冷原子系综中光存储效率与耦合光强的关系研究

我们在87Rb冷原子团中利用电磁感应透明(EIT)动力学过程实现了光学信号的存储与释放,研究了光存储效率与耦合光强的关系.实验结果表明,在较低的耦合光强下,存储效率随耦合光强增大而增大,当耦合光达到一定强度时,存储效率不再增高,然后随着耦合光强增大而降低.通过分析光脉冲在冷原子团中传播时的吸收和色散特性,我们对存储效率随耦合光强的变化趋势给出了定性解释.     

冷原子系综的光学厚度对腔增强量子存储读出效率的影响

本文在⁸⁷Rb冷原子系综中开展了腔增强量子存储,研究了光学厚度对读出效率的影响。我们通过改变冷却光的失谐量得到了原子系综不同的Optical Depth(OD),测量了读出效率随着原子系综光学厚度的变化。结果显示：当冷却光处于负失谐18.5 MHz的情况时,可使冷原子系综的OD达到最大值14,读出效率的增强倍数为1.6倍,本质恢复效率最大可达43.2%。     

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.     

冷原子系综中量子关联光子对的产生

本文在87 Rb冷原子系综中通过自发拉曼散射过程进行了量子关联光子对产生的实验研究,测量了关联光子对的产生率和二阶关联函数g(2)随写光激发率、读光功率以及写光失谐等实验参数的关系曲线。结果表明:随着写光激发率的增加,光子对产生率线性增加,g(2)则不断减小;在小的读光功率处,光子对产生率和g(2)随着读光功率增加而增大,当读光功率超过30 mW后,产生率和g(2)随着读光功率增加反而有所下降;写光频率失谐为-5 MHz时光子对产生率和g(2)均达到最低,增大或减小写光失谐产生率和g(2)均逐渐增加。通过选取合适的实验参数,光子对产生率达到18对/秒,同时g(2)为106。本文的研究结果为产生高质量的纠缠源提供了实验基础。     

冷原子系综中偏振关联光子对的产生和测量

我们在~(87)Rb冷原子系综中进行了关联光子对的产生和测量。通过自发拉曼散射过程,产生了一个Stokes光子和一个原子自旋波激发。利用电磁感应透明效应将存储在原子系综中的自旋波转化为一个anti-Stokes光子。在两相互正交的偏振基下,测量了Stokes光子和anti-Stokes光子之间的交叉关联函数。实验结果表明,交叉关联函数g~(2)达到~75,表现了强的非经典关联。基于现场可编程门阵列(FPGA)的自编程的多通道符合器被用于光子脉冲信号的采集和逻辑处理,大大提高了偏振关联光子对的产生和测量速度,为以后量子中继技术中的高效信息处理提供了重要基础。     

宏观人工原子相关的量子相干操纵

文章着重介绍作者及其合作者于2005年8月份发表在Physical Review Letters的2篇文章及其相关的系列研究工作．其中包括：（1）利用超导电荷量子比特的“Bang—Bang”控制，去冷却与之相耦合的纳米机械谐振器件，使之接近基态，以展示其丰富的量子效应；（2）具有△型能级结构超导人工原子的物理实现及其绝热量子操纵的奇异特性．为了深入了解这些工作的物理和潜在的应用背景，文章同时也介绍了超导量子比特物理实现及其相关的“全固态系统腔量子电动力学”研究的新进展，强调了基于超导人工原子的强耦合量子光学结构形成的物理机制．     

宏观人工原子相关的量子相干操纵

文章着重介绍作者及其合作者于2005年8月份发表在Physical Review Letters的2篇文章［ 1,2］及其相关的系列研究工作［ 3—5］.其中包括：（1）利用超导电荷量子比特的“Bang-Bang”控制,去冷却与之相耦合的纳米机械谐振器件,使之接近基态,以展示其丰富的量子效应;（2）具有Δ型能级结构超导人工原子的物理实现及其绝热量子操纵的奇异特性.为了深入了解这些工作的物理和潜在的应用背景,文章同时也介绍了超导量子比特物理实现及其相关的 “全固态系统腔量子电动力学”研究的新进展,强调了基于超导人工原子的强耦合量子光学结构形成的物理机制.     

单原子激光操控研究进展

文章评述了激光操控单个中性原子研究方面的进展,报道了作者及其合作者近年来在单原子激光囚禁与冷却、单原子量子比特的相干转移、单原子物质波干涉仪、两个异核原子的受控碰撞等方面所取得的最新研究结果,最后对未来发展进行了展望。     

基于原子系综集体激发的量子存贮

近几年来,人们从理论和实验两个方面探讨了利用自由原子系综作为量子记忆体的可能性. 基于固定于格点上Λ型“原子”系综的准自旋波激发的研究, 提出了实现光子信息量子存贮的新方案. 这个方案的目标是为了克服自由原子系综量子存贮方案中存在的量子态漏损诱导退相干的致命问题. 研究过程中发现了系综型量子记忆体中隐藏动力学的对称性和其绝热暗态演化的选择定则. 为了确切地表述这个工作的意义,还扼要地介绍这个方案建立的前期关于原子系综对称集体激发的系列工作. In this article we reviews a new protocol of quantum memory based on the quasi pin wave excitation of the Λ systems fixed the sites of lattice. This protocol tries to avoid the disadvantage of the quantum memory scheme based on free atom ensemble, the quantum decoherence induced by the quantum leakage of collective state. Especially,we discover a universal dynamic symmetry hidden in various ensemble based quantum storage scheme. To understand the significance of this work exactly, we also give a brief introduction to our systematical studies on collective symmetric excitons in quantum ensemble,which is the necessary to propose the present scheme for quantum memory.     

单原子脉塞和单原子激光——腔量子电动力学的实验平台

单原子脉塞和单原子激光是腔量子电动力学研究的重要实验平台。本文综述关于几类单原子脉塞和单原子激光的研究工作,包括德国Walther小组的单原子脉塞、超冷原子注入的单原子脉塞,法国Haroche小组的腔量子电动力学系统、注入原子的单原子激光、以及囚禁原子和囚禁离子的单原子激光。我们介绍相关的理论工作、实验系统、以及主要结果。     

Quantum leakage of collective excitations of atomic ensemble induced by spatial motion

We generalize the conception of quantum leakage for the atomic collective excitation states. By making use of the atomic coherence state approach, we study the influence of the atomic spatial motion on the symmetric collective states of 2-level atomic ensemble due to inhomogeneous coupling. In the macroscopic limit, we analyze the quantum decoherence of the collective atomic state by calculating the quantum leakage for a very large ensemble at a finite temperature. Our investigations show that the fidelity of the atomic system will not be good in the case of atom numberN→∞. Therefore, quantum leakage is an inevitable problem in using the atomic ensemble as a quantum information memory. The detailed calculations shed theoretical light on quantum processing using atomic ensemble collective qubit.     

Scheme for implementing frequency up-conversion between two collective atomic modes

This paper presents a scheme for realizing the frequency up-conversion between two collective atomic modes. In the scheme two atomic samples are coupled to a cavity mode. Under the large detuning condition, the two collective atomic modes are coupled via the virtual excitation of the cavity mode and the effective Hamiltonian corresponds to the frequency up-conversion. In the scheme the cavity mode is only virtually excited and thus the process is insensitive to cavity decay.     

The remote implementation of all possible generalized quantum measurement on single atomic qubit in a quantum network

To implement generalized quantum measurement (GQM) one has to extend the original Hilbert space. Generally speaking, the additional dimensions of the ancilla space increase as the number of the operators of the GQM n increases. This paper presents a scheme for deterministically implementing all possible n-operator GQMs on a single atomic qubit by using only one 2-dimensional ancillary atomic qubit repeatedly, which remarkably reduces the complexity of the realistic physical system. Here the qubit is encoded in the internal states of an atom trapped in an optical cavity and single-photon pulses are employed to provide the interaction between qubits. It shows that the scheme can be performed remotely, and thus it is suitable for implementing GQM in a quantum network. What is more, the number of the total ancilla dimensions in our scheme achieves the theoretic low bound.     

Non-classical correlation between a single photon and the collective spin excited state of a cold atomic ensemble

We experimentally establish a non-classical correlation between a single Stokes photon and the collective spin excited state of a cold atomic ensemble by using a spontaneous Raman scattering process. The correlation between them can be proved by transferring the spin excited state of the atomic ensemble into an anti-Stokes photon and checking the Cauchy-Schwarz inequality between the Stokes and the anti-Stokes photons. The non-classical correlation can be kept for at least 300 ns.     

基于量子系综的准自旋波激发的量子存贮研究

如何利用实际的物理系统存贮光子的量子信息，是当前量子计算和量子通信研究的核心问题之一，近几年来，人们从理论和实验两个方面探讨了利用自由原子系综作为量子记忆体的可能性，最近，基于固定于格点上A型“原子”系综的准自旋波激发的研究，我们提出了实现光子信息量子存贮的新方案，这个方案的目标是为了克眼自由原子系综量子存贮方案中存在的量子态漏损诱导退相干的致命问题，在研究过程中，作者发现了系综型量子记忆体中隐藏动力学的对称性和其绝热暗态演化的选择定则，为了使读者能确切地了解这个工作的意义，文章除了报道这个发表在Physical Review Letters(2003，91：147903)的研究工作外，还扼要地介绍这个方案建立的前期关于原子系综对称集体激发的系列工作。     

Parametric conversion and maximally entangled photon pair via collective excitations in a cycle atomic ensemble

We study the propagation of two quantized optical fields via considering the collective effects of photonic emissions and excitations of a three-level cyclic-type system (such as atomic ensemble with symmetry broken, or the chiral molecular gases, or manual “atomic” array with symmetry broken), where the quantum transitions is driven by two quantized fields and a classical one. The results show that the parametric conversion and maximally entangled photon pair generation can be achieved by means of the collective excitation of the two upper energy levels induced by the classic optical field. This investigation may be used for the generated coherent short-wavelength quantum radiation and quantum information processing.     

Inelastic light scattering by collective excitations in the fractional quantum Hall regime

We report an inelastic light scattering study of long wavelength collective gap excitations of fractional quantum Hall (FQH) states at ν=p/(2p+1) for . The ν-dependence of the gap energy suggests a collapse of the collective excitation gap near . In a range of filling factors close to , where the FQH gap is believed to collapse, we observe a collective excitation mode that exists only at temperatures below 150 mK.     

Coherent manipulation of spin squeezing in atomic Bose-Einstein condensate via electromagnetically induced transparency

We propose a scheme to coherently control spin squeezing of atomic Bose-Einstein condensate (BEC) via the technique of electromagnetically induced transparency (EIT). We study quantum dynamics of the mean spin vector and spin squeezing. It is shown that the mean spin vector and spin squeezing of the BEC can be controlled and manipulated by adjusting the external coupling fields or/and internal nonlinear interactions of the BEC. It is indicated that the spin squeezing can be generated rapidly in the dynamical process and maintained in a long time interval. It is found that a larger effective Rabi coupling between atoms and lasers can produce a stronger spin squeezing, and the squeezing can maintain a longer time interval.     

Coherent modes in an ensemble of quantum dots generated by dynamic dipole–dipole interactions

The generation and stabilization of coherent modes induced by electric dipole–dipole interactions in a quantum dot ensemble are verified, by means of a standard interaction Hamiltonian scheme, and also by way of a full quantum mechanical path integral on the coherent state basis.