电磁诱导光透明过程中两原子的自旋极化特性

本文研究了电磁诱导光透明过程中两原子的自旋极化矢量.对于数态探测场,在光信息转移过程中,两原子自旋极化矢量始终固定在z方向,大小变化非常明显,两原子一般处于混合态;对于相干态探测场,自旋极化矢量大小变化很微弱,两量子比特基本处于纯态,这有利于改善系综内原子之间的相干性,但自旋极化矢量的方向在x-z平面内.     

电磁诱导光透明过程中两原子的自旋极化特性

本文研究了电磁诱导光透明过程中两原子的自旋极化矢量。对于数态探测场,在光信息转移过程中,两原子自旋极化矢量始终固定在z方向,大小变化非常明显,两原子一般处于混合态;对于相干态探测场,自旋极化矢量大小变化很微弱,两量子比特基本处于纯态,这有利于改善系综内原子之间的相干性,但自旋极化矢量的方向在x-z平面内。     

基于电磁诱导透明的原子自旋波读出效率实验研究

光信息的存储与读出效率对量子信息领域有着重要的意义, 电磁诱导透明是最常用的方法. 本论文基于电磁诱导透明方法首次在热原子系综中实验研究了反 向和同向读取原子自旋波信息的回复效率, 并对实验结果进行了理论分析, 实验和理论的结果都表明同向和反向读出效率的高低取决于原子系综中原子自旋波的强度空间分布. 这一实验研究将会对热原子系综中的光与原子信号相互转换过程的应用有着一定的意义. 关键词： 电磁诱导透明 反向传播 原子系综     

~(87)Rb玻色-爱因斯坦凝聚体中的电磁诱导透明现象

通过吸收成像方法,统计探测光对原子气体的加热损耗,在~(87)Rb玻色-爱因斯坦凝聚体中观察到电磁诱导透明现象。观察电磁诱导透明现象的常规方法是测量探测光的透射光谱,但是对于玻色-爱因斯坦凝聚体这种易受外界干扰的研究对象,应用吸收成像的方法更加简单适用。实验发现,经过透镜聚焦后耦合光的光斑大小强烈影响电磁诱导透明的线型。改变探测光的偏振状态,在电磁诱导透明的峰值处观察到拉曼跃迁过程。拉曼跃迁过程使电磁诱导透明窗口出现极窄的吸收线。相比于电磁诱导透明效应,在拉曼跃迁机制下,探测光的色散曲线更加陡峭,可实现更高效的光减速效果。在双光子共振处,通过改变探测光的偏振状态从而改变其吸收与色散性质,可构造光学开关和光偏振的光致旋转。     

倒Y型四能级原子系统中电磁诱导透明的相干控制

研究了倒Y型四能级原子系统中相干控制电磁诱导透明.应用微扰理论给出一阶近似条件下,不同控制场原子系统对探测光场响应的解析式及电磁诱导透明窗宽度的解析式.在探测场为弱场时,分析了系统中控制光场强度、耦合光场强度及其失谐量对电磁诱导透明窗的影响.研究发现电磁诱导透明窗随控制光场强度的增强变宽,而随耦合光场强度的增强变窄,当探测光场与耦合光场偏离双光子共振时,透明窗外的吸收增加,透明窗变宽.在弱探测场、弱控制场条件下,分析了初始时刻原子处在相干叠加态时的相对相位、相对强度与探测场和控制场的相对相位等对系统吸收特性及透明窗的影响.结果表明探测场与控制场的相对相位对吸收的影响与相干叠加态中两能级之间几率幅的相对相位对吸收的影响作用相反,系统在两下能级干涉极大时,存在一个很宽的透明带.应用缀饰态理论和量子相干理论对所得结果进行了解释.     

四能级原子系统中的电磁诱导吸收

以三个电偶极跃迁(中间跃迁作为探测跃迁、两边分别是耦合跃迁和控制跃迁)构成N型四能级系统为例，研究介质对探测光的吸收.揭示了在N型四能级系统中，既可产生电磁诱导吸收又可产生电磁诱导透明.这不仅取决于非相干转移率的大小，而且还取决于耦合场、控制场的强度和第四个简并能级的衰减速率大小. 关键词： 电磁诱导吸收 电磁诱导透明 原子相干的自发转移 能级的衰减速率     

单模辐射场作用下两二能级原子的Wigner-Yanse偏振信息

研究了与单模辐射场作用过程中两二能级原子的Wigner-Yanse信息.结果表明：两原子总Wigner-Yanse偏振信息依赖于处于基态|gg>, 第一激发态(|ge>+|eg>)/2^(1/2)和第二激发态|ee>的概率. 由于光场的作用, 总Wigner-Yanse偏振信息呈现出周期性的丢失与恢复现象, 但原子间的偶极-偶极相互作用对Wigner-Yanse信息的丢失有阻抗作用.     

单模辐射场作用下两二能级原子的Wigner-Yanse偏振信息

研究了与单模辐射场作用过程中两二能级原子的Wigner-Yanse信息.结果表明：两原子总Wigner-Yanse偏振信息依赖于处于基态|gg>, 第一激发态(|ge>+|eg>)/2^(1/2)和第二激发态|ee>的概率. 由于光场的作用, 总Wigner-Yanse偏振信息呈现出周期性的丢失与恢复现象, 但原子间的偶极-偶极相互作用对Wigner-Yanse信息的丢失有阻抗作用.     

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。     

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 memory based on $\mathsf{\Lambda}$-atoms ensemble with two-photon resonance EIT

We study the -atoms ensemble based quantum memory for the quantum information carried by a probe light field. Two atomic Rabi transitions of the ensemble are coupled to the quantum probe field and classical control field respectively with a same detuning. Our analysis shows that the dark states and dark-state polaritons can still exist for the present case of two-photon resonance EIT. Starting from these dark states we can construct a complete class of eigen-states of the total system. A explicit form of the adiabatic condition is also given in order to achieve the memory and retrieve of quantum information.Received: 8 June 2004, Published online: 10 August 2004PACS: 03.67.-a Quantum information - 42.50.Gy Effects of atomic coherence on propagation, absorption, and amplification of light; electromagnetically induced transparency and absorption - 03.65.Fd Algebraic methods     

Cavity linewidth narrowing with dark-state polaritons

We present a quantum-theoretical treatment of cavity linewidth narrowing with intracavity electromagnetically induced transparency(EIT). By means of intracavity EIT, the photons in the cavity are in the form of cavity polaritons:bright-state polariton and dark-state polariton. Strong coupling of the bright-state polariton to the excited state induces an effect known as vacuum Rabi splitting, whereas the dark-state polariton decoupled from the excited state induces a narrow cavity transmission window. Our analysis would provide a quantum theory of linewidth narrowing with a quantum field pulse.     

Dark-state polaritons in electromagnetically induced transparency

We identify form-stable coupled excitations of light and matter ("dark-state polaritons") associated with the propagation of quantum fields in electromagnetically induced transparency. The properties of dark-state polaritons such as the group velocity are determined by the mixing angle between light and matter components and can be controlled by an external coherent field as the pulse propagates. In particular, light pulses can be decelerated and "trapped" in which case their shape and quantum state are mapped onto metastable collective states of matter. Possible applications of this reversible coherent-control technique are discussed.     

Electromagnetically induced transparency and electromagnetically induced absorption in Y-type system

The propagation of a probe field through a four-level Y-type atomic system is described in the presence of two additional coherent radiation fields, namely, the control field and the coupling field. An expression for the probe response is derived analytically from the optical Bloch equations under steady state condition to study the absorptive properties of the system under probe field propagation through an ensemble of stationary atoms as well as in a Doppler broadened atomic vapor medium. The most striking result is the conversion of electromagnetically induced transparency(EIT) into electromagnetically induced absorption(EIA) as we start switching from weak probe regime to strong probe regime. The dependence of this conversion on residual Doppler averaging due to wavelength mismatch is also shown by choosing the coupling transition as a Rydberg transition.     

Adiabatic propagation of quantized light pulses in an atomic medium with the tripod level configuration

We consider adiabatic propagation of a pair of quantized light pulses in a coherently prepared atomic medium with the tripod level configuration. We find that under conditions of electromagnetically induced transparency, two distinct polariton modes are simultaneously formed in the medium. These polaritons, represented by certain coherent superpositions of the quantized fields, have different group velocities; the fast one propagates at essentially the speed of light, while the group velocity of the slow polariton can be dynamically reduced to zero. The state mapping between the electromagnetic field and atomic ensemble is also demonstrated. The text was submitted by the author in English.     

基于量子相干控制吸收的准Λ型四能级原子局域化研究

提出了一种基于量子相干控制吸收的对准Λ型四能级原子进行二维局域化方案. 利用密度矩阵微扰理论, 得到了确定原子空间位置信息的筛选函数解析表达式. 在缀饰态表象中, 分析了在相干控制场作用下原子初始状态对原子局域的影响. 数值模拟了控制场参量对原子局域化结果的影响. 研究发现原子局域化结果与初始时刻在控制场作用下原子在下能态的布局、下能级间产生的极化密切相关; 不管探测场与耦合场是否满足电磁感应透明配置条件, 通过改变控制场中的行波场的振幅和探测场的失谐量, 均可实现高精度原子局域化, 在亚波长范围内测量到原子的概率达到100%.     

An experimental approach for investigating many-body phenomena in Rydberg-interacting quantum systems

Recent developments in the study of ultracold Rydberg gases demand an adwanced level of experimental sophistication, in which high atomic and optical densities must be combined with excellent control of external fields and sensitive Rydberg atom detection. We describe a tailored experimental system used to produce and study Rydberg-interacting atoms excited from dense ultracold atomic gases. The experiment has been optimized for fast duty cycles using a high flux cold atom source and a three beam optical dipole trap. The latter enables tuning of the atomic density and temperature over several orders of magnitude, all the way to the Bose--Einstein condensation transition. An elec- trode structure surrounding the atoms allows for precise control over electric fields and single-particle sensitive field ionization detection of Rydberg atoms. We review two experiments which highlight the influence of strong Rydberg---Rydberg interactions on different many-body systems. First, the Rydberg blockade effect is used to pre-structure an atomic gas prior to its spontaneous evolution into an ultracold plasma. Second, hybrid states of photons and atoms called dark-state polaritons are studied. By looking at the statistical distribution of Rydberg excited atoms we reveal correlations between dark-state polaritons. These experiments will ultimately provide a deeper understanding of many-body phenomena in strongly-interacting regimes, including the study of strongly-coupled plasmas and interfaces between atoms and light at the quantum level.     

Enhanced Deflection of Light Ray by Atomic Ensemble on Coherent Population Oscillation

In recent experiments [e.g., Nature Physics 2 (2006) 332], the enhancedlight deflection in an atomic ensemble due to inhomogeneous fields is demonstrated by the electromagnetically induced transparency (EIT) based mechanism. In this paper, we explore a different mechanism for the similar phenomenon of the enhanced light deflection. This mechanism is based on the coherent population oscillation, which leads to the hole burning in the absorption spectrum. The medium causing the deflection of probe light is an ensemble of two-level atoms manipulated by a strong controlled field on the two photon resonances. In the large detuning condition, the response of the medium to the pump field and signal field is obtained with steady state approximation. And it is found that after the probe field travels across the medium, the signal ray bends due to the spatial-dependent profile of thecontrol beam.