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.     

Quantum teleportation and entanglement distribution over 100-kilometre free-space channels

Transferring an unknown quantum state over arbitrary distances is essential for large-scale quantum communication and distributed quantum networks. It can be achieved with the help of long-distance quantum teleportation and entanglement distribution. The latter is also important for fundamental tests of the laws of quantum mechanics. Although quantum teleportation and entanglement distribution over moderate distances have been realized using optical fibre links, the huge photon loss and decoherence in fibres necessitate the use of quantum repeaters for larger distances. However, the practical realization of quantum repeaters remains experimentally challenging. Free-space channels, first used for quantum key distribution, offer a more promising approach because photon loss and decoherence are almost negligible in the atmosphere. Furthermore, by using satellites, ultra-long-distance quantum communication and tests of quantum foundations could be achieved on a global scale. Previous experiments have achieved free-space distribution of entangled photon pairs over distances of 600?metres (ref. 14) and 13?kilometres (ref. 15), and transfer of triggered single photons over a 144-kilometre one-link free-space channel. Most recently, following a modified scheme, free-space quantum teleportation over 16?kilometres was demonstrated with a single pair of entangled photons. Here we report quantum teleportation of independent qubits over a 97-kilometre one-link free-space channel with multi-photon entanglement. An average fidelity of 80.4?±?0.9 per cent is achieved for six distinct states. Furthermore, we demonstrate entanglement distribution over a two-link channel, in which the entangled photons are separated by 101.8?kilometres. Violation of the Clauser-Horne-Shimony-Holt inequality is observed without the locality loophole. Besides being of fundamental interest, our results represent an important step towards a global quantum network. Moreover, the high-frequency and high-accuracy acquiring, pointing and tracking technique developed in our experiment can be directly used for future satellite-based quantum communication and large-scale tests of quantum foundations.     

自由空间量子隐形传态实验中远程光符合的电子学系统研制

在两光子自由空间量子隐形传态实验方案中,设计了一套远程光符合的电子学系统,系统最小符合时间窗口约2ns.整个系统包括编码、解码、延时、高压驱动、符合、计数等模块.此系统实现了将实验中的经典信息和量子信息耦合在一起并通过自由空间传送到另一端进行在线符合来判定纠缠光子对.系统中编码模块将Alice端Bell基测量结果的时间标记信息和通道信息叠加在一起,通道信息的编码使用了奇偶校验和汉明码来降低事例丢失率和出错率,解码模块则分离出经典的时间标记信息和通道信息并做相应操作.完成了800m距离的两光子自由空间量子隐形传态实验,实验结果表明用本系统进行远程的实验是可行的.     

EPR粒子对与量子隐形传态

量子隐形传态,由发送者Alice将准备传送的信息分离成一部分纯粹经典的信息和另外一部分纯粹非经典的信息,通过2条不同的信道传送给接收者Bob。首先传送非经典部分,这需要借助于EPR粒子对,考虑由2个自旋皆为12的费密子构成,其中一个分配给Alice,另一个分配给Bob。Alice选择对她一方的原始粒子和她的EPR粒子一并进行冯.罗曼类型的测量,这个在贝尔算符的4个本征态中的测量,导致系统的波函数的波包坍缩为相互关联的4个贝尔基矢。Bob通过对他的EPR粒子的状态进行适当的幺正变换,能够重新构造出在Alice一方被＂毁灭＂了的原始粒子的状态。此外,这个贝尔测量产生2个比特的经典信息,传送给Bob,从而完成一个量子隐形传态。文章中研究了EPR粒子对与量子隐形传态的内在联系。     

三粒子W纠缠态的概率量子隐形传态

提出一个对未知三粒子W纠缠态的量子隐形传态方案.该方案用一个非最大GHZ纠缠态和一个非最大EPR纠缠态作为量子信道实现对未知经典W纠缠态的概率量子隐形传态和对未知一般W纠缠态受控的概率量子隐形传态.     

利用四粒子纠缠态实现的两量子位一般态的概率隐形传态

提出了以四粒子部分纠缠态作为量子信道的两量子位一般态的概率隐形传输方案,它通过联合么正变换和引入辅助量子位的方法来实现.     

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.     

Quantum teleportation between light and matter

Quantum teleportation is an important ingredient in distributed quantum networks, and can also serve as an elementary operation in quantum computers. Teleportation was first demonstrated as a transfer of a quantum state of light onto another light beam; later developments used optical relays and demonstrated entanglement swapping for continuous variables. The teleportation of a quantum state between two single material particles (trapped ions) has now also been achieved. Here we demonstrate teleportation between objects of a different nature--light and matter, which respectively represent 'flying' and 'stationary' media. A quantum state encoded in a light pulse is teleported onto a macroscopic object (an atomic ensemble containing 10 caesium atoms). Deterministic teleportation is achieved for sets of coherent states with mean photon number (n) up to a few hundred. The fidelities are 0.58 +/- 0.02 for n = 20 and 0.60 +/- 0.02 for n = 5--higher than any classical state transfer can possibly achieve. Besides being of fundamental interest, teleportation using a macroscopic atomic ensemble is relevant for the practical implementation of a quantum repeater. An important factor for the implementation of quantum networks is the teleportation distance between transmitter and receiver; this is 0.5 metres in the present experiment. As our experiment uses propagating light to achieve the entanglement of light and atoms required for teleportation, the present approach should be scalable to longer distances.     

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。     

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.     

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.     

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．     

Plasmon-assisted transmission of entangled photons

The state of a two-particle system is said to be entangled when its quantum-mechanical wavefunction cannot be factorized into two single-particle wavefunctions. This leads to one of the strongest counter-intuitive features of quantum mechanics, namely non-locality. Experimental realization of quantum entanglement is relatively easy for photons; a starting photon can spontaneously split into a pair of entangled photons inside a nonlinear crystal. Here we investigate the effects of nanostructured metal optical elements on the properties of entangled photons. To this end, we place optically thick metal films perforated with a periodic array of subwavelength holes in the paths of the two entangled photons. Such arrays convert photons into surface-plasmon waves--optically excited compressive charge density waves--which tunnel through the holes before reradiating as photons at the far side. We address the question of whether the entanglement survives such a conversion process. Our coincidence counting measurements show that it does, so demonstrating that the surface plasmons have a true quantum nature. Focusing one of the photon beams on its array reduces the quality of the entanglement. The propagation of the surface plasmons makes the array effectively act as a 'which way' detector.     

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.     

N粒子纠缠GHZ态的控制性概率传送

提出了一种把N粒子纠缠GHZ态从发送者Alice传送给远方的接收者Bob的控制性隐形传送方案.在传送过程中,N对非最大纠缠EPR态和m粒子GHZ态被选择作为量子通道.发送者Alice先对它所拥有的粒子做贝尔态测量,然后每一位控制者对它们各自所拥有的GHZ态粒子先进行Hadamard变换再做投影测量,之后将它们的最终测量结果通过经典通道告知给接收者Bob,根据接收到的经典信息,Bob通过引入一个辅助粒子并且对它所拥有的粒子做唯一的通用幺正变换,就可以重现原始态.结果显示,传送成功的总概率为2N+1-m∏N k=1|ak|2,任何一个控制者的信息缺失都将导致传送的失败.     

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.     

基于隐形传态的量子通信流量与用户监测

利用隐形传态需要传递测量信息的特点,由经典帧监控通信对象,并监测其流量.分析了基于隐形传态的量子网络通信过程,设计了一种监控通信用户与监测流量的方案.对通信过程中资源消耗进行研究,并提出粒子对消耗监控方案.     

原子腔场系统中弱相干态的几率传送

提出了一个用原子与腔场纠缠态作为量子信道,非贝尔态测量和腔量子电动力学技术实现弱相干态的隐形传送的方案,分析了平均光子数对传送相干态的影响,发现平均光子数在0.25附近时效果最佳.该方案的最大优点是在某种程度上可以避免退相干现象.     

通过一个EPR对和一个GHZ态实现任意两粒子态的概率传递

我们提出了一个非最大纠缠EPR对和一个GHZ态作量子通道实现一个任意两粒子量子态隐形传递的方案。利用Bell态测量和Hadamard门测量,如果接收者引入一个联合幺正变换,就能实现一个任意两粒子态的概率传递,这个联合幺正变换是唯一的。同时,我们还给出了实现该传态过程的量子电路。     

Creation of a six-atom 'Schrödinger cat' state

Among the classes of highly entangled states of multiple quantum systems, the so-called 'Schr?dinger cat' states are particularly useful. Cat states are equal superpositions of two maximally different quantum states. They are a fundamental resource in fault-tolerant quantum computing and quantum communication, where they can enable protocols such as open-destination teleportation and secret sharing. They play a role in fundamental tests of quantum mechanics and enable improved signal-to-noise ratios in interferometry. Cat states are very sensitive to decoherence, and as a result their preparation is challenging and can serve as a demonstration of good quantum control. Here we report the creation of cat states of up to six atomic qubits. Each qubit's state space is defined by two hyperfine ground states of a beryllium ion; the cat state corresponds to an entangled equal superposition of all the atoms in one hyperfine state and all atoms in the other hyperfine state. In our experiments, the cat states are prepared in a three-step process, irrespective of the number of entangled atoms. Together with entangled states of a different class created in Innsbruck, this work represents the current state-of-the-art for large entangled states in any qubit system.