Robust Generation of Three-Particle W State with Atoms Trapped in Separate Cavities

This paper presents a scheme for generating three-particle W state of remote atoms trapped in leaky cavities.The scheme uses cavity decay to inject photons into a setup of optical devices which consist of a series of beam splitters and photon detectors.Photon detection on the output mode projects the atomic state into the W state.In the condition of “weakly driven approach”,it shows that the scheme is robust and has high fidelity.It also points out that the scheme is scalable to generate multi-atomic W state.     

An efficient scheme for preparation of multipartite entanglement of atomic ensembles

We propose an efficient scheme to prepare multipartite entanglement of atomic ensembles trapped in separate cavities. Our scheme has high fidelity even with realistic noise based on the repeat-until-success strategy. By employing the quantum memory of the atomic internal state, the scaling efficiency decreases only with the number of atomic ensembles by a slow polynomial law. Moreover, the atomic ensembles also can function as quantum repeaters, which enable our system to compatible with the current experimental technique for quantum communication using atomic ensembles.     

Self-error-rejecting photonic qubit transmission in polarization-spatial modes with linear optical elements

We present an original self-error-rejecting photonic qubit transmission scheme for both the polarization and spatial states of photon systems transmitted over collective noise channels. In our scheme, we use simple linear-optical elements, including half-wave plates, 50:50 beam splitters, and polarization beam splitters, to convert spatial-polarization modes into different time bins. By using postselection in different time bins, the success probability of obtaining the uncorrupted states approaches 1/4 for single-photon transmission, which is not influenced by the coefficients of noisy channels. Our self-error-rejecting transmission scheme can be generalized to hyperentangled n-photon systems and is useful in practical high-capacity quantum communications with photon systems in two degrees of freedom.     

Entangling many atomic ensembles through laser manipulation

We propose an experimentally feasible scheme to generate the Greenberger-Horne-Zeilinger-type of maximal entanglement between many atomic ensembles based on laser manipulation and single-photon detection. The scheme, with inherent fault tolerance to the dominant noise and efficient scaling of the efficiency with the number of ensembles, allows one to maximally entangle many atomic ensembles within the reach of current technology. Such a maximum entanglement of many ensembles has wide applications in the demonstration of quantum nonlocality, high-precision spectroscopy, and quantum information processing.     

Unconditional two-mode squeezing of separated atomic ensembles

We propose schemes for the unconditional preparation of a two-mode squeezed state of effective bosonic modes realized in a pair of atomic ensembles interacting collectively with optical cavity and laser fields. The scheme uses Raman transitions between stable atomic ground states and under ideal conditions produces pure entangled states in the steady state. The scheme works both for ensembles confined within a single cavity and for ensembles confined in separate, cascaded cavities.     

Quantum Theory of Atom-Wave Beam Splitters and Application to Multidimensional Atomic Gravito-Inertial Sensors

We review the theory of atom-wave beam splitters using atomic transitions induced by electromagnetic interactions. Both the spatial and temporal dependences of the e.m.³ fields are introduced in order to compare the differences in momentum transfer which occur for pulses either in the time or in the space domains. The phases imprinted on the matter-wave by the splitters are calculated in the limit of weak e.m. and gravitational fields and simple rules are derived for practical atom interferometers. The framework is applicable to the Lamb-Dicke regime. Finally, a generalization of present 1D beam splitters to 2D or 3D is considered and leads to a new concept of multidimensional atom interferometers to probe inertial and gravitational fields especially well-suited for space experiments.     

Quantum Nondemolition Measurement of Entangled Atomic Ensembles in Coupled Cavity System

Quantum nondemolition measurement scheme for entangled atomic ensembles was presented when they are restricted in two distant coupled cavities. Under certain conditions, we showed that the entanglement evolution of both the atomic ensembles and cavity fields are of the same periods, which provides us with potential nondemolitionmeasurement of entangled atomic ensembles by experimentally detecting cavity fields. Including the dissipation of the cavity fields, the same steps of evolution still hold. Furthermore, it is shown that we can have the two-mode squeezing steady-states of both the cavity fields and the atomic ensembles, which really provides us with the quantum nondemolition measurement of the entangled atomic state.     

Scheme for preparation of multi-partite entanglement of atomic ensembles

We show a scheme of preparing multipartite W type of maximally entangled states among many atomic ensembles with the generation time increasing with the party number only polynomially. The scheme is based on laser manipulation of atomic ensembles and single-photon detection, and fits well the status of the current experimental technology. We also show one of the applications of this kind of W state, demonstrating Bell theorem without inequalities.     

Adiabatic Passage of Collective Excitations in Atomic Ensembles

We describe a theoretical scheme that allows for transfer of quantum states of atomic collective excitation between two macroscopic atomic ensembles localized in two spatially-separated domains. The conception is based on the occurrence of double-exciton dark states due to the collective destructive quantum interference of the emissions from the two atomic ensembles. With an adiabatically coherence manipulation for the atom-field couplings by stimulated Rmann scattering, the dark states will extrapolate from an exciton state of an ensemble to that of another. This realizes the transport of quantum information among atomic ensembles.     

Adiabatic Passage of Collective Excitations in Atomic Ensembles

We describe a theoretical scheme that allows for transfer of quantum states of atomic collective excitation between two macroscopic atomic ensembles localized in two spatially-separated domains. The conception is based on the occurrence of double-exciton dark states due to the collective destructive quantum interference of the emissions from the two atomic ensembles. With an adiabatically coherence manipulation for the atom-field couplings by stimulated Ramann scattering, the dark states will extrapolate from an exciton state of an ensemble to that of another. This realizes the transport of quantum information among atomic ensembles.     

一种基于双偏振分束器的量子秘密共享方案

提出一个基于双偏振分束器的单量子比特全光纤量子秘密共享方案, 该方案具有自动补偿光纤及光学器件的双折射效应和相位抖动的功能, 在干涉对比度测试和稳定性测试时, 该方案在5 km通信距离中, 获得的干涉对比度优于993%, 且可长时间保持稳定. 关键词： 量子秘密共享 偏振分束器 单光子干涉     

Preparation of Arbitrary Four-Qubit W State with Atomic Ensembles via Rydberg Blockade

The generation of various entangled states is an essential task in quantum information processing. Recently, a scheme （PRA 79, 022304） has been suggested for generating Greenberger-Horne-Zeilinger state and cluster state with atomic ensembles based on the Rydberg blockade. Using similar resources as the earlier scheme, here we propose an experimentally feasible scheme of preparing arbitrary four-qubit W class of maximally and non- maximally entangled states with atomic ensembles in a single step. Moreover, we carefully analyze the realistic noises and predict that four-qubit W states can be produced with high fidelity （F - 0.994） via our scheme.     

A quantum computer in the scheme of an atomic quantum transistor with logical encoding of qubits

A scheme of a multiqubit quantum computer on atomic ensembles using a quantum transistor implementing two qubit gates is proposed. We demonstrate how multiatomic ensembles permit one to work with a large number of qubits that are represented in a logical encoding in which each qubit is recorded on a superposition of single-particle states of two atomic ensembles. The access to qubits is implemented by appropriate phasing of quantum states of each of atomic ensembles. An atomic quantum transistor is proposed for use when executing two qubit operations. The quantum transistor effect appears when an excitation quantum is exchanged between two multiatomic ensembles located in two closely positioned QED cavities connected with each other by a gate atom. The dynamics of quantum transfer between atomic ensembles can be different depending on one of two states of the gate atom. Using the possibilities of control for of state of the gate atom, we show the possibility of quantum control for the state of atomic ensembles and, based on this, implementation of basic single and two qubit gates. Possible implementation schemes for a quantum computer on an atomic quantum transistor and their advantages in practical implementation are discussed.     

Influence of optical aberrations in an atomic gyroscope

In atom interferometry based on light-induced diffraction, the optical aberrations of the laser beam splitters are a dominant source of noise and systematic effect. In an atomic gyroscope, this effect is dramatically reduced by the use of two atomic sources. But it remains critical while coupled to fluctuations of atomic trajectories, and appears as a main source of noise to the long term stability. Therefore we measure these contributions in our set-up, using cold cesium atoms and stimulated Raman transitions.     

Teleportation of a two-particle four-component squeezed vacuum state by linear optical elements

We present a linear optical scheme for achieving a unity fidelity teleportation of a two-particle four- component squeezed vacuum state using two entangled squeezed vacuum states as quantum channel. The devices used are beam splitters and ideal photon detectors capable of distinguishing between odd and even photon numbers. Moreover, we also obtain the success probability of the teleportation scheme.     

Scheme for the preparation of macroscopicW-type state of atomic ensembles in cavity QED coulped with optical fibers

We propose a potentially practical scheme to generate macroscopic W-type state of N atomic ensembles in cavity QED system consisting of N atomic ensembles trapped in N single-mode cavities connected by(N 1)optical fibers.We show that the N-qubit W-type state of atomic ensembles can be realized with high success probabilities if the coulping strength of the cavity-fiber is much stronger than that of cavity-atom.We also show that both the growth of atomic number in each ensemble and the increase of the number of atomic ensembles can diminish the detrimental influence from dissipative processes.This idea provides a scalable way to an atomic-ensemble-based quantum network,which is plausible with current available technology.     

Scalable Implementation of Multi-qubit Quantum Grover Search with Atomic Ensembles by Adiabatic Passage

We propose a simple scheme to implement multi-qubit quantum Grover search with atomic ensembles by adiabatic passage. The scheme is immune to the atomic spontaneous emission, cavity decay and fiber decay. Furthermore, the process can be speeded up with atomic ensemble instead of single atom, which is important in view of decoherence. With each atomic ensemble confined in an individual cavity, our scheme is experimentally scalable to multi-qubit cases.     

六光子超纠缠态制备方案

自发参量下转换对应于一种非线性光学过程, 实验上作为一种标准方法, 人们利用自发参量下转换源产生纠缠光子对. 本文考虑由自发参量下转换源产生三对纠缠光子的情况. 通过使用由几组偏振光 束分束器、分束器和半波片等线性光学器件组成的量子线路演化三对光子, 给出了一个高效制备 包含偏振纠缠和空间纠缠的六光子超纠缠态方案. 因为方案中包含了参量下转换源产生三对纠缠光子 的所有可能情况, 所以本方案有很高的效率. 基于弱非线性介质构建了一个量子非破坏性测量装置, 用于区分光子在两指定的空间模中的两种分布情况. 特别地, 方案中可以通过合理约束在量子非破坏性测量过程中引入的非线性强度来达到实际实验所限定的数量级, 因此, 该方案易于在实验上实现.     

How to measure squeezing and entanglement of Gaussian states without homodyning

We propose a scheme for measuring the squeezing, purity, and entanglement of Gaussian states of light that does not require homodyne detection. The suggested setup needs only beam splitters and single-photon detectors. Two-mode entanglement can be detected from coincidences between photodetectors placed on the two beams.     

Continuous-Variable Quantum Teleportation of Entangled Coherent States

We propose a quantum teleportation scheme for tripartite entangled coherent state （ECS） with continuous variable. Our scheme is feasible and economical in the sense that we need only linear optical devices such as beam splitters, phase shifters and photon detectors and employ three bipartite maximally ECSs as quantum channels. We also generalize the tripartite scheme into multipartite ease and calculate the minimum average fidelity for the schemes in tripartite and multipartite cases.