different locations enables private communication and quantum teleportation, and facilitates quantum information processing. Distributed entanglement is established by preparing an entangled pair of quantum particles in one location, and transporting one member of the pair to another location. However, decoherence during transport reduces the quality (fidelity) of the entanglement. A protocol to achieve entanglement 'purification' has been proposed to improve the fidelity after transport. This protocol uses separate quantum operations at each location and classical communication to distil high-fidelity entangled pairs from lower-fidelity pairs. Proof-of-principle experiments distilling entangled photon pairs have been carried out. However, these experiments obtained distilled pairs with a low probability of success and required destruction of the entangled pairs, rendering them unavailable for further processing. Here we report efficient and non-destructive entanglement purification with atomic quantum bits. Two noisy entangled pairs were created and distilled into one higher-fidelity pair available for further use. Success probabilities were above 35 per cent. The many applications of entanglement purification make it one of the most important techniques in quantum information processing.     

Experimental entanglement of four particles

Quantum mechanics allows for many-particle wavefunctions that cannot be factorized into a product of single-particle wavefunctions, even when the constituent particles are entirely distinct. Such 'entangled' states explicitly demonstrate the non-local character of quantum theory, having potential applications in high-precision spectroscopy, quantum communication, cryptography and computation. In general, the more particles that can be entangled, the more clearly nonclassical effects are exhibited--and the more useful the states are for quantum applications. Here we implement a recently proposed entanglement technique to generate entangled states of two and four trapped ions. Coupling between the ions is provided through their collective motional degrees of freedom, but actual motional excitation is minimized. Entanglement is achieved using a single laser pulse, and the method can in principle be applied to any number of ions.     

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.     

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.     

An entanglement criterion for states in infinite-dimensional multipartite quantum systems

In(Phys Lett A,2002,297:4-8) an entanglement criterion for finite-dimensional bipartite systems is proposed:If ρ AB is a separable state,then Tr(ρA2) Tr(ρ2) and Tr(ρB2) Tr(ρ2).In the present paper this criterion is extended to infinite-dimensional bipartite and multipartite systems.The reduction criterion presented in(Phys Rev A,1999,59:4206-4216) is also generalized to infinitedimensional case.Then it is shown that the former criterion is weaker than the later one.     

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.     

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.     

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.     

无错区分两个任意分布的未知纬线态

量子态分辨在量子信息和量子计算领域中是一个非常重要的问题.在量子通讯理论和实验的研究过程中,经常要用到赤道态,它是一种比较特殊的纬线态.本文主要利用Neumark定理和可编程量子态分辨器的光学实现方法来研究如何无错区分两个任意分布的未知纬线态,推导出了测量算符和对应的成功概率,并且发现本文得到的测量算符与无错区分均匀分布纬线态得到的最优测量算符一致.这些结果对实验室中构造可编程量子分辨器具有重要参考意义.     

A quantum algorithm that deletes marked states from an arbitrary database

We present a general quantum deletion algorithm that deletes M marked states from an N-item quantum database with arbitrary initial distribution. The general behavior of this algorithm is analyzed, and analytic result is given. When the number of marked states is no more than 3N/4 , this algorithm requires just a single query, and this achieves exponential speedup over classical algorithm.     

双模奇、偶相干态的反聚束效应与纠缠度的关系

为研究双模奇、偶相干态(或双模Schodinger猫态)的反聚束效应与纠缠度的关系,主要讨论了双模奇、偶相干态的二阶关联函数g(2)(τ),得出该量子态具有明显反聚束效应,并利用约化密度算符的本征值求Von Neumann熵的办法给出该态模间纠缠度.数值分析表明:双模奇、偶相干态的纠缠程度与反聚束效应都受到相位和振幅的影响,纠缠度和二阶关联函数都是相位φ的周期函数,且当纠缠度增大时其反聚束效应也逐渐增大.     

XXX模型在与时间有关的任意外场下量子态的纠缠测度

基于代数动力学和纠缠测度的基本概念,研究了时间相关的任意外场驱动的海森堡XXX-模型下的双量子比特纠缠态随时间演化的性质.可以证明,纠缠测度仅仅由耦合常量和外场的强度决定的.通过进一步研究发现,在任意恒定外场的驱动下,热域纠缠测度是一个 与耦合常量J,温度T以及外场大小有关的函数.     

热电集成直接蒸馏式水净化系统的实验

为了避免热电集成膜蒸馏系统(TIMES)结构复杂,可靠性差,能耗高的缺点,提出了热电集成直接蒸馏式水净化系统,并通过实验对该系统进行了可行性验证和性能分析.与TIMES系统相比,直接蒸馏式水净化系统的能耗较低,净化水中无机盐成分较少;随着系统工作电流增大,系统产水率提高,但单位质量产水所消耗的能量(单耗)也增大;增加半导体热泵数量既可以提高系统的产水率也可以降低净化水的单耗;然而增大半导体热泵热端的液膜厚度会使系统的产水率降低且单耗增大.     

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．     

稳定核^91Zr高自旋态实验研究

利用能量为44MeV的13C束流,通过重离子熔合蒸发反应82Se（13C,4n）91Zr布居了稳定核91Zr的高自旋态.基于γ-γ符合关系扩展了91Zr21/2＋同核异能态上的能级纲图,根据实验提取的γ跃迁的角分布系数对部分能级的自旋进行了指认.前人对21/2＋同核异能态下正负宇称能级已指认为质子在π（p1/2,g9/2）轨道的激发耦合价中子vd5/2;在π（f5/2,p3/2,p1/2,g9/2）和v（d5/2,s1/2,d3/2,g7/2,h11/2）空间内,经过系统性的分析和比较91Zr邻近核高自旋态能级结构,指出同核异能态上正宇称态激发机制依次为质子跨越38子壳π（f5/2,p3/2）→πp1/2激发和中子跨越50闭壳vg9/2→vd5/2激发,同核异能态上负宇称态可能涉及质子跨越38子壳π（f5/2,p3/2）→πg9/2激发与价中子vd5/2→vh11/2激发的竞争.     

两非等同纠缠原子与单模光场相互作用系统的纠缠特性

研究了在考虑腔场有能量损耗、原子有自发辐射时,两非等同纠缠原子与单模腔场相互作用过程中两原子之间的纠缠特性。结果表明;两纠缠原子的纠缠特性与两原子初态、腔场耗散系数k、原子的自发辐射率Γ及两原子与腔、场的耦合系数g1g2有一定的联系。     

量子纠缠的新理论

利用独立态矢特性,首次提出了一个纠缠态的定义及构造纠缠的理论,并进一步给出了多粒子系统的纠缠方式,进而给出基本纠缠方式的纠缠度的定义式,并推导出其公式,得出了三粒子系统的非纠缠判据.     

润滑状态下凹口螺栓力学特性试验研究

对MoS2干膜润滑和航空润滑油润滑的凹口螺栓进行了施加扭力时的预紧力测试,试验结果表明:施加相同扭力时MoS2干膜润滑凹口螺栓较航空润滑油润滑凹口螺栓的预紧力大,亦拧紧螺栓时施加较小的力矩可获得较大的预紧力,这是很有工程价值的现象。     

^139Pm高自旋态的实验研究

通过重离子核反应^116Cd（^27Al,4n）与在束γ谱的实验技术,对A=130-140核区的奇A核139Pm的高自旋态进行了研究.根据γ-γ符合关系、γ射线的相对强度和各向异性度的测量结果,扩展并更新了^139Pm的能级纲图.实验观测到基于πh11/2和πg7/2 [πh11/2]^2（或者πd5/2 [πh11/2]^2）组态的转动带,利用已有的理论计算结果对这些转动带进行了解释.同时还观测到三个具有很强M1跃迁、旋称劈裂很小的△I=1的带.通过简单分析和系统学比较,指认了它们的组态,发现它们具备磁转动带的特性,很可能是磁转动带.