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.     

Experimental violation of a Bell's inequality with efficient detection

Local realism is the idea that objects have definite properties whether or not they are measured, and that measurements of these properties are not affected by events taking place sufficiently far away. Einstein, Podolsky and Rosen used these reasonable assumptions to conclude that quantum mechanics is incomplete. Starting in 1965, Bell and others constructed mathematical inequalities whereby experimental tests could distinguish between quantum mechanics and local realistic theories. Many experiments have since been done that are consistent with quantum mechanics and inconsistent with local realism. But these conclusions remain the subject of considerable interest and debate, and experiments are still being refined to overcome 'loopholes' that might allow a local realistic interpretation. Here we have measured correlations in the classical properties of massive entangled particles (9Be+ ions): these correlations violate a form of Bell's inequality. Our measured value of the appropriate Bell's 'signal' is 2.25 +/- 0.03, whereas a value of 2 is the maximum allowed by local realistic theories of nature. In contrast to previous measurements with massive particles, this violation of Bell's inequality was obtained by use of a complete set of measurements. Moreover, the high detection efficiency of our apparatus eliminates the so-called 'detection' loophole.     

Bell不等式及其局域性假设

文中首先证明Bell不等式成立的充足条件是几率的Kolmogorov模型.不等式破缺只说明对量子关联系统Kolmogorov模型失效.其次,Bell的局域性假设不能与 Einstein 的“分离性”条件等价,它隐含有苛刻的几率约束条件,最后指出了 Bell 不等式也不是完全地符合量子理论的预测.因此,量子理论基础问题的讨论,并没有由于 Bell 不等式的实验验证作出判决.     

Hardy定理的一种证明

在两个自旋1 2粒子形成纠缠态的条件下不用不等式论证了量子力学具有Ein stein,Podolsky,和Rosen的局域实在论所无法说明的非局域关联性。从而证明了EPR局域实在论不能重现量子力学的独特的非经典结论。     

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 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.     

Violation of a Bell-like inequality in single-neutron interferometry

Non-local correlations between spatially separated systems have been extensively discussed in the context of the Einstein, Podolsky and Rosen (EPR) paradox and Bell's inequalities. Many proposals and experiments designed to test hidden variable theories and the violation of Bell's inequalities have been reported; usually, these involve correlated photons, although recently an experiment was performed with (9)Be(+) ions. Nevertheless, it is of considerable interest to show that such correlations (arising from quantum mechanical entanglement) are not simply a peculiarity of photons. Here we measure correlations between two degrees of freedom (comprising spatial and spin components) of single neutrons; this removes the need for a source of entangled neutron pairs, which would present a considerable technical challenge. A Bell-like inequality is introduced to clarify the correlations that can arise between observables of otherwise independent degrees of freedom. We demonstrate the violation of this Bell-like inequality: our measured value is 2.051 +/- 0.019, clearly above the value of 2 predicted by classical hidden variable theories.     

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．     

对EPR悖论的一个分析——分析理性与辩证理性之联手

1.悖论产生的要害或根源在于,背景知识是＂有缺陷的＂却又被公认为正确的。2.EPR悖论是量子力学领域中的一个著名悖论。爱因斯坦等人借助于理想实验和EPR论证,发掘了量子力学背后在元理论层次的奇特的非定域性假设,从表面上看,似乎违背相对论的要求。3.爱因斯坦想维护实在论和非超距作用并不错,但量子整体性或远程关联性也不容忽视,它将展示有待进一步挖掘的深刻含义。4.分析理性与辩证理性的联手,是化解科学上悖论的强有力的思想工具。     

量子通信研究的若干问题

1935 年A. Einstein、B. Podolsky 和N. Rosen 三人发表合写文章"量子力学对物理实在的描述是否完备?"该文的局域性原则与他的狭义相对论(SR) 相呼应,但与量子力学(QM) 不一致.1951 年D. Bohm 对EPR 思维作了现代意义上的陈述,实际上启动了量子纠缠态研究.在此基础上,1965 年J. Bell 提出了后来称为Bell 不等式的隐变量理论,而在1981 年-1982 年A. Aspect 做了多个精确实验,结果与Bell 不等式不符,而与QM 一致.因此,双粒子系统中存在QM 预期的奇异相关,其中的超空间(超距) 作用却与EPR 思维矛盾.在后来数十年中,Bell 型实验常盛不衰,互相纠缠的光子间隔由Aspect 时的15m 逐步加大到144km,而在2017 年由中国量子卫星扩展到1200km,十分惊人.EPR 论文的错误对科学研究提供了深刻的教益.近年来量子通信技术的发展,就是建筑在量子非局域性和量子纠缠的基础上的.量子信息学(QIT) 有三个主要研究方向——量子计算、量子通信和量子雷达.量子通信的关键点在于必须有绝对保密性.但这在实际上很难,故迄今不能说这问题已经解决.因此,我们认为在2017 年量子通信才获很大成就.     

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.     

定域隐变量理论的实验检验

<正> 一、历史上两种观点的争论量子力学的形式体系是完美的,它经受了半个多世纪的各种严格的实验检验,但是关于量子力学理论基础的解释,无论物理上还是哲学上都存在重大的原则分岐。假若把量子力学的形式体系彻底地贯彻到对现实物理世界的认识中去,那么势必导致与经典物理概念和传统哲学观念格格不入的新概念和新观点     

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.     

Deterministic quantum teleportation of atomic qubits

Quantum teleportation provides a means to transport quantum information efficiently from one location to another, without the physical transfer of the associated quantum-information carrier. This is achieved by using the non-local correlations of previously distributed, entangled quantum bits (qubits). Teleportation is expected to play an integral role in quantum communication and quantum computation. Previous experimental demonstrations have been implemented with optical systems that used both discrete and continuous variables, and with liquid-state nuclear magnetic resonance. Here we report unconditional teleportation of massive particle qubits using atomic (9Be+) ions confined in a segmented ion trap, which aids individual qubit addressing. We achieve an average fidelity of 78 per cent, which exceeds the fidelity of any protocol that does not use entanglement. This demonstration is also important because it incorporates most of the techniques necessary for scalable quantum information processing in an ion-trap system.     

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.     

量子力学中电磁场矢势(-A)的再认识

通过引用英国钱伯斯(Chambers R.G.)所做的实验探讨了电磁AB效应,并根据量子论的基本概念,计算两种情况的干涉条纹的移动值,其计算结果与实验数据完全一致,从而论证了矢势A在量子理论中具有基本的物理意义和确定的物理效应.由此,带电粒子在外磁场中的动力学行为会受到矢势A的直接影响.为了更好地认识矢势A是一个基本的物理实体,类比于电场强度E、磁感应强度B的定义,在此定义了一个新的物理量磁场矢势A.     

实在性、定域性和见尔不等式的若干讨论

本文介绍和讨论了实在性和定域性观念以及其数学化的表述——Bell不等式中的若干问题,指出某些与Bell不等式相联系的实验存在严重的缺陷.对实在性观念和量子力学的分歧重新给予了评论。     

Decoherence in crystals of quantum molecular magnets

Quantum decoherence is a central concept in physics. Applications such as quantum information processing depend on understanding it; there are even fundamental theories proposed that go beyond quantum mechanics, in which the breakdown of quantum theory would appear as an 'intrinsic' decoherence, mimicking the more familiar environmental decoherence processes. Such applications cannot be optimized, and such theories cannot be tested, until we have a firm handle on ordinary environmental decoherence processes. Here we show that the theory for insulating electronic spin systems can make accurate and testable predictions for environmental decoherence in molecular-based quantum magnets. Experiments on molecular magnets have successfully demonstrated quantum-coherent phenomena but the decoherence processes that ultimately limit such behaviour were not well constrained. For molecular magnets, theory predicts three principal contributions to environmental decoherence: from phonons, from nuclear spins and from intermolecular dipolar interactions. We use high magnetic fields on single crystals of Fe(8) molecular magnets (in which the Fe ions are surrounded by organic ligands) to suppress dipolar and nuclear-spin decoherence. In these high-field experiments, we find that the decoherence time varies strongly as a function of temperature and magnetic field. The theoretical predictions are fully verified experimentally, and there are no other visible decoherence sources. In these high fields, we obtain a maximum decoherence quality-factor of 1.49?×?10(6); our investigation suggests that the environmental decoherence time can be extended up to about 500 microseconds, with a decoherence quality factor of ～6?×?10(7), by optimizing the temperature, magnetic field and nuclear isotopic concentrations.     

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.     

可控的量子密集编码

提出了一种利用三粒子非最大纠缠态作为量子信道的可控的概率密集编码方案.第3方通过局域测量控制着量子信道.通过引入辅助粒子和做适当的幺正变换,就能以一定概率实现量子密集编码.