Quantum pattern retrieval by qubit networks with Hebb interactions

Qubit networks with long-range interactions inspired by the Hebb rule can be used as quantum associative memories. Starting from a uniform superposition, the unitary evolution generated by these interactions drives the network through a quantum phase transition at a critical computation time, after which ferromagnetic order guarantees that a measurement retrieves the stored pattern. The maximum memory capacity of these qubit networks is reached at a memory density alpha=p/n=1.     

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.     

Quantum interference of electromagnetic fields from remote quantum memories

We observe quantum, Hong-Ou-Mandel, interference of fields produced by two remote atomic memories. High-visibility interference is obtained by utilizing the finite atomic memory time in four-photon delayed coincidence measurements. Interference of fields from remote atomic memories is a crucial element in protocols for scalable entanglement distribution.     

多模式固态量子存储

量子存储器是光子与物质系统之间的接口,允许存入和读出加载了量子信息的光子,是构建实用化量子网络的核心器件.基于稀土掺杂晶体可以实现固态的量子存储器,较长的相干时间和较宽的存储带宽使其成为目前最有潜力的量子物理系统之一.本文综述近年来基于稀土掺杂晶体的多模式固态量子存储方面的实验进展.主要内容包括频率自由度的多模式量子存储、时间自由度的多模式量子存储、空间自由度的多模式量子存储和多个自由度并行复用的多模式量子存储.在多自由度复用的多模式存储的基础上进一步介绍基于量子存储器的量子模式变换和实时的任意操作.该系列工作为构建高速率的实用化量子网络奠定基础,其中超越存储器本身的脉冲操作功能还有望在未来量子信息处理过程中获得广泛的应用.     

基于拉曼协议的量子存储

量子存储器是实现按照需要存储/读出诸如单光子、纠缠或者压缩态等非经典量子态的系统,是实现量子通信和量子计算必不可少的核心器件.量子存储协议多种多样,其中拉曼方案由于具有存储宽带大、可用于存储短脉冲信号的优点而引起了人们的广泛关注.然而实现真正单光子和光子纠缠的拉曼存储具有挑战性.本文简要介绍了量子存储器的主要性能和评价指标,在回顾了量子存储器特别是拉曼量子存储器的发展现状后,重点介绍了本研究组最近基于拉曼协议实现各种量子态存储的系列研究,取得的研究成果对于构建高速量子网络具有重要参考价值.     

Single-photon-level quantum memory at room temperature

Room-temperature, easy-to-operate quantum memories are essential building blocks for future long distance quantum information networks operating on an intercontinental scale, because devices like quantum repeaters, based on quantum memories, will have to be deployed in potentially remote, inaccessible locations. Here we demonstrate controllable, broadband and efficient storage and retrieval of weak coherent light pulses at the single-photon level in warm atomic cesium vapor using the robust far off-resonant Raman memory scheme. We show that the unconditional noise floor of this technically simple quantum memory is low enough to operate in the quantum regime, even in a room-temperature environment.     

量子图像存储

量子通讯的实现离不开量子存储器。量子存储器是一个能够按照需要存储和读出量子态的系统,而被存储的是非经典的量子态,如单光子、纠缠、压缩态等。多模存储能力是衡量量子存储器工作性能的一个重要指标。空间域的多模存储可以极大地减小对存储器存储时间的要求,因此空间多模存储引起了国内外学者的广泛关注,并在近几年取得了许多重要进展。文章简述了量子存储器的主要性质与评价指标,回顾了近年来量子存储的研究进程,特别介绍了量子图像存储方面的最新实验进展。     

Theoretical assessments of major physical processes involved in the depth resolution in sputter profiling

Two of the more important physical processes which militate against high depth resolution capabilities for ion-induced sputter sectioning associated with compositional analysis techniques are surface topography development and recoil atomic mixing. This review describes earlier, simplistic, theoretical modelling of such processes and describes new approaches based upon empirical evidence of the nature and magnitude of these processes. It is shown that, in general, the depth resolution of the sputtering technique will be a complex function of the depth probed and that both “broadenings” and “shifts” in depth evaluation are to be expected.     

基于电磁诱导透明机制的压缩光场量子存储

光场的量子存储不仅是构建量子计算机的重要基础,而且是实现量子中继和远距离量子通信的核心部分.由于存在不可避免的光学损耗,光学参量放大器产生的压缩真空态光场将变为压缩热态光场,不再是最小不确定态.因此,压缩热态光场的量子存储是实现量子互联网的关键.在原子系综中利用电磁诱导透明机制能够实现量子态在光场正交分量和原子自旋波之间的相互映射,即受控量子存储.本文根据量子存储的保真度边界,研究了实现压缩热态光场量子存储的条件.量子存储的保真度边界是通过经典手段能够达到的最大保真度,当保真度大于该边界时,就实现了量子存储.通过数值计算分析了不同情况下压缩热态光场的量子存储保真度边界,以及存储保真度随存储效率的变化关系,得到了实现量子存储的条件,为连续变量量子存储实验设计提供了直接参考.     

Rare-earth-doped materials for applications in quantum information storage and signal processing

Realization of practical quantum memory and optical signal processing systems critically depends on suitable materials that offer specific combinations of properties. Solid-state materials such as rare-earth ions doped into dielectric crystals are one of the most promising candidates for several quantum information storage protocols, including quantum storage of single photons. This article provides an overview of rare-earth-doped material properties and summarizes some of the most promising materials studied in our laboratory and by other groups for applications in quantum information storage and for ultra-wide bandwidth signal processing. Understanding and controlling spectral diffusion in these materials, which ultimately limits the achievable performance of any quantum memory system, is also briefly reviewed. Applications in quantum information impose stringent requirements on laser phase and frequency stability, and employing a narrow spectral hole in the inhomogeneous absorption profile in these materials as a frequency reference can dramatically improve laser stability. We review our work on laser frequency and phase stabilization and report our recent results on using a narrow spectral hole as a passive dynamic spectral filter for laser phase noise suppression, which can dramatically narrow the laser linewidth with or without the requirement of active feedback.     

Self field electromagnetism and quantum phenomena

Abstract

Quantum Electrodynamics (QED) has been extremely successful inits predictive capability for atomic phenomena. Thus the greatest hope for any alternative view is solely to mimic the predictive capability of quantum mechanics (QM), and perhaps its usefulness will lie in gaining a better understanding of microscopic phenomena. Many “paradoxes” and problematic situations emerge in QED. To combat the QED problems, the field of Stochastics Electrodynamics (SE) emerged, wherein a random “zero point radiation” is assumed to fill all of space in an attmept to explain quantum phenomena, without some of the paradoxical concerns. SE, however, has greater failings. One is that the electromagnetic field energy must be infinit eto work. We have examined a deterministic side branch of SE, “self field” electrodynamics, which may overcome the probelms of SE. Self field electrodynamics (SFE) utilizes the chaotic nature of electromagnetic emissions, as charges lose energy near atomic dimensions, to try to understand and mimic quantum phenomena. These fields and charges can “interact with themselves” in a non-linear fashion, and may thereby explain many quantum phenomena from a semi-classical viewpoint. Referred to as self fields, they have gone by other names in the literature: “evanesccent radiation”, “virtual photons”, and “vacuum fluctuations”. Using self fields, we discuss the uncertainty principles, the Casimir effects, and the black-body radiation spectrum, diffraction and interference effects, Schrodinger's equation, Planck's constant, and the nature of the electron and how they might be understood in the present framework. No new theory could ever replace QED. The self field view (if correct) would, at best, only serve to provide some understanding of the processes by which strange quantum phenomena occur at the atomic level. We discuss possible areas where experiments might be employed to test SFE, and areas where future work may lie.     

Entanglement of atomic ensembles by trapping correlated photon states

We describe a general technique that allows for an ideal transfer of quantum correlations between light fields and metastable states of matter. The technique is based on trapping quantum states of photons in coherently driven atomic media, in which the group velocity is adiabatically reduced to zero. We discuss possible applications such as quantum state memories, generation of squeezed atomic states, preparation of entangled atomic ensembles, quantum information processing, and quantum networking.     

Efficient solid state memories for quantum cryptography

Long distance quantum cryptography requires quantum repeaters which use quantum memories. The latter are designed to store and retrieve photon quantum states on demand. Although quantum memories have been demonstrated in atomic vapors and ultra cold gases, a solid state alternative may better fulfill quantum memories requirements. Rare earth based crystals, which exhibit long coherence lifetimes, are actively studied for this purpose. Memory efficiency, i.e. the probability to retrieve a photon after storage, should be close to unity for practical applications. This can be achieved in highly doped crystals. Although Pr-Pr interactions could be detrimental in this case, we show that in a 3% Pr³⁺ doped La₂(WO₄)₃ crystal ground state hyperfine coherence lifetime is still close to that measured at low Pr concentration. Since the latter determines the memory storage time, this result suggests that highly doped crystals may be useful for efficient quantum memories.     

Phase coherent transmission through interacting mesoscopic systems

This is a review of the phase coherent transmission through interacting mesoscopic conductors. As a paradigm we study the transmission amplitude and the dephasing rate for electron transport through a quantum dot in the Coulomb blockade regime. We summarize experimental and theoretical work devoted to the phase of the transmission amplitude. It is shown that the evolution of the transmission phase may be dominated by non-universal features in the short-time dynamics of the quantum dot. The controlled dephasing in Coulomb-coupled conductors is investigated. Examples comprise a single or multiple quantum dots in close vicinity to a quantum point contact. The current through the quantum point contact “measures” the state of the dots and causes dephasing. The dephasing rate is derived using widely different theoretical approaches. The Coulomb coupling between mesoscopic conductors may prove useful for future work on electron coherence and quantum computing.     

稀土掺杂固态量子存储研究进展

量子存储对量子信息网络的实现至关重要,是当前量子信息领域的研究前沿和热点.在实现量子存储的多种媒质中,稀土掺杂固体材料由于具有较长的光学相干时间和较宽的光学吸收带宽而备受研究人员的关注.本文将系统地介绍稀土掺杂固态量子存储在材料体系、存储协议、应用范围等方面的重要进展,着重从物理机理、实验方法和新近成果等方面,阐述基于原子频梳协议的稀土掺杂固态量子存储方案,并对该方案的未来发展做简要展望.     

Generation of a superposition of odd photon number states for quantum information networks

We report on the experimental observation of quantum-network-compatible light described by a nonpositive Wigner function. The state is generated by photon subtraction from a squeezed vacuum state produced by a continuous wave optical parametric amplifier. Ideally, the state is a coherent superposition of odd photon number states, closely resembling a superposition of weak coherent states |alpha > - |-alpha >. In the limit of low squeezing the state is basically a single photon state. Light is generated with about 10,000 and more events per second in a nearly perfect spatial mode with a Fourier-limited frequency bandwidth which matches well atomic quantum memory requirements. The generated state of light is an excellent input state for testing quantum memories, quantum repeaters, and linear optics quantum computers.     

Preparation of narrow-band photons for atomic-based quantum memory with a type-I phase matched periodical poled KTP crystal

Transferring a quantum state between a photon and a quantum memory is the key point for realizing a long-distance quantum communication, and is also a basic ingredient of linear optical quantum computation. In an atomic-based network, the efficient coupling between a photon and an atomic system is a prerequisite for realizing the transfer of information between them, which requires that the photon should have a comparable bandwidth with the natural bandwidth of an atom. Therefore, generating a narrow-band photon has become a very important topic in the quantum information field. One simple and efficient way is cavity-enhanced spontaneously parametric down-conversion. In this paper, we will review and introduce a series of experiments done in our group for realizing this goal. We believe these works are very useful for the research in this direction.     

Hybrid quantum processors: molecular ensembles as quantum memory for solid state circuits

We investigate a hybrid quantum circuit where ensembles of cold polar molecules serve as long-lived quantum memories and optical interfaces for solid state quantum processors. The quantum memory realized by collective spin states (ensemble qubit) is coupled to a high-Q stripline cavity via microwave Raman processes. We show that, for convenient trap-surface distances of a few microm, strong coupling between the cavity and ensemble qubit can be achieved. We discuss basic quantum information protocols, including a swap from the cavity photon bus to the molecular quantum memory, and a deterministic two qubit gate. Finally, we investigate coherence properties of molecular ensemble quantum bits.