里德堡钾原子的布居数在受激拉曼绝热通道中的相干迁移

本文用含时多态展开方法研究了在两束部分重叠的脉冲场驱动下布居数在钾原子量子态中的相干迁移。结果表明，布居数能否实现完全迁移取决于两个脉冲的持续时间和重叠程度，当脉冲的持续时间和重叠合适时，将发生布居数的完全迁移，并形成布居数囚禁。     

耗散对受激拉曼绝热转移过程的影响

应用最近发展的量子耗散理论 ,研究了耗散对简单三能级体系的受激拉曼绝热转移过程的影响 ,并与pump dump过程比较 .计算结果表明 ,受激拉曼绝热转移的机制能很好地抑制中间态的弛豫与涨落的影响 .数值结果也表明了新的量子耗散理论可以正确地描述场与耗散相互耦合的动力学问题     

非旋波近似下Λ型三能级原子与相干态光场的量子纠缠

在非旋波近似下,利用相干态正交化展开方法,对相干态光场与Λ型三能级原子相互作用的量子场熵进行了精确求解.利用量子熵理论讨论了耦合强度、平均光子数以及初始时刻原子处于不同的能级对量子纠缠的影响.数值计算的结果表明:当初始时刻原子处于激发态时,量子纠缠在较短的时间内就能演化到最大值,随着平均光子数的增大,纠缠演化的周期性逐渐明显;原子初始时刻处于三个能级的叠加态会使初始阶段量子纠缠显著降低;与旋波近似下的结果不同的是,随着耦合强度以及平均光子数的增加,非旋波项的贡献显著增强,使得量子纠缠演化曲线出现小锯齿状的振荡.     

倒Y型四能级系统中自发辐射诱导相干对弱探测场的色散和吸收特性的影响

本文研究了倒Y型四能级系统中自发辐射诱导相干对探测场的色散和吸收特性的影响。在稳态条件下利用密度矩阵微扰理论推导出了密度矩阵的迭代解。数值分析了自发辐射诱导相干对线性和非线性折射率和吸收系数的影响,分析了泵浦场和耦合场偏离双光子共振对弱探测场色散和吸收特性的影响。研究发现自发辐射诱导相干使弱探测光的线性和非线性折射率增强,同时形成一个较宽的透明窗。在没有自发辐射诱导相干的情况下,泵浦场和耦合场偏离双光子共振对探测场的线性和非线性折射率影响明显,在有自发辐射诱导相干的情况下,泵浦场和耦合场偏离双光子共振使得探测场的线性透明窗变窄,非线性吸收增加。     

在多能级∧系统中运用“部分受激拉曼绝热通道”制备相干叠加态(英文)

我们给出了在A型五能级系统中制备| 3态和| 5态的连续叠加态的方法,这种方法是部分受激拉曼绝热通道的扩充。但是它又不同于部分受激拉曼绝热通道,我们设计了新的斯托克斯脉冲和相应的泵浦脉冲。进而应用数值方法讨论了脉冲驰豫时间和失谐量对该方法的影响.数值结果表明要制备任意的叠加态要满足双光子共振.这种方法可以推广到多能级A系统中制备任意两个下面能级的叠加态.     

库仑势对抛物量子点量子比特消相干的影响

采用Pekar类型的变分方法,在电子与体纵光学声子强耦合的条件下,计算得出了抛物量子点中电子的基态能量和第一激发态能量及其相应的本征波函数.量子点中这样的二能级体系可以作为一个量子比特.由于声子的自发辐射,造成量子比特的消相干,讨论了消相干时间与库仑结合参数,耦合强度,受限长度,色散系数的变化关系. 关键词： 量子点 量子比特 量子信息 消相干     

准一维混合自旋(1/2,5/2)Ising-XXZ模型的量子相干和互信息

准一维混合自旋(1/2, 5/2) Ising-XXZ模型可以用来研究一些材料(如异质三金属化合物Fe-Mn-Cu)的磁性质,该研究也有助于这类材料在量子信息等领域的应用.本文利用转移矩阵法计算了该模型的量子相干和互信息,讨论了Ising作用、温度和磁场对其的影响.结果表明,在极低温度下随Ising作用的增强量子相干逐渐减小,而互信息在各向同性系统中存在一个极小值,在各向异性系统(?=4)中存在多个极小值.进一步研究发现,量子相干和互信息在量子临界点存在突变,其一阶导数在该点存在奇异行为.还研究了有限温度下的量子相干和互信息,当磁场较弱时,两者随温度的升高单调减小;当磁场较强时,热涨落与磁场的竞争使得两者随温度的升高先增大后减小.相比于量子互信息,量子相干存在于更大的磁场和温度范围内,有利于在实验中对其进行调控.     

真空诱导相干对电磁感应左手效应的影响

研究了具有超精细结构的四能级原子系统在电磁感应下的左手效应.讨论了由交叉耦合自发辐射路径引起的真空诱导相干(VIC)对左手效应的影响.研究表明,在出现左手效应的频率区间VIC效应强弱对介质相对介电常数和相对磁导率实部取值有显著影响,介质的左手效应随VIC效应的增强而增强. 关键词： 量子干涉 真空诱导相干 电磁感应 左手效应     

基于绝热快速通道控制原子布居数及其相干性的研究

本文在理论上研究了纳秒数量级内，运用绝热快速通道（ARP）来控制基态原子的相干性及 其布居数在不同能态间的转移.理论计算结果表明抽运激光脉冲相对斯托克斯（Stokes）脉 冲延迟时间的微小改变会导致原子相干性及其布居数的显著变化.运用密度矩阵方程和麦克 斯韦方程联合的自洽方程数值解，我们找到了实现原子最大相干性及其布居数在不同能态间 完全转移的条件. 关键词： 绝热快速通道 布居数 原子相干     

Population transfer via adiabatic passage in the Rydberg potassium atom

Using the time-dependent multilevel approach, we have calculated the coherent population transfer between the quantum states of potassium atom by a single frequency-chirped laser pulse. The result shows that a pair of sequential `broadband' frequency-chirped laser pulses can efficiently transfer population from the initial state of the ladder system to the target state. It is also found that the population can be efficiently transferred to a target state and trapped there by using an `intuitive' or a `counterintuitive' frequency sweep laser pulse in the case of `narrowband' frequency-chirped laser pulse. Our research shows that the complete population transfer is related to the pulse duration, chirp rate, and amplitude of the laser pulse.     

Population coherent control of Rydberg potassium atom via adiabatic passage

The time-dependent multilevel approach(TDMA) and B-spline expansion technique are used to study the coherent population transfer between the quantum states of a potassium atom by a single frequency-chirped microwave pulse.The Rydberg potassium atom energy levels of n=6-15,l=0-5 states in zero field are calculated and the results are in good agreement with other theoretical values.The time evolutions of the population transfer of the six states from n=70 to n=75 in different microwave fields are obtained.The results show that the coherent control of the population transfer from the lower states to the higher ones can be accomplished by optimizing the microwave pulse parameters.     

Population and coherence transfer induced by double frequency sweeps in half-integer quadrupolar spin systems

We have recently shown that the sensitivity of single- and multiple-quantum NMR experiments of half-integer (N/2) quadrupolar nuclei can be increased significantly by introducing so-called double frequency sweeps (DFS) in various pulse schemes. These sweeps consist of two sidebands generated by an amplitude modulation of the RF carrier. Using a time-dependent amplitude modulation the sidebands can be swept through a certain frequency range. Inspired by the work of Vega and Naor (J. Chem. Phys. 75, 75 (1981)), this is used to manipulate +/-(m - 1) <--> +/-m (3/2 < or = m < or = N/2) satellite transitions in half-integer spin systems simultaneously. For (23)Na (I = 3/2) and (27)Al (I = 5/2) spins in single crystals it proved possible to transfer the populations of the outer +/-m spin levels to the inner +/-1/2 spin levels. A detailed analysis shows that the efficiency of this process is a function of the adiabaticity with which the various spin transitions are passed during the sweep. In powders these sweep parameters have to be optimized to satisfy the appropriate conditions for a maximum of spins in the powder distribution. The effects of sweep rate, sweep range, and RF field strength are investigated both numerically and experimentally. Using a DFS as a preparation period leads to significantly enhanced central transition powder spectra under both static and MAS conditions, compared to single pulse excitation. DFSs prove to be very efficient tools not only for population transfer, but also for coherence transfer. This can be exploited for the multiple- to single-quantum transfer in MQMAS experiments. It is demonstrated, theoretically and experimentally, that DFSs are capable of transferring both quintuple-quantum and triple-quantum coherence into single-quantum coherence in I = 5/2 spin systems. This leads to a significant enhancement in signal-to-noise ratio and strongly reduces the RF power requirement compared to pulsed MQMAS experiments, thus extending their applicability. This is demonstrated by (27)Al 3QMAS experiments on 9Al(2)O(3). 2B(2)O(3) and the mineral andalusite. In the latter compound, Al experiences a quadrupolar-coupling constant of 15.3 MHz in one of the sites. Finally a 5QMAS spectrum on 9Al(2)O(3). 2B(2)O(3) demonstrates the sensitivity enhancement of this experiment using a double frequency sweep.     

Quantum state transfer via adiabatic passage

Based on adiabatic passage, we propose a scheme for implementing the quantum transfer of an unknown atomic state. In our scheme, we utilize photons for ideal quantum transmission between two cavities with the successful probability being about 1. Meanwhile, the scheme is robust against the effects of atomic spontaneous emission. It may be useful for transferring quantum information among spatially distant atoms.     

实现粒子布居高效转移的两种激光脉冲时序方案的理论研究

基于最近实验工作的结果(2010 Nat.Phys.6 265)即Danzl等在五能级M型级联系统中分别利用连续型和四光子型受激拉曼绝热通道(stimulated Raman adiabatic passage,STIRAP)实现了将Feshbach态上弱束缚的Cs2有效转移到其振转基态,本文理论研究了两种STIRAP方案实施的基本条件,解析推导系统的准暗态、绝热参数的具体形式并分析其存在的必要性,详细讨论布居转移效率对相关参量的依赖关系.通过比较激光脉冲的时序、中间能级的失谐量和自发辐射率、光场脉冲的幅值等诸多参量的不同影响,讨论方案各自的优缺点,找到了参量优化的方法以实现最高效的粒子布居数转移.与前人的实验结果相比,本文研究表明,实验观测值(约0.60)均低于理论预估最佳值(约0.97)的主要原因是受限于激发态能级的自发辐射率过大.该理论方案还可用于制备量子纠缠态,在量子逻辑门操控、量子信息传输等领域都有潜在的应用.     

Quantum coherence in A-three-level system driven by bichromatic coupling field

<正>We present a theoretical study of quantum coherent effects in a A-three-level system with a strong bichromatic coupling field and a weak probe field.When one component of the strong bichromatic coupling field is resonant with a corresponding transition and the other is detuning with an integer fraction of the Rabi frequency of the resonant field, the absorption spectrum exhibits a series of symmetrical doublets.While two frequencies of the strong bichromatic coupling field are symmetrically detuned from the transition,the position and the relative intensity of the absorption peak are both affected by the coupling field intensity and detuning.An explanation of the spectrum is given in term of the dressed-state formalism.     

Population transfer of sodium in a single analytical laser pulse

The population transfer of sodium in a single analytical laser pulse was studied in three models:two-level sodium,three-level sodium and many-level sodium.The effect of a third state on a two-level system was studied by investigating a ladder three-level system.Two effects were found in the vicinity of the resonance frequency.     

Quantum coherence in λ-three-level system driven by bichromatic coupling field

We present a theoretical study of quantum coherent effects in a λ-three-level system with a strong bichromatic coupling field and a weak probe field. When one component of the strong bichromatic coupling field is resonant with a corresponding transition and the other is detuning with an integer fraction of the Rabi frequency of the resonant field, the absorption spectrum exhibits a series of symmetrical doublets. While two frequencies of the strong bichromatic coupling field are symmetrically detuned from the transition, the position and the relative intensity of the absorption peak are both affected by the coupling field intensity and detuning. An explanation of the spectrum is given in term of the dressed-state formalism.