Spontaneous emission properties of a driven five-level atom embedded in photonic crystals

The spontaneous emission spectra of a five-level double tripod-type atom embedded in photonic crystals (PCs) are investigated by means of two external fields driving different atomic transitions. We find that due to the quantum interference effects caused by two driving fields, the spontaneous emission spectra exhibit different features from the case of only one driving field. The influences of the parameters of two external driving fields, photonic band-gap (PBG), as well as the atomic initial states on the spectra are analyzed in detail. It is shown that some interesting phenomena such as spectral-line enhancement, spectral-line suppression, spectral-line narrowing, the appearance of dark lines, and multi-peak structures can be observed in the spectra by appropriately modulating the available system parameters. These investigations may find applications in high-precision spectroscopy.     

原子自发辐射中偶极矩的涨落与最大纠缠态的保持

研究了二能级原子与电磁场相互作用体系的自发辐射与量子纠缠态.在原子的自发辐射过程中,其偶极矩的期待值总是零,但偶极矩的涨落恒等于一个不为零的常量,因此原子的自发辐射是由真空起伏导致偶极矩的涨落引起的.Jaynes-Cummings模型是产生量子纠缠态的重要体系,研究发现,原子与场纠缠态的信息熵和纠缠度随时间作周期性的振荡,量子态在非纠缠与纠缠态之间变化.更为重要的是,在失谐量适当时,量子态将长时间停留在最大纠缠态.     

Efficient two-dimensional atom localization via spontaneous emission in a single decay channel

We investigate the two-dimensional atom localization behaviors in a four-level atomic system via controlled spontaneous emission in a single decay channel. It is found that the detecting probability and precision of atom localization behaviors can be significantly improved via adjusting the system parameters. More importantly, the two-dimensional atom localization patterns reveal that the maximal probability of finding an atom within the sub-half-wavelength domain of the standing waves can reach unity when the corresponding conditions are satisfied. As a result, our scheme may be helpful in laser cooling or the atom nano-lithography via atom localization.     

Coherent effects under suppressed spontaneous emission

An ensemble of resonance atoms is considered, which are doped into a medium with well developed polariton effect, when in the spectrum of polariton states there is a band gap. If an atom with a resonance frequency inside the polariton gap is placed into the medium, the atomic spontaneous emission is suppressed. However, a system of resonance atoms inside the polariton gap can radiate when their coherent interaction is sufficiently strong. Thus the suppression of spontaneous emission for a single atom can be overcome by a collective of atoms radiating coherently. Conditions when such collective effects can appear and their dynamics are analysed. Received 7 June 2000     

Coherence-controlled stationary entanglement between two atoms embedded in a bad cavity injected with squeezed vacuum

We investigate the entanglement between two atoms in an overdamped cavity injected with squeezed vacuum when these two atoms are initially prepared in coherent states. It is shown that the stationary entanglement exhibits a strong dependence on the initial state of the two atoms when the spontaneous emission rate of each atom is equal to the collective spontaneous emission rate, corresponding to the case where the two atoms are close together. It is found that the stationary entanglement of two atoms increases with decreasing effective atomic cooperativity parameter. The squeezed vacuum can enhance the entanglement of two atoms when the atoms are initially in coherent states. Valuably, this provides us with a feasible way to manipulate and control the entanglement, by changing the relative phases and the amplitudes of the polarized atoms and by varying the effective atomic cooperativity parameter of the system, even though the cavity is a bad one. When the spontaneous emission rate of each atom is not equal to the collective spontaneous emission rate, the steady-state entanglement of two atoms always maintains the same value, as the amplitudes of the polarized atoms varies. Moreover, the larger the degree of two-photon correlation, the stronger the steady-state entanglement between the atoms.     

Modification of radiation pressure due to cooperative scattering of light

Cooperative spontaneous emission of a single photon from a cloud of N atoms modifies substantially the radiation pressure exerted by a far-detuned laser beam exciting the atoms. On one hand, the force induced by photon absorption depends on the collective decay rate of the excited atomic state. On the other hand, directional spontaneous emission counteracts the recoil induced by the absorption. We derive an analytical expression for the radiation pressure in steady-state. For a smooth extended atomic distribution we show that the radiation pressure depends on the atom number via cooperative scattering and that, for certain atom numbers, it can be suppressed or enhanced. Cooperative scattering of light by extended atomic clouds can become important in the presence of quasi-resonant light and could be addressed in many cold atoms experiments.     

Spontaneous emission from a microwave-driven four-level atom in an anisotropic photonic crystal

The spontaneous emission from a microwave-driven four-level atom embedded in an anisotropic photonic crystal is studied. Due to the modified density of state(DOS) in the anisotropic photonic band gap(PBG) and the coherent control induced by the coupling fields, spontaneous emission can be significantly enhanced when the position of the spontaneous emission peak gets close to the band gap edge. As a result of the closed-loop interaction between the fields and the atom,the spontaneous emission depends on the dynamically induced Autler–Townes splitting and its position relative to the PBG.Interesting phenomena, such as spectral-line suppression, enhancement and narrowing, and fluorescence quenching, appear in the spontaneous emission spectra, which are modulated by amplitudes and phases of the coherently driven fields and the effect of PBG. This theoretical study can provide us with more efficient methods to manipulate the atomic spontaneous emission.     

A giant atom with modulated transition frequency

Giant atoms are known for the frequency-dependent spontaneous emission and associated interference effects. In this paper, we study the spontaneous emission dynamics of a two-level giant atom with dynamically modulated transition frequency. It is shown that the retarded feedback effect of the giant-atom system is greatly modified by a dynamical phase arising from the frequency modulation and the retardation effect itself. Interestingly, such a modification can in turn suppress the retarded feedback such that the giant atom behaves like a small one. By introducing an additional phase difference between the two atom-waveguide coupling paths, we also demonstrate the possibility of realizing chiral and tunable temporal profiles of the output fields. The results in this paper have potential applications in quantum information processing and quantum network engineering.     

Spontaneous emission spectrum of a three-level atom embedded in photonic crystal

The two models of three-level (one upper level and two lower levels, or two upper levels and one lower level) atom embedded in a double-band photonic crystal are adopted. The atomic transitions from the upper levels to the lower levels are assumed to be coupled by the same reservoir which are respectively the isotropic photonic band gap (PBG) modes, the anisotropic PBG modes and the free vacuum modes. The effects of the fine structure of the atomic ground state levels in the model with one upper level and two lower levels, and the quantum interferences in the model with two upper levels and one lower level on the spontaneous emission spectrum of an atom are investigated in detail. Most interestingly, it is shown that new spontaneous emission lines are produced from the fine splitting of atomic ground state levels in the isotropic PBG case. The quantum interferences induce additional narrow spontaneous lines near the transition from the empty upper level to the lower level.     

原子与热库相互作用系统中的三体纠缠特性

利用Ket-Bra纠缠态方法,求解了原子与热库相互作用系统中的密度矩阵主方程,得到了密度矩阵的演化表达式.考虑三个二能级原子独立与热库相互作用的情况,利用负本征值度量三体纠缠,研究了系统中原子间的三体纠缠特性.采用数值计算方法,讨论了热库平均光子数和原子自发辐射率对原子间三体纠缠特性的影响.研究结果表明:随原子自发辐射率和热库平均光子数的增大,原子间的三体纠缠衰减加快.     

Casimir-Polder力对左手材料板附近的原子的动力学作用

本文研究了初始处于激发态的两能级原子在左手材料附近运动时Casimir-Polder力对原子动力学的影响. 左手材料有两个的作用: 一是在距离界面波长区域内提供了较强的Casimir-Polder共振力, 二是在这一范围原子的自发辐射受到抑制, 延长了作用时间. 这两种效应使得依靠原子自发辐射这一过程中的Casimir-Polder力能对原子的运动学产生影响, 并将一定初速度的原子排斥远离界面. 由于原子偶极矩的取向会影响Casimir-Polder力的性质, 因此对于某些初始条件(初速度和初始位置), 不同偶极矩取向的原子有不同的运动学结果, 会被吸引到界面或反射出去, 从而对具有不同偶极矩方向的原子进行筛选. 当然由于Casimir-Polder力很小, 能够反射的初速度也很小, 但是已经可以反抗极低温的热涨落, 我们的理论预估值约为15 μupK. 如果和其他约束手段同时作用, 便能对原子的动力学产生更为有利的控制.     

Generation of compressed sub-poisson Bose condensate by an atom laser

The dynamics of Bose-condensate generation by a cw atom laser with simultaneous stimulated evaporative cooling in a magnetic trap was analyzed using a quantum-mechanical master equation. The model of the atom laser includes irreversible processes of incoherent trap mode pumping and spontaneous atomic transitions due to the interaction of the atomic ensemble with heat reservoirs. The inelastic atomic collisions in the trap and the continual coherent Bose-condensate output coupling from the trap were considered. At certain values of parameters, the Bose condensate created in this laser scheme occurs in a compressed sub-Poisson state. For large Bose condensates with a mean number of atoms ～10⁶, the Fano factor may be as high as ?0.5. The influence of spontaneous transitions from the excited trap modes on the statistics of Bose condensate was analyzed.     

Spontaneous emission spectrum of a four-level atom in a double-band photonic crystal

The spontaneous emission spectrum from a four-level atom in a double-band photonic crystal has been investigated.We use the model which assumes three atomic transitions. One of the transitions interacts with the free vacuum modes,and the other two transitions couple to the modes of the isotropic photonic band gap (PBG), the anisotropic PBG and another free vacuum. The effects of the fine structure of the lower levels on the spontaneous emission spectrum of an atom are investigated in detail in the three cases. New features of four (two) transparencies with two (one) spontaneous emission peaks, resulting from the fine structure of the lower levels of an atom, are predicted in the case of isotropic PBG modes.     

Properties of the spontaneous emission of a driven four-level atom coupled to separate reservoirs

We investigate the spontaneous emission properties of a driven four-level atom coupled to separate reservoirs. Both the effect of the external field and the influence of the photonic bandgap reservoir on the atomic spontaneous emission are discussed. It is shown that a few interesting phenomena such as a laser-induced spectral narrowing dark line can be achieved and its physical understanding can be given by invoking the dressedatom approach.     

Spontaneous emission from an atom in a photonic crystal with two coherent bands

The dynamic and the radiative properties of an excited three-level atom embedded in an anisotropic photonic crystal with two coherent bands are investigated.The relative position of the atom in a Wigner-Seitz cell is described with a position-dependent parameter θ(r0),which is used as the coherent parameter for the two bands.The result shows that the dynamic properties of the atomic system are not only determined by atomic transition frequencies,but also affected by the gap width and the coherence of the two bands.In addition,the spontaneous emission spectrum of the atomic transition in free space is discussed.The center and the intensity of the spectrum can be obviously manipulated via the coherent parameter.     

运用理想光子禁带模型实现对激发态原子系统演化的调控

通过调节动静态理想光子禁带模型库的结构参数,研究了初态处于激发态的两能级原子系统的演化.在静态无调制下研究理想光子禁带模型库环境的半宽度、中心谐振频率及比重对原子布居数演化的影响.在理想光子禁带库环境的中心共振频率受动态调制下,其调制形式分别取为:矩形单次脉冲、矩形周期性脉冲和缓变连续周期.在此基础上讨论动态调制形式的不同对原子布居数演化的影响.无论怎样的动态调制形式,衰减抑制在原子系统的演化过程还是有较明显的体现.这样就使得利用环境变化对原子布居数和原子系统相干性演化调制的想法得以实现.     

动态各向同性光子晶体中二能级原子的自发辐射

研究了动态各向同性光子晶体中二能级原子自发辐射的性质,主要讨论了光子晶体能带带边频率随时间作阶跃调制和三角函数周期调制两种情况下,原子上能级占据数随时间的演化特性.当光子晶体能带带边频率随时间作阶跃调制时,原子上能级占据数随时间的演化不仅和上能级与能带带边的相对位置δ有关,更依赖于阶跃调制发生的时刻.调制发生时刻不同,调制后原子上能级占据数随时间的演化也不同.当光子晶体能带带边频率随时间作三角函数周期调制时,二能级原子上能级占据数随时间作总体衰减的准周期振荡.通过选择调制频率和调制初相位可调控准周期振荡的频率、峰值与谷值的大小以及占据数的总体衰减速度等.     

Effect of mode mode-competition on atom-atom entanglement

A system consisting of two atoms interacting with a two-mode vacuum is considered, where each atom is resonant with the two cavity modes through two different competing transitions. The effect of mode--mode competition on the atom--atom entanglement is investigated. We find that the entanglement between the two atoms can be induced by the mode--mode competition. For the initial atomic state |\varPsi(0)\rangle, whether the atoms are initially separated or entangled, a large or even maximal entanglement between them can be obtained periodically by introducing the mode--mode competition. For the initial atomic state |\varPhi(0)\rangle, the strong mode--mode competition can prevent the two atoms entangled initially from suffering entanglement sudden death; besides, it makes them in a more stable and longer-lived entanglement than in the non-competition case.     

光子闭合轨道理论对单镜面附近原子自发辐射率的解释

利用腔量子电动力学计算了原子在全反射镜面附近的自发辐射率。结果表明,所得的震荡谱和原子到镜面的距离有关。通过傅立叶变换,得到与这些振荡谱相对应的频率谱。为了解释这种现象,我们首次将闭合轨道理论用于研究辐射原子在单镜面附近的自发辐射率,给出了对原子自发辐射现象新的理论解释。