在光腔中利用自然非均匀展宽的原子系综进行光量子态存储

在非对称的光腔中,我们提出了一个利用自然非均匀展宽的原子系综实现高效量子存储方案.在这个存储方案里我们使用了类似于传统光子回声技术中强π脉冲,而不是烧孔技术.这使得我们的方案可以在更多的材料体系中实现.在阻抗匹配的条件下,我们发现进入光腔的光脉冲可以完全被原子系综吸收.而且在光深度很小时,量子存储效率就可以达到1.     

脉冲激光激发的原子共振荧光

本文从含有弛豫项的光与二能级原子相互作用的布洛赫(Block)方程出发,利用计算机进行数值求解,在强共振激发条件下,给出了几种不同的激光脉冲(包括常见的高斯脉冲)激发的原子共振荧光谱,获得了原子在瞬态辐射过程中的一些新现象。     

星型网络中的两比特量子相位门

我们提出一个方案,在星型耦合腔QED网络中一步实现任意两个非直接耦合光腔中原子的两比特量子相位门操作。该操作是通过虚光子交换技术,使两个非直接耦合光腔中的原子与星型网络中心光腔中的辅助原子形成有效的非对称海森堡XY模型。通过数值模拟验证了方案的可行性。在系统的整个演化过程中腔模始终处于真空态,并且原子不被激发,原子的自发辐射与腔的衰减得到有效抑制,通过数值求解主方程可以证明操作对耗散不敏感。     

囚禁在不同腔中的两原子相位门的实现

逻辑门操作特别是远距离qubit间态的控制是实现量子通讯和量子计算的一个关键组元.这里我们利用两个光腔的单向耦合和光子的干涉效应,提出了实现分别被囚禁在光腔中的两原子相位门方案.用两束与腔共轴的经典光驱动一个双模腔原子系统,使腔场和原子纠缠起来.携带着第一个原子信息的腔场从腔中泄漏出来,经PBS分成两束光脉冲,一束进入第二个腔并和第二个原子相互作用.当再次被腔反射后又和第二束光脉冲在另一个PBS上发生干涉.用两个单光子探测器观测,依据测量结果对第一个原子进行某些局域操作,这样就实现了两原子的相位门操作.该方案的可扩展性强,同时数值计算的结果表明可不受Lamb-Dick极限限制.     

用共振激发延迟电离方法测定原子激发态的光电离截面

用两束脉冲激光,一束使原子共振激发,另一束使激发态原子电离,当电离光脉冲能量约达1J/cm~2时,可使电离饱和.测量被电离原子数的相对大小与电离光脉冲能量之间的函数关系,可以方便地得出原子激发态的光电离截面.延迟电离光至激发光脉冲结束后到达作用区,可以修正激发光对电离的贡献,并避免电离光引起的Stark位移对共振激发的影响.应用共振激发延迟电离方法测定了Cs原子7~2P_(3/2)态的光(λ=5320(?))电离截面,测量值σ_I=6.5×10~(-18)cm~2与Manson的计算值非常接近.     

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。     

腔内混合态原子系统中辐射场的压缩效应

本文应用熵最大原理，假定腔内二能级原子起初就没有关于系统所处态的任何信息，研究了腔内原子与腔场相互作用的压缩行为，得出了原子初始处于任意统计混合态下，当初始激发足够强时，辐射场将产生极其接近于纯态情况在相同激发时的强压缩现象的重要结果，同时还讨论了产生压缩的混态范围。     

基于腔增强的磁光旋转铯原子磁强计

谐振腔通过增加光与原子相互作用的长度而增大探测光偏转角,大大增强了原子磁强计的灵敏度。我们基于磁光旋转的铯原子磁强计,在理论与实验上研究了磁场测量灵敏度的腔增强因子与腔镜反射率的关系,得到在一定原子气室损耗条件下获得最优磁测灵敏度的最佳腔镜反射率。利用平凹驻波腔,在特定的腔镜反射率,原子气室对光功率的损耗分别为9.5%和8.5%,对应的腔逃逸效率为38%和41%,腔对磁场灵敏度的增强因子为4.4和5.1;在保持光与原子相互作用强度的情况下,随着原子气室对光场损耗的降低,磁场灵敏度进一步增强,腔增强效果更显著。     

基于低Q腔光子Faraday旋转的远程态制备

基于低Q腔中单光子的输入与输出关系,提出了利用偏振光Faraday旋转分别遥远制备单原子态和两原子纠缠态的可行方案.研究结果表明,当初始原子态的系数为实数时,通过选择合适的偏振光、腔场与原子相互作用系统的参数,单原子态与两原子纠缠态的远程制备均可确定性地得以实现.与以前的原子态远程制备方案相比,本文方案采用光子作为飞行比特来传递量子信息,故原则上可实现原子态的真正长距离制备.由于原子态的信息编码在耗散单边腔囚禁的Λ型三能级原子的两个基态能级,且原子仅虚激发,因此本文方案对腔衰减和原子自发辐射不敏感.此外,本文所提出的两种方案不需要两体或多体正交测量,仅涉及单体直积态测量,而且两种方案都工作在低Q腔,不需要原子与光腔的强耦合,从而有效降低了实验难度.     

腔场耦合的二能级原子荧光谱北大核心

通过光与物质相互作用的主方程计算了强场激励的二能级原子与单模腔耦合的稳态荧光谱.在腔场的强耦合作用下,三峰Mollow谱的每个成分都分裂为多重的,这种现象由腔场使原子修饰态能级漂移而导致的,荧光谱的具体结构则依赖于驱动场的拉比频率和原子-腔的耦合系数.     

Excitation of hydrogen atom by ultrashort laser pulses in optically dense plasma

The features of excitation of a hydrogen atom by ultrashort laser pulses (USP ) with a Gaussian envelope in optically dense plasma at a Lyman‐beta transition are studied theoretically. The problem is of interest for diagnostics of optically dense media. USP have two doubtless advantages over conventional laser excitation: (a) the USP carrier frequency is shifted to the region of short wavelengths allowing exciting atoms from the ground state and (b) the wide spectrum of USP allows them to penetrate into optically dense media to much longer distances as compared with monochromatic radiation. As actual realistic cases, two examples are considered: hot rarefied plasma (the coronal limit) and dense cold plasma (the Boltzmann equilibrium). Universal expressions for the total probability of excitation of the transition under consideration are obtained in view of absorption of radiation in a medium. As initial data for the spectral form of a line, the results of calculations by methods of molecular dynamics are used. The probability of excitation of an atom is analysed for different values of problem parameters: the pulse duration, the optical thickness of a medium, and the detuning of the pulse carrier frequency from the eigenfrequency of an electron transition.     

碱金属原子的光激发与光电离

本文研究了碱金属原子在三步激光脉冲作用下的光激发和光电离过程的动力学特性, 重点关注和比较了锂和铯原子的异同. 针对多种激发模式, 本文不但建立了其原子布居数在各个跃迁态的速率方程组, 还给出了各相关态的光激发和光电离过程的解析解. 通过精心设计并选择了特殊情况, 显著简化了解析解的数学表达式, 从而凸显和讨论了其物理内涵. 通过自行编程, 系统地计算和观察了各种激发模式对锂原子的光激发和光电离过程的可能影响, 研究和讨论了电离率随激光参数的变化规律. 在相同激发模式下, 比较和分析了采用两种不同激发路径所导致的各态原子布居率的变化, 凸显了改变原子参数所产生的作用. 探讨了锂和铯原子在类似的激发条件下在电离率方面的差别. 最后, 基于本文的研究结果, 本文指出了优化电离率的多种途径.     

三色圆偏振激光组合脉冲驱动氦原子产生椭圆偏振的阿秒脉冲

我们提出利用频率为ω,3ω和强度较弱的2ω组成的三色圆偏振激光组合脉冲驱动氦原子能够得到椭偏率较大的阿秒脉冲链的一种方法.通过强场近似方法,计算了氦原子在两色和三色圆偏振激光组合脉冲驱动下发射高次谐波谱及其合成阿秒脉冲链,比较了氦原子(初态为s态)在这两种情况下发射高次谐波谱的特点及其合成阿秒脉冲链椭偏率的大小,结果发现,在反旋的两色ω,3ω激光脉冲基础上加入了频率为2ω的第三色激光脉冲联合作用到氦原子上,所得到的阿秒脉冲链的椭偏率相对于双圆场情况下有所增加,通过调整ω,3ω激光的强度比,并且选择适当的第三色激光的强度,对初态为s态的原子,仍能够得到具有较大椭偏率的阿秒脉冲链.     

Deterministic single-photon source for distributed quantum networking

A sequence of single photons is emitted on demand from a single three-level atom strongly coupled to a high-finesse optical cavity. The photons are generated by an adiabatically driven stimulated Raman transition between two atomic ground states, with the vacuum field of the cavity stimulating one branch of the transition, and laser pulses deterministically driving the other branch. This process is unitary and therefore intrinsically reversible, which is essential for quantum communication and networking, and the photons should be appropriate for all-optical quantum information processing.     

Local blockade of Rydberg excitation in an ultracold gas

In the laser excitation of ultracold atoms to Rydberg states, we observe a dramatic suppression caused by van der Waals interactions. This behavior is interpreted as a local excitation blockade: Rydberg atoms strongly inhibit excitation of their neighbors. We measure suppression, relative to isolated atom excitation, by up to a factor of 6.4. The dependences of this suppression on both laser irradiance and atomic density are in good agreement with a mean-field model. These results are an important step towards using ultracold Rydberg atoms in quantum information processing.     

Control and Transfer of Entanglement between Two Atoms Driven by Classical Fields under Dressed-State Representation

We have studied the dynamics and transfer of the entanglement of the two identical atoms simultaneously interacting with vacuum field by employing the dressed-state representation. The two atoms are driven by classical fields. The influence of the initial entanglement degree of two atoms, the coupling strength between the atom and the classical field and the detuning between the atomic transition frequency and the frequency of classical field on the entanglement and atomic linear entropy is discussed. The initial entanglement of the two atoms can be transferred into the entanglement between the atom and cavity field when the dissipation is neglected. The maximally entangled state between the atoms and cavity field can be obtained under some certain conditions. The time of disentanglement of two atoms can be controlled and manipulated by adjusting the detuning and classical driving fields. Moreover, the larger the cavity decay rate is, the more quickly the entanglement of the two atoms decays.     

Forces and spatial ordering of driven atoms in a resonator in the regime of fluorescence suppression

An atom in a high-Q cavity, which is coherently driven at the frequency of a cavity mode, exhibits strong suppression of fluorescence when the atomic decay rate exceeds the cavity linewidth. This effect is due to destructive interference of cavity and pump field, such that at the atomic position the total field intensity has a local minimum. For atomic ensembles the magnitude of the interference effect grows with atom number and depends on the relative atomic positions. It is strongest for a wavelength spaced array of atoms placed at the antinodes of the cavity mode. This suppresses fluorescence and enhanced collective scattering into the cavity mode. We analyze the mechanical forces in the regime where the interference condition is fulfilled. We show that the atomic pattern is mechanically stable whenever the driving frequency is red detuned with respect to the cavity frequency, irrespective of the atomic transition frequency. Hence atomic selforganization, as predicted in [6] can also occur in the parameter regime where superradiant scattering is suppressed by collective interference. PACS 32.80.Pj; 42.50.Pq; 42.50.Fx     

Photoelectron angular distributions from strong-field coherent electronic excitation

Photoelectron Angular Distributions (PADs) resulting from nonperturbative excitation of potassium atoms using shaped femtosecond laser pulses are presented. We study control exerted by (1) the polarization of an unshaped, i.e., a bandwidth-limited light pulse, (2) shaped linearly polarized light, and (3) a combination of both degrees of freedom, i.e., polarization-shaped laser pulses. A theoretical approach to describe PADs from nonperturbative Resonance Enhanced Multi-Photon Ionization (REMPI) with ultrashort polarization-shaped laser pulses is presented and compared to experimental results. Applications of this technique to the generation and observation of atomic ring currents are discussed.     

Storing optical information as a mechanical excitation in a silica optomechanical resonator

We report the experimental demonstration of storing optical information as a mechanical excitation in a silica optomechanical resonator. We use writing and readout laser pulses tuned to one mechanical frequency below an optical cavity resonance to control the coupling between the mechanical displacement and the optical field at the cavity resonance. The writing pulse maps a signal pulse at the cavity resonance to a mechanical excitation. The readout pulse later converts the mechanical excitation back to an optical pulse. The storage lifetime is determined by the relatively long damping time of the mechanical excitation.     

Efficiency of frequency-pulsed excitation of a micron-sized spherical microcavity by chirped ultrashort laser radiation

Peculiarities of internal optical field resonant excitation inside micron-sized spherical transparent dielectric cavity illuminated by a train of ultrashort laser pulses are investigated. On an example of water microdroplet is shown that optimal tuning of incident radiation to a selected high-Q resonance electromagnetic cavity mode can be realized by varying temporal interpulse interval in a laser train together with linear frequency modulation of every pulse (chirp). Efficiency of resonant particle excitation by a chirped picosecond pulses train can be considerably increased as compared to unchirped pulse train and CW excitation also.