Entanglement purification for quantum communication

The distribution of entangled states between distant locations will be essential for the future large-scale realization of quantum communication schemes such as quantum cryptography and quantum teleportation. Because of unavoidable noise in the quantum communication channel, the entanglement between two particles is more and more degraded the further they propagate. Entanglement purification is thus essential to distil highly entangled states from less entangled ones. Existing general purification protocols are based on the quantum controlled-NOT (CNOT) or similar quantum logic operations, which are very difficult to implement experimentally. Present realizations of CNOT gates are much too imperfect to be useful for long-distance quantum communication. Here we present a scheme for the entanglement purification of general mixed entangled states, which achieves 50 per cent of the success probability of schemes based on the CNOT operation, but requires only simple linear optical elements. Because the perfection of such elements is very high, the local operations necessary for purification can be performed with the required precision. Our procedure is within the reach of current technology, and should significantly simplify the implementation of long-distance quantum communication.     

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。     

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.     

Experimental demonstration of a BDCZ quantum repeater node

Quantum communication is a method that offers efficient and secure ways for the exchange of information in a network. Large-scale quantum communication (of the order of 100 km) has been achieved; however, serious problems occur beyond this distance scale, mainly due to inevitable photon loss in the transmission channel. Quantum communication eventually fails when the probability of a dark count in the photon detectors becomes comparable to the probability that a photon is correctly detected. To overcome this problem, Briegel, Dür, Cirac and Zoller (BDCZ) introduced the concept of quantum repeaters, combining entanglement swapping and quantum memory to efficiently extend the achievable distances. Although entanglement swapping has been experimentally demonstrated, the implementation of BDCZ quantum repeaters has proved challenging owing to the difficulty of integrating a quantum memory. Here we realize entanglement swapping with storage and retrieval of light, a building block of the BDCZ quantum repeater. We follow a scheme that incorporates the strategy of BDCZ with atomic quantum memories. Two atomic ensembles, each originally entangled with a single emitted photon, are projected into an entangled state by performing a joint Bell state measurement on the two single photons after they have passed through a 300-m fibre-based communication channel. The entanglement is stored in the atomic ensembles and later verified by converting the atomic excitations into photons. Our method is intrinsically phase insensitive and establishes the essential element needed to realize quantum repeaters with stationary atomic qubits as quantum memories and flying photonic qubits as quantum messengers.     

Quantum key distribution using four-level particles

We present a quantum key distribution protocol based on four-level particle entanglement. Furthermore, a controlled quantum key distribution protocol is proposed using three four-level particles. We show that the two protocols are secure.     

Demonstration of a quantum teleportation network for continuous variables

Quantum teleportation involves the transportation of an unknown quantum state from one location to another, without physical transfer of the information carrier. Although quantum teleportation is a naturally bipartite process, it can be extended to a multipartite protocol known as a quantum teleportation network. In such a network, entanglement is shared between three or more parties. For the case of three parties (a tripartite network), teleportation of a quantum state can occur between any pair, but only with the assistance of the third party. Multipartite quantum protocols are expected to form fundamental components for larger-scale quantum communication and computation. Here we report the experimental realization of a tripartite quantum teleportation network for quantum states of continuous variables (electromagnetic field modes). We demonstrate teleportation of a coherent state between three different pairs in the network, unambiguously demonstrating its tripartite character.     

叠加态量子纠错的5位编码

利用叠加态量子纠错思想，设计了由H门和CNOT门实现的5位量子编码纠错线路，实现了在量子Hamming界条件下用最少位的叠加态编码．     

Demonstration of an all-optical quantum controlled-NOT gate

The promise of tremendous computational power, coupled with the development of robust error-correcting schemes, has fuelled extensive efforts to build a quantum computer. The requirements for realizing such a device are confounding: scalable quantum bits (two-level quantum systems, or qubits) that can be well isolated from the environment, but also initialized, measured and made to undergo controllable interactions to implement a universal set of quantum logic gates. The usual set consists of single qubit rotations and a controlled-NOT (CNOT) gate, which flips the state of a target qubit conditional on the control qubit being in the state 1. Here we report an unambiguous experimental demonstration and comprehensive characterization of quantum CNOT operation in an optical system. We produce all four entangled Bell states as a function of only the input qubits' logical values, for a single operating condition of the gate. The gate is probabilistic (the qubits are destroyed upon failure), but with the addition of linear optical quantum non-demolition measurements, it is equivalent to the CNOT gate required for scalable all-optical quantum computation.     

Mapping photonic entanglement into and out of a quantum memory

Developments in quantum information science rely critically on entanglement-a fundamental aspect of quantum mechanics that causes parts of a composite system to show correlations stronger than can be explained classically. In particular, scalable quantum networks require the capability to create, store and distribute entanglement among distant matter nodes by means of photonic channels. Atomic ensembles can play the role of such nodes. So far, in the photon-counting regime, heralded entanglement between atomic ensembles has been successfully demonstrated through probabilistic protocols. But an inherent drawback of this approach is the compromise between the amount of entanglement and its preparation probability, leading to intrinsically low count rates for high entanglement. Here we report a protocol where entanglement between two atomic ensembles is created by coherent mapping of an entangled state of light. By splitting a single photon and performing subsequent state transfer, we separate the generation of entanglement and its storage. After a programmable delay, the stored entanglement is mapped back into photonic modes with overall efficiency of 17%. Together with improvements in single-photon sources, our protocol will allow 'on-demand' entanglement of atomic ensembles, a powerful resource for quantum information science.     

Measuring-basis-encrypted six?state quantum key distribution scheme

A six-state quantum key distribution scheme with measuring-basis encryption technique, which is based on the six?state protocol and the MBE protocol, is proposed. In this modified six-state quantum key distribution protocol, Alice and Bob use a control key to synchronize the use of their measuring-basis so that they always use the same measuring?basis. This modified six-state quantum key distribution protocol retains the advantage of higher security and at the same time has a higher efficiency.     

量子安全直接通信?

"墨子"号量子科学实验卫星的发射,进一步吸引了人们对量子通信的关注.自1984年IBM的Bennett和加拿大蒙特利尔大学的Brassard提出第一个量子密钥随机分配协议以来,量子通信能否进行机密信息的安全直接传输一直是困扰量子通信界的一个难题.2000年,清华大学龙桂鲁等萌芽了量子安全直接通信的初步思想,随后的几年,我们建立了量子安全直接通信的完整理论,创建了4个判据,构造了2个最早期完整的量子安全直接通信代表性方案,为量子安全直接通信方案设计及其应用提供了物理原理依据,吸引了众多学者的关注与投入,促进了这一领域的发展.本文首先以第一个量子秘钥随机分配协议为例介绍量子通信如何为经典保密通信服务;随后,以最早期完整的2个量子安全直接通信方案为基础,阐述了为什么可以进行量子安全直接通信,并介绍量子安全直接通信的物理原理.     

基于纠缠交换的量子密钥分发

为了提高量子密钥分发的效率,提出了一种基于纠缠交换的密钥分配方案。该方案无需交换经典信息且不要进行任何酉操作,通信双方通过纠缠交换并利用贝尔测量即可生成密钥;除去少量用于检测量子信道安全的量子位,其余量子位都可以用来生成密钥,且每两对纠缠粒子就可以生成密钥的两个比特位。利用Stinespring Dilation定理证明了该方案的安全性并给出了效率分析。     

基于量子测量的量子计算模型综述

基于测量的单向量子计算是重要的通用量子计算模型,可以模拟一般量子计算任务。单向量子计算基于量子簇态作为计算资源,利用每个量子位的局部量子测量和经典通信执行一般量子计算。单向量子计算是与量子线路模型等价的量子计算模型。近年来,研究者们对单向量子计算的量子资源、纠缠度量、局部操作简化,以及量子通信等给出一系列研究成果,并基于光学平台开展了一些量子模拟实验。量子簇态与单向量子计算为一般量子计算提供非常好的量子任务处理方式,受到研究者们的广泛关注。该文主要总结基于测量的单向量子计算模型,包括重要的量子资源态、局部信息处理方式,以及与单向量子计算相关的研究;该文对单向量子计算存在的问题和前沿研究方法进行展望,为研究者提供借鉴。     

Realization of quantum error correction

Scalable quantum computation and communication require error control to protect quantum information against unavoidable noise. Quantum error correction protects information stored in two-level quantum systems (qubits) by rectifying errors with operations conditioned on the measurement outcomes. Error-correction protocols have been implemented in nuclear magnetic resonance experiments, but the inherent limitations of this technique prevent its application to quantum information processing. Here we experimentally demonstrate quantum error correction using three beryllium atomic-ion qubits confined to a linear, multi-zone trap. An encoded one-qubit state is protected against spin-flip errors by means of a three-qubit quantum error-correcting code. A primary ion qubit is prepared in an initial state, which is then encoded into an entangled state of three physical qubits (the primary and two ancilla qubits). Errors are induced simultaneously in all qubits at various rates. The encoded state is decoded back to the primary ion one-qubit state, making error information available on the ancilla ions, which are separated from the primary ion and measured. Finally, the primary qubit state is corrected on the basis of the ancillae measurement outcome. We verify error correction by comparing the corrected final state to the uncorrected state and to the initial state. In principle, the approach enables a quantum state to be maintained by means of repeated error correction, an important step towards scalable fault-tolerant quantum computation using trapped ions.     

地空量子通信流程构建及策略分析

针对预警机的地空量子通信问题，首先对影响其传输信道的主要因素进行了分析。基于分析结果，构建一地空量子通信流程，可使地面站与预警机在双向对准和跟踪后进行量子态的无线传输；基于该通信流程搭建一地空量子广域网，网络由空中平台和地面通信端等设备组成，根据不同的通信节点剖析3种路由策略，验证其可行性；最后对量子比特传输错误率进行分析，仿真结果表明通过提高信道系数，缩小测量基的偏差角度以及减少纠缠交换、经典信息引起的差错率，可有效减小传输的误码率。     

基于EPR粒子对的信息签名协议

研究了利用Einstein-Podolsky-Rosen (EPR)对的纠缠特性进行量子数字签名的方案,提出了一个基于测量结果比较的签名协议以及一个基于隐形传态的签名协议. 在基于测量结果比较的签名协议中,通信双方测量各自的EPR并将测量结果与消息M进行比较可以实现M的签名和验证,同时通过量子单向函数来保证协议的安全. 在基于隐形传态的签名协议中,消息发送者将M通过隐形传态传递给接收者来实现对M签名并验证,其安全性由量子隐形传态的物理特性来保证. 这两种协议均可以脱离可信赖的第三方,且通信过程中不需要密钥,具有无条件安全性.     

两种典型的量子通信技术

量子通信是量子信息中的一个重要分支．而其中最典型的量子通信技术是量子隐形传态和量子密码通信．文章介绍了量子通信中量子叠加和量子纠缠的概念．并介绍两种量子通信技术的理论框架,同时也涉及了这个领域的实验研究进展．     

传输距离对实际量子密钥分发系统的影响

基于BB84协议原理,搭建偏振编码的长距离点对点实际量子密钥分发实验系统.利用该实验平台完成了5,10,15km 3种不同传输距离的量子密钥分发实验,并对系统主要参数实时采集进行数据分析.实验结果表明:随着发送端与接收端量子光纤信道距离的增加,在统计时间内,系统分段成码率降低,信号态和诱骗态误码率上升,系统产生的密钥量降低.     

Communications: quantum teleportation across the Danube

Efficient long-distance quantum teleportation is crucial for quantum communication and quantum networking schemes. Here we describe the high-fidelity teleportation of photons over a distance of 600 metres across the River Danube in Vienna, with the optimal efficiency that can be achieved using linear optics. Our result is a step towards the implementation of a quantum repeater, which will enable pure entanglement to be shared between distant parties in a public environment and eventually on a worldwide scale.     

Broadband waveguide quantum memory for entangled photons

The reversible transfer of quantum states of light into and out of matter constitutes an important building block for future applications of quantum communication: it will allow the synchronization of quantum information, and the construction of quantum repeaters and quantum networks. Much effort has been devoted to the development of such quantum memories, the key property of which is the preservation of entanglement during storage. Here we report the reversible transfer of photon-photon entanglement into entanglement between a photon and a collective atomic excitation in a solid-state device. Towards this end, we employ a thulium-doped lithium niobate waveguide in conjunction with a photon-echo quantum memory protocol, and increase the spectral acceptance from the current maximum of 100?megahertz to 5?gigahertz. We assess the entanglement-preserving nature of our storage device through Bell inequality violations and by comparing the amount of entanglement contained in the detected photon pairs before and after the reversible transfer. These measurements show, within statistical error, a perfect mapping process. Our broadband quantum memory complements the family of robust, integrated lithium niobate devices. It simplifies frequency-matching of light with matter interfaces in advanced applications of quantum communication, bringing fully quantum-enabled networks a step closer.