弱场条件下两囚禁离子质心量子态运动

用微扰的方法计算了一个外加周期驱动激光场所组成的,两离子系统质心量子态跃迁几率的解析表达式.以两Ca+作为囚禁离子,模拟出了在弱场的作用下声子态跃迁几率随激光束频偏和相互作用时间变化的分布图形.结果表明:随着外加周期驱动激光场的频率和相互作用时间的调节,系统质心量子态随时间周期性的坍塌与复原;在频偏范围0.85×107...     

弱场条件下两囚禁离子质心量子态运动

用微扰的方法计算了一个外加周期驱动激光场所组成的,两离子系统质心量子态跃迁几率的解析表达式.以两Ca+作为囚禁离子,模拟出了在弱场的作用下声子态跃迁几率随激光束频偏和相互作用时间变化的分布图形.结果表明:随着外加周期驱动激光场的频率和相互作用时间的调节,系统质心量子态随时间周期性的坍塌与复原|在频偏范围0.85×107~1.2×107 Hz内,质心量子态跃迁几率最高能达到0.97,操控周期最长为0.6×10-6 s,从理论上质心量子态操控周期有很大的缩短|若频偏值大于1.2×107 Hz时,质心量子态跃迁几率约为0.49,激光不能有效控制质心量子态的跃迁.所得结论对实现两比特量子逻辑门等实验研究有一定的参考意义.     

多组分纠缠在光子晶体中的存储

研究了激子的多组分纠缠相干态保真度在各向异性光子晶体中的演化行为.结果表明,当激子的跃迁频率处于光子晶体带隙时,保真度随时间变化作周期振荡,这与激子处于真空环境时,保真度振荡衰减的演化行为不同.此外,当激子跃迁频率离光子晶体带边较远时,其多组分纠缠相干态越容易被保存.     

利用基频与倍频脉冲控制扩展“ladder”式跃迁

利用含时量子波包动力学方法研究了两束基频与倍频脉冲控制下的扩展"ladder"式跃迁.通过基频与2倍频脉冲控制分子布居从|0,0态跃迁至|5,0与|5,2态;基频与3倍频脉冲控制分子布居从|0,0态跃迁至|5,3?与|6,2态.计算结果表明,利用两束基频与倍频脉冲,通过"ladder"式跃迁,可以得到近100%的布居跃迁概率.两束脉冲间的相对相位角影响分子的布居分布.当脉冲频率比为1:2时,布居以π为周期变化;当脉冲频率比为1:3时,布居以2π为周期变化.     

室温下正己烷本底中PbSe量子点的荧光寿命

利用透射电镜、近红外吸收谱、荧光光谱和时间分辨光谱等技术,在室温下测量离散在正己烷有机溶剂中,不同粒径的Pb Se量子点的吸收谱和荧光谱,给出了第一吸收峰和荧光峰随粒径变化的经验公式。通过对瞬态荧光衰减曲线的测量和分析,得到了Pb Se量子点的荧光寿命,其寿命与量子点的表面缺陷有关。在研究的粒径范围内,由于荧光跃迁仅为带间直接跃迁和仅为缺陷态跃迁两种极端情况,可得荧光寿命最宽分布区间位于1.44~11.96μs。荧光寿命弱关联于粒径,并随粒径的增大而呈线性减小。当Pb Se量子点的粒径为2.7~5.7 nm时,其实测的平均荧光寿命为7.17~6.72μs。     

原子光激发过程中的选择性和相干性

通过求解含时Ｓｃｈｒｏｄｉｎｓｅｒ方程，对于原子在激光场的共振和非共振驱动下向各个态的跃迁几率进行了计算和分析，探讨了跃迁几率的时间特性以及随激光参数的变化规律，揭示了光场与原子系统相互作用的相干性和选择性。计算并验证由解析解所预测的激发态粒子数布居的峰值和实现粒子数反转所应满足的条件。     

线型三原子分子在强激光场中多光子激发的李代数方法

使用二次非谐振子模型和Magnus近似方法构造了在强激光场下线型三原子分子的振动哈密顿并推导出时间演化算符的代数形式,在此基础上研究了线型DCN分子的跃迁几率随外场的频率和强度的变化关系,表明了多光子跃迁谱的特点.     

CO（A 1Π～e 3Σ-）混合激发态碰撞传能过程的量子干涉效应———跃迁振幅和相位角的时间特性

从含时微扰的一级波恩近似出发 ,进一步研究了CO(A～e)混合态在以He或HCl为碰撞伴时其跃迁振幅和相位角的时间特性 .计算显示跃迁振幅随时间显现衰减的振荡 .其振荡周期反比于初末态的能量差而衰减时间则决定于碰撞体系的分子间相互作用势能 .对于给定的碰撞参数和碰撞速度 ,CO分子从初始转动态J到末态J′的跃迁可经过许多不同的通道 ,包括CO的不同量子数M和碰撞伴的各种J和M .所以实验的跃迁振幅及其实部和虚部是所有不同通道的平均结果 .当He为碰撞伴时 ,所有的单重态和三重态通道的跃迁振幅矢量都处于复平面的第一象限 ,而当HCl为碰撞伴时 ,则单重态和三重态通道的跃迁振幅矢量分别处在第四和第一象限 ,这就解释了以He为碰撞伴时干涉相位角小于 90°而以HCl为碰撞伴时则大于 90°的实验事实 .     

超短激光脉冲的绝对相位对三能级Λ型原子布居的影响

通过数值求解含时薛定谔方程,研究超短激光脉冲与三能级Λ型原子相互作用过程中影响原子电离态跃迁几率的几个因素,如:绝对相位,啁啾率等.研究结果表明激光绝对相位、啁啾率的大小是关系态跃迁几率的重要因素,通过调节超短激光脉冲的绝对相位和啁啾率,可以将态跃迁几率控制到较大的水平.本文研究结果为实验上对态跃迁几率的控制提供了相关理论基础.     

高阶多量子跃迁的一维直接检测——Raman磁共振方法

研究了异核AX₃自旋体系的初始态为最高阶量子相干态时的Raman磁共振谱.研究表明,唯有制备高阶多量子相干的初始态时才有可能对高阶多量子跃迁进行有效检测.积算符理论的精确性使得理论上准确地预言了Raman磁共振谱中各阶多量子跃迁频率、谱线强度,使得各阶多量子跃迁频率相互交错时仍有可能解析谱图.在Raman磁共振实验中采用了正负频偏交替变化采样的方法,对单量子信号进行压制,使其更有效地观察高阶多量子跃迁.     

Gauging a quantum heat bath with dissipative Landau-Zener transitions

We calculate the exact Landau-Zener transition probabilities for a qubit with an arbitrary linear coupling to a bath at zero temperature. The final quantum state exhibits a peculiar entanglement between the qubit and the bath. In the special case of diagonal coupling, the bath does not influence the transition probability, whatever the speed of the Landau-Zener sweep. It is proposed to use Landau-Zener transitions to determine both the reorganization energy and the integrated spectral density of the bath. Possible applications include circuit QED and molecular nanomagnets.     

Quantum phenomena via complex measure: Holomorphic extension

The complex measure theoretic approach proposed earlier is reviewed and a general version of density matrix as well as conditional density matrix is introduced. The holomorphic extension of the complex measure density (CMD) is identified to be the Wigner distribution function of the conventional quantum mechanical theory. A variety of situations in quantum optical phenomena are discussed within such a holomorphic complex measure theoretic framework. A model of a quantum oscillator in interaction with a bath is analyzed and explicit solution for the CMD of the coordinate as well as the Wigner distribution function is obtained. A brief discussion on the assignment of probability to path history of the test oscillator is provided.     

Dynamics of a spin-boson model with structured spectral density

We report the results of a study of the dynamics of a two-state system coupled to an environment with peaked spectral density. An exact analytical expression for the bath correlation function is obtained. Validity range of various approximations to the correlation function for calculating the population difference of the system is discussed as function of tunneling splitting, oscillator frequency, coupling constant, damping rate and the temperature of the bath. An exact expression for the population difference, for a limited range of parameters, is derived.     

Peculiarities of two-photon impurity absorption spectra in a quantum molecule with a tunnel-transparent potential barrier

The effect of dissipative tunneling parameters on the spectral dependence of the probability of two-photon impurity absorption in a quantum molecule is theoretically investigated. The effect of the tunnel barrier transparency on two-photon impurity absorption in a quantum molecule is shown to manifest itself as a change in the width of energy levels of the virtual and final states due to variations in such parameters of dissipative tunneling as temperature, photon-mode frequency, and the constant of interaction with a heat bath.     

The renormalization group and fractional Brownian motion

We find that in generic field theories the combined effect of fluctuations and interactions leads to a probability distribution function which describes fractional Brownian motion (fBM) and “complex behavior”. To show this we use the renormalization group as a tool to improve perturbative calculations, and check that beyond the classical regime of the field theory (i.e., when no fluctuations are present) the non-linearities drive the probability distribution function of the system away from classical Brownian motion and into a regime which to the lowest order is that of fBM. Our results can be applied to systems away from equilibrium and to dynamical critical phenomena. We illustrate our results with two selected examples: a particle in a heat bath, and the KPZ equation.     

Strong interaction at finite temperature

We review two methods discussed in the literature to determine the effective parameters of strongly interacting particles as they move through a heat bath. The first one is the general method of chiral perturbation theory, which may be readily applied to this problem. The other is the method of thermal QCD sum rules. We show that, when the spectral sides of the sum rules are calculated correctly, they do not lead to any new results, but reproduce those of the vacuum sum rules.     

On the strength distribution of a field created by randomly distributed dipoles

The probability distribution of the strength of a field created by parallel dipoles arranged randomly, on average, uniformly is considered. The problem of divergence at small distances is analyzed. It is shown that, for any, including an arbitrarily small, number of dipoles ε in the cutoff sphere, the average field value is zero. In the case ε ? 1, the field distribution consists of a central part, whose half-width is of an order of the average polarization of a medium, and a very wide and structured background. An interpolation formula for the Fourier image of the field distribution applicable for arbitrary values of ε is proposed. An analogy with the theory of spectral line broadening is established. The connection with the problems of molecular optics and inhomogeneous Stark broadening of spectral lines is discussed.     

Mechanism of electron-positron pair production by high-energy photons in a single crystal

A mechanism of pair production by photons in an electric field of the axes (planes) of a single crystal is discussed. Simple expressions for the process probability and spectral distribution at κ?1 are presented. An analysis of conditions to observe the effect is made.     

Nonequilibrium dynamics of scalar fields in a thermal bath

We study the approach to equilibrium for a scalar field which is coupled to a large thermal bath. Our analysis of the initial value problem is based on Kadanoff-Baym equations which are shown to be equivalent to a stochastic Langevin equation. The interaction with the thermal bath generates a temperature-dependent spectral density, either through decay and inverse decay processes or via Landau damping. In equilibrium, energy density and pressure are determined by the Bose-Einstein distribution function evaluated at a complex quasi-particle pole. The time evolution of the statistical propagator is compared with solutions of the Boltzmann equations for particles as well as quasi-particles. The dependence on initial conditions and the range of validity of the Boltzmann approximation are determined.