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.     

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.     

A new approach for the justification of ensembles in quantum statistical mechanics — I

In this and the following paper, a new approach for the justification of ensembles in statistical mechanics is given. The essential physical idea is that a measurement is an average of values arising from disjoint regions in three-space. This idea is given a mathematical basis in terms of a class of operators called local operators, and the first paper is devoted primarily to the development of the properties of local operators. In particular, a complete characterization of the bounded local operators on ₂ spaces of finite measure is given. Two results of importance for statistical mechanics are also derived. First, it is shown that the observables of quantum mechanics are local operators. Second, it is shown that the expectation value of an observable for a pure state can be written formally as an ensemble average. In the following paper, these results are used to develop a new approach for the justification of statistical ensembles.This work was supported in part by research grants from the National Science Foundation and the U.S. Public Health Service. The material of this paper is contained in a doctoral dissertation submitted by the author to the University of Oregon (1969).     

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.     

Probabilistic and robust preparation of a GHZ-type state via atomic ensembles and linear optics

This paper proposes two simple and robust schemes to generate an atomic-ensemble Greenberger--Horne--Zeilinger-type (GHZ-type) entangled state via linear optics and single photon detection. These schemes are based on two-photon Hong--Ou--Mandel-type interference, therefore they are insensitive to the phase fluctuation. This advantage will make the realizations of these two schemes easier. One scheme can scale efficiently with the number of ensembles because of the used quantum memory. Both schemes are also robust to the noise and within the reach of current technology.     

多个量子节点确定性纠缠的建立

量子纠缠是一种重要的量子资源,在多个空间分离的量子存储器间建立确定性的量子纠缠,然后在用户控制的时刻将所存储的量子纠缠转移到量子信道中进行信息的分发和传送,这对于实现量子信息网络是至关重要的.本文介绍了用光学参量放大器制备与铷原子D1吸收线对应的非经典光场,而且在三个空间分离的原子系综中确定性量子纠缠的产生、存储和转移.利用电磁感应透明光和原子相互作用的原理,将制备的多组分光场纠缠态模式映射到三个远距离的原子系综以建立原子自旋波之间的纠缠.然后,存储在原子系综中的纠缠态通过三个量子通道,纠缠态的量子噪声被转移到三束空间分离的正交纠缠光场.三束释放的光场间纠缠的存在验证了该系统具有保持多组分纠缠的能力.这个方案实现了三个量子节点间的纠缠,并且可以直接扩展到具有更多节点的量子网络,为未来实现大型量子网络通信奠定了基础.     

Hamiltonian Formulation of Statistical Ensembles and Mixed States of Quantum and Hybrid Systems

Representation of quantum states by statistical ensembles on the quantum phase space in the Hamiltonian form of quantum mechanics is analyzed. Various mathematical properties and some physical interpretations of the equivalence classes of ensembles representing a mixed quantum state in the Hamiltonian formulation are examined. In particular, non-uniqueness of the quantum phase space probability density associated with the quantum mixed state, Liouville dynamics of the probability densities and the possibility to represent the reduced states of bipartite systems by marginal distributions are discussed in detail. These considerations are used to study ensembles of hybrid quantum-classical systems. In particular, nonlinear evolution of a single hybrid system in a pure state and unequal evolutions of initially equivalent ensembles are discussed in the context of coupled hybrid systems.     

光腔耦合阵列中四模方形Cluster态的制备

提出一种四模方形cluster态的制备方案,方案选用四个分别包含一个原子系综的独立单模光腔,光腔之间用短光纤实现耦合.讨论证明在合适外加激光脉冲的驱动下,可确定性的制备得到稳定的四模方形cluster态.通过调节驱动激光的频率和相位,该方案可以拓展到多模和其他形cluster态的制备.     

Long-distance quantum communication with “polarization” maximally entangled states

We propose a scheme for long-distance quantum communication where the elementary entanglement is generated through two-photon interference and quantum swapping is performed through one-photon interference. Local “polarization” maximally entangled states of atomic ensembles are generated by absorbing a single photon from on-demand single-photon sources. This scheme is robust against phase fluctuations in the quantum channels, moreover speeds up long-distance high-fidelity entanglement generation rate.     

Average mutual information of the random block-message ensembles for many-letters

By making use of the theoretical framework presented by Bostroem(K.J.Bostroem,LANL quant-ph/0009052),we generalize the standard quantum information theory of block messages with fixed block length to the variable one.We show that the states belonging to a sufficiently large hilbert space are the highly distinguishable states.We also consider the collection states (product states of more than one qubit state) and seek a “pretty good measurement” (PGM) with measurement vectors to improve the mutual information.The average mutual information over random block-message ensembles with variable block length n is discussed in detail.     

光腔耦合阵列中四模方形Cluster态的制备

提出一种四模方形cluster态的制备方案,方案选用四个分别包含一个原子系综的独立单模光腔,光腔之间用短光纤实现耦合.讨论证明在合适外加激光脉冲的驱动下,可确定性的制备得到稳定的四模方形cluster态.通过调节驱动激光的频率和相位,该方案可以拓展到多模和其他形cluster态的制备.     

Quantum SWAP gate with atomic ensembles in two distant cavities

A scheme is presented to realize remote quantum SWAP gate with atomic ensembles. During the process, the fiber mode is only virtually excited. The quantum information is encoded in two degenerate ground states, so the atoms' spontaneous emission can be omitted approximately. Moreover, the time needed to complete the gate is proportional to the inverse of the number of atoms and thus the gate is greatly speeded up as the number of atoms increases.     

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.     

冷原子系综内单集体激发态的相干操纵

原子系综内部分原子发生相干态转移后所处量子态被称为集体激发态.如果激发数目在单原子量级则被称为单激发态.在量子存储过程中,单光子以单激发态的形式在原子系综内进行存储.因此,研究单激发态的制备、演化、转化、干涉等过程是量子存储及其应用研究的关键.本文总结了近年来作者所在研究团队针对冷原子系综体系在此研究方向取得的若干成果.主要包括采用动量模式调控、三维光晶格等手段抑制单激发态的退相干,采用环形腔增强原子至光子的转化效率,发展基于拉曼光的单激发态相干转移技术,利用单量子态不同模式间干涉制备光与原子纠缠,利用里德伯阻塞机制提升纠缠制备效率等.此外,简要回顾了基于多个单激发态的量子中继及量子网络实验.     

超冷原子系综的非高斯纠缠态与精密测量

量子精密测量是基于量子力学的基本原理对特定物理量实施测量,并利用量子效应提高测量精度的交叉科学.随着超冷原子实验技术的发展,超冷原子为量子精密测量提供了一个优异的研究平台.利用发展成熟的量子调控技术,人们可以基于超冷原子系综制备一些新奇的非高斯多粒子纠缠态.基于多体量子干涉,利用这些非高斯纠缠态作为输入,可以实现超越标准量子极限的高精度测量.本文简要综述这一研究领域的进展.     

Pre- and Post-Selected Ensembles and Time-Symmetry in Quantum Mechanics

It has recently been argued (Shimony, Erkenntnis 45:337, 1997) that time-symmetry does not hold for pre- and post-selected ensembles in quantum mechanics. That conclusion depends on what is meant by “time-symmetry” in relation to those types of ensembles. It is shown that on the conventional view of time-symmetry, pre- and post-selected ensembles are time-symmetric as was originally proposed.     

Density Matrix in Quantum Mechanics and Distinctness of Ensembles Having the Same Compressed Density Matrix

We clarify different definitions of the density matrix by proposing the use of different names, the full density matrix for a single-closed quantum system, the compressed density matrix for the averaged single molecule state from an ensemble of molecules, and the reduced density matrix for a part of an entangled quantum system, respectively. We show that ensembles with the same compressed density matrix can be physically distinguished by observing fluctuations of various observables. This is in contrast to a general belief that ensembles with the same compressed density matrix are identical. Explicit expression for the fluctuation of an observable in a specified ensemble is given. We have discussed the nature of nuclear magnetic resonance quantum computing. We show that the conclusion that there is no quantum entanglement in the current nuclear magnetic resonance quantum computing experiment is based on the unjustified belief that ensembles having the same compressed density matrix are identical physically. Related issues in quantum communication are also discussed.     

Teleportation of an atomic ensemble quantum state

We propose a protocol to achieve high fidelity quantum state teleportation of a macroscopic atomic ensemble using a pair of quantum-correlated atomic ensembles. We show how to prepare this pair of ensembles using quasiperfect quantum state transfer processes between light and atoms. Our protocol relies on optical joint measurements of the atomic ensemble states and magnetic feedback reconstruction.     

Controversial problems of measurements within quantum mechanics

It is explained why quantum mechanics applies principally to single systems and not to ensembles. A thorough analysis of thought-experiments shows clearly that irreversibility is connected with the storing of information rather than with the act of measurement. In order to avoid paradoxes one has to admit that the wave function does not represent the state of the system in itself, but information acquired in consequence of a complete measurement. The meaning of the time-energy uncertainty relation for stable systems is discussed.     

Extended Clausius Relation and Entropy for Nonequilibrium Steady States in Heat Conducting Quantum Systems

Recently, in their attempt to construct steady state thermodynamics (SST), Komatsu, Nakagawa, Sasa, and Tasaki found an extension of the Clausius relation to nonequilibrium steady states in classical stochastic processes. Here we derive a quantum mechanical version of the extended Clausius relation. We consider a small system of interest attached to large systems which play the role of heat baths. By only using the genuine quantum dynamics, we realize a heat conducting nonequilibrium steady state in the small system. We study the response of the steady state when the parameters of the system are changed abruptly, and show that the extended Clausius relation, in which “heat” is replaced by the “excess heat”, is valid when the temperature difference is small. Moreover we show that the entropy that appears in the relation is similar to von Neumann entropy but has an extra symmetrization with respect to time-reversal. We believe that the present work opens a new possibility in the study of nonequilibrium phenomena in quantum systems, and also confirms the robustness of the approach by Komatsu et al.