闭合轨道理论在二层介质中原子自发辐射问题上的应用

利用量子电动力学计算了二层介质中电磁场零点涨落,并用原子物理中发展成熟的闭合轨道理论探讨了原子自发辐射速率随发光原子离开界面的距离的增加而变化的特性.研究结果表明:当发光原子远离界面时,原子自发辐射速率呈现衰减的周期振荡现象.通过傅立叶变换,得到了原子自发辐射速率随发光原子离开界面的距离的增加而振荡的频率,为了解释这些现象,我们首次将闭合轨道理论应用于研究两层介质中原子的自发辐射,给出了一种对原子自发辐射现象新的理解方式.     

半无限大介质板夹层对空腔中原子自发辐射率的影响

研究了半无限大介质板夹层对空腔中激发态原子的自发辐射率的影响.引人标度变量,采用腔量子电动力学方法计算了原子的自发辐射率,并用闭合轨道理论对结果进行分析:由于介质界面的影响,光子经界面反射后返回到辐射原子上,并与出射的光子相干,辐射率呈现出拟周期的阻尼振荡;通过Fourier变换抽取振荡频率,每一频率对应一条光子经典轨迹的作用量,从而给出了夹层空腔对激发态原子自发辐射率的影响的新解释.     

氢负离子在非均匀电场中的光剥离

在闭合轨道理论的基础上对氢负离子在非均匀电场中的光剥离进行了研究,推导给出了体系的光剥离截面的计算公式,并且对体系的光剥离截面进行了计算和分析.研究表明,除了氢负离子在均匀电场中的闭合轨道外,在非均匀电场中会产生一些额外的闭合轨道.与氢负离子在均匀电场中的光剥离截面相比较,体系中的光剥离截面的振荡情况更加复杂,表现出一个多周期振荡结构.为了清晰的表明这个系统中氢负离子的光剥离截面振荡和剥离电子的闭合轨道的关系,对这个体系的光剥离截面做了傅里叶变换.经过傅里叶变换,得到了一系列峰,其中每一个峰都对应着一个闭合轨道.     

磁场对电场和金属面附近氢负离子光剥离的影响

利用闭合轨道理论,研究了负离子在电场和金属面附近光剥离的磁场效应.结果表明磁场对负离子在金属面附近的光剥离可以产生很重要的影响.除了杜孟利等人在研究电场和金属面附近负离子的光剥离中发现的闭合轨道以外,还存在由于磁场的影响而产生的更多的闭合轨道.当给定负离子到金属面的距离和电场强度之后,光剥离截面随着磁场强度的变化非常敏感.磁场强度很小时,其影响可以忽略.但是随着磁场强度的增大,产生的闭合轨道数目增多,导致光剥离截面中的振荡结构变得非常复杂.因此,可以通过改变磁场的强度来调控负离子的光剥离截面.该理论结果对于将来实验研究负离子体系在表面和外电场存在时的光剥离问题提供一定的参考.     

氢负离子在外强场及表面附近光剥离的研究

随着光剥离显微技术和表面物理的发展,原子或负离子体系在外场和表面附近光剥离的研究引起了人们的广泛关注.本文利用闭合轨道理论研究了平行电场和磁场以及金属面附近氢负离子的光剥离问题.保持负离子到金属面的距离和磁场强度大小不变,我们主要研究了电场对光剥离截面的影响.结果发现:光剥离截面不仅与电场强度大小有关而且与电场方向有关.当电场方向沿+z轴时,不论电场强度大小如何都对光剥离截面有影响.当电场方向沿-z轴时,情况更加复杂,此时既可以形成向上的闭合轨道,还可以形成向下的闭合轨道,截面中的振荡也变得更复杂.此研究不仅有助于理解负离子在外场和表面附近的光剥离问题,也为将来进一步开发光剥离显微镜提供一定的参考,同时对丰富和发展该理论本身意义重大.在等离子体物理、表面科学、核聚变、激光分离同位素等高科技领域中也有重要的应用前景.     

激光脉冲对锂原子在强外场中光吸收谱的影响

利用含时微扰论和强外场中原子的单脉冲和双脉冲光吸收谱的计算公式, 计算了对应不同脉冲宽度的锂原子在强磁场中的单脉冲和双脉冲光吸收谱. 计算结果表明: 一定宽度的激光脉冲能够减弱周期短的闭合轨道的贡献, 对于周期远大于脉冲宽度的闭合轨道的贡献则完全消除, 和单脉冲光吸收谱进行比较, 发现对于某些相位差, 双脉冲吸收谱增强; 而对于另外一些相位差, 吸收谱减弱. 因此, 可以利用脉冲激光来控制光吸收谱的振荡, 从而达到优化的目的.     

纳米银球附近三能级原子的自发辐射动力学特性

基于格林函数预解算子方法，研究了纳米银球附近V型三能级原子的自发辐射动力学特性.结果表明，原子的自发辐射动力学强烈依赖于原子的跃迁频率和所处位置.原子动力学演化谱呈现出双主峰的特征，距离较近时，高频峰宽度远小于低频峰宽度；自发辐射动力学振荡频率由演化谱双峰频率差决定，而振荡衰减快慢主要由演化谱峰宽决定；距离越近，相干振荡越强；2个跃迁频率越接近自发辐射增强谱峰值频率，演化谱峰高越低，尤其是低频峰，相干振荡越弱.     

激光极化方向对氢负离子在金属面附近光剥离的调控研究

利用闭合轨道理论,研究了激光极化方向对氢负离子在金属面附近光剥离截面的影响。结果表明,当激光极化方向和金属面垂直时,光剥离截面的振荡振幅最大;当激光极化方向和金属面平行时,光剥离截面的振荡几乎消失。当激光极化沿其它方向时,光剥离截面的振幅在两种极限值之间变化。因此,可以通过改变激光的极化方向对负离子体系的光剥离截面进行调控研究。计算结果对于研究负离子体系在表面附近的光剥离问题具有一定的参考价值。     

带电粒子在平行电场和磁场及弹性界面附近闭合轨道的模拟

从哈密顿正则方程出发，推导了带电粒子在平行电场和磁场中的运动方程，并讨论了在平行电场和磁场中存在两个弹性界面时粒子的闭合轨道形成的条件，最后借助于计算机编程，对带电粒子的一些闭合轨道进行了模拟．     

氢负离子的光剥离研究

利用半经典闭合轨道理论研究了氢负离子在矩形微腔中的光剥离,推导出了体系的光剥离截面,利用反射定律,找出该体系下剥离电子运动的闭合轨道.光剥离截面中的振荡是由与闭合轨道相联系的返回波与波源函数发生干涉引起的,而闭合轨道的长度和数目与矩形腔尺寸有关,所以通过改变矩形腔尺寸可以来调控光剥离截面.同时,麻志君用量子力学方法讨论了矩形腔的光剥离,本文的计算结果与其进行了详细比较分析,希望为量子力学方法和半经典理论研究光剥离问题的正确性提供一定依据,对于实验研究负离子在外腔中的光剥离提供参考.     

Light interference from single atoms and their mirror images

A single atom emitting single photons is a fundamental source of light. But the characteristics of this light depend strongly on the environment of the atom. For example, if an atom is placed between two mirrors, both the total rate and the spectral composition of the spontaneous emission can be modified. Such effects have been observed using various systems: molecules deposited on mirrors, dye molecules in an optical cavity, an atom beam traversing a two-mirror optical resonator, single atoms traversing a microwave cavity and a single trapped electron. A related and equally fundamental phenomenon is the optical interaction between two atoms of the same kind when their separation is comparable to their emission wavelength. In this situation, light emitted by one atom may be reabsorbed by the other, leading to cooperative processes in the emission. Here we observe these phenomena with high visibility by using one or two single atom(s), a collimating lens and a mirror, and by recording the individual photons scattered by the atom(s). Our experiments highlight the intimate connection between one-atom and two-atom effects, and allow their continuous observation using the same apparatus.     

Cavity cooling of a single atom

All conventional methods to laser-cool atoms rely on repeated cycles of optical pumping and spontaneous emission of a photon by the atom. Spontaneous emission in a random direction provides the dissipative mechanism required to remove entropy from the atom. However, alternative cooling methods have been proposed for a single atom strongly coupled to a high-finesse cavity; the role of spontaneous emission is replaced by the escape of a photon from the cavity. Application of such cooling schemes would improve the performance of atom-cavity systems for quantum information processing. Furthermore, as cavity cooling does not rely on spontaneous emission, it can be applied to systems that cannot be laser-cooled by conventional methods; these include molecules (which do not have a closed transition) and collective excitations of Bose condensates, which are destroyed by randomly directed recoil kicks. Here we demonstrate cavity cooling of single rubidium atoms stored in an intracavity dipole trap. The cooling mechanism results in extended storage times and improved localization of atoms. We estimate that the observed cooling rate is at least five times larger than that produced by free-space cooling methods, for comparable excitation of the atom.     

Measurement of the internal state of a single atom without energy exchange

A measurement necessarily changes the quantum state being measured, a phenomenon known as back-action. Real measurements, however, almost always cause a much stronger back-action than is required by the laws of quantum mechanics. Quantum non-demolition measurements have been devised that keep the additional back-action entirely within observables other than the one being measured. However, this back-action on other observables often imposes its own constraints. In particular, free-space optical detection methods for single atoms and ions (such as the shelving technique, a sensitive and well-developed method) inevitably require spontaneous scattering, even in the dispersive regime. This causes irreversible energy exchange (heating), which is a limitation in atom-based quantum information processing, where it obviates straightforward reuse of the qubit. No such energy exchange is required by quantum mechanics. Here we experimentally demonstrate optical detection of an atomic qubit with significantly less than one spontaneous scattering event. We measure the transmission and reflection of an optical cavity containing the atom. In addition to the qubit detection itself, we quantitatively measure how much spontaneous scattering has occurred. This allows us to relate the information gained to the amount of spontaneous emission, and we obtain a detection error below 10 per cent while scattering less than 0.2 photons on average. Furthermore, we perform a quantum Zeno-type experiment to quantify the measurement back-action, and find that every incident photon leads to an almost complete state collapse. Together, these results constitute a full experimental characterization of a quantum measurement in the 'energy exchange-free' regime below a single spontaneous emission event. Besides its fundamental interest, this approach could significantly simplify proposed neutral-atom quantum computation schemes, and may enable sensitive detection of molecules and atoms lacking closed transitions.     

Bi熔体粘滞性的非连续变化现象

金属熔体的粘滞性是液态金属原子迁移能力的一种表现，反映了原子间结合力的大小，是重要的熔体敏感物理性质之一，从中能得到许多关于液态结构的信息，同时也是重要的铸造工艺参数。Bi熔体的粘度一温度曲线关系表明，粘度随着温度的降低而增加，但并不连续，计算可知，高温区域的粘流活化能E值最小，低温区域的最大，中温区域的处于两者之间。随温度升高，流团尺寸vm在减小。结合DSC曲线分析认为，粘度异常变化区域是Bi熔体由不均匀向均匀原子结构非连续变化所引起的，与熔体中原子健的转变密切相关。     

由量子场驱动的双原子发射谱

给出了与单模场相互作用的两个不同二能级原子发射谱的精确表达式．研究了系统初始状态对原子发射谱的影响,并得出了系统出现相干捕获的条件。     

功能化氮化硼纳米管磁性的理论研究

本文采用密度泛函理论计算研究了氢化/氟化BN纳米管的自发磁化现象.研究结果表明：单个H原子/F原子倾向于吸附在B位,且均能够诱导自发磁化.两个H原子/F原子吸附在B位上时也可以使体系发生自发磁化,而具有磁性的氢化BN纳米管是一种亚稳结构.与此相反,具有磁性的氟化BN纳米管是稳定结构,其磁矩随着覆盖率的增加而增大,其电子学性质与氟原子在BN纳米管表面的覆盖结构有关.因此,具有磁性的氟化BN纳米管比氢化BN纳米管在实验上更容易实现.可以预见,磁性氟化BN纳米管将在自旋电子器件的制作中发挥巨大的应用价值.     

CO吸附于PtRu合金(100)面上的DFT理论研究

采用DFT方法对CO吸附在PtRu(100)表面的吸附行为进行系统性的研究,分析了键参数及电子结构．结果表明Ru的加入确实能削弱CO在合金上的吸附并且活化CO分子．当Ru的含量达50％时,PtRu催化剂抗CO中毒能力最强．另外CO分子在合金表面的吸附还和其吸附位及表面原子排布有关,CO吸附在Pt上较吸附于Ru上更强,吸附位周围分布Ru较分布Pt更促进CO分子的活化．