InAs量子环中类氢杂质能级

在有效质量近似下,利用微扰法研究了InAs量子环内类氢杂质基态及低激发态的能级分布。受限势采用抛物形势,在二维平面极坐标下,用薛定谔方程的精确解析解进行计算。数值计算结果显示,电子能级敏感地依赖于量子环半径,能级存在极小值,这是由于限制势采用抛物势的结果。如果减小环的半径,可以增加能级间距。第一激发态类氢杂质能级的简并没有消除,n≥2时简并的能级发生分裂并且间距随半径的增大而增大。电子能级间距还敏感地依赖于角频率并随角频率的增大而增大。第一激发态的简并没有消除,第二激发态的简并被部分地消除。在计算InAs量子环中类氢杂质的基态和低激发态的能级时,角频率改变的影响也是很深刻的。文章结果对研究量子环的光跃迁及光谱结构有重要指导意义。     

有限深抛物势量子环类氢杂质能级

在有效质量近似下,用微扰法研究InAs量子环内类氢杂质基态及低激发态的能级.受限势采用有限深抛物型势,在二维平面极坐标下,用薛定谔方程的解析解计算.数值结果显示:在抛物势平台区,类氢杂质能级不随电子径向坐标改变,并具有二维氢原子能级的特征;在有限深抛物势区,电子能级敏感地依赖于量子环半径,能级存在极小值,这是由于限制势采用抛物势的结果.如果减小环的半径,可以增加能级间距;简并能级发生分裂并且间距随半径增大而增大,第一激发态的简并没有消除,第二激发态的简并被部分地消除.本文结果对研究量子环的光跃迁及光谱结构有指导意义.     

激子自旋弛豫对简并量子点发射光子对纠缠度的影响

利用粒子数运动方程和量子回归理论，计算了单个半导体量子点双激子体系脉冲激发下粒子在各能级间辐射跃迁的二阶交叉相关函数以及系统发射光子对的偏振密度矩阵.分析了激子态能级简并量子点体系发射光子对偏振纠缠特性，讨论了纠缠度随激子态间自旋弛豫的变化关系.研究表明，激子自旋弛豫会破坏该系统发射光子对的纠缠度. 关键词： 纠缠光子对 半导体量子点 二阶相关函数     

微波场驱动下V型三能级原子的吸收和色散特性

当用微波场作用到V型三能级原子的两个激发态能级时,系统跃迁路径之间发生交叉耦合导致了量子相干效应.通过调节微波场的强度,可实现对原子吸收和色散性质的改变,并呈现零吸收高折射率现象.此外,微波场诱导的量子相干也可实现相对相位对探测光增益的控制.     

微波场驱动下V型三能级原子的吸收和色散特性

当用微波场作用到V型三能级原子的两个激发态能级时,系统跃迁路径之间发生交叉耦合导致了量子相干效应.通过调节微波场的强度,可实现对原子吸收和色散性质的改变,并呈现零吸收高折射率现象.此外,微波场诱导的量子相干也可实现相对相位对探测光增益的控制.     

超短激光脉冲在多能级系统中的传输特性

运用数值方法研究了超短激光脉冲在多能级系统中的传输特性.当激光脉冲能量相对较弱时,适当的脉冲面积可以实现自感应透明;激光脉冲能量逐渐增强,脉冲将产生分裂.脉冲分裂的基本面积（即脉冲实现稳定传播的最小面积）与二能级系统不同,而是A₀=2π/κ,与激发态的能级个数和跃迁偶极矩密切相关.由于分裂和吸收,超短激光脉冲传输了一段距离以后,将演化为一个脉冲宽度更窄、峰值强度更高且面积为A₀的超短激光脉冲. 关键词： 超短激光脉冲 多能级系统 特征时间     

引入量子干涉的双光子共振非简并四波混频

研究了级联四能级系统中双光子共振非简并四波混频(NFWM)由于加入耦合光场而产生的量子干涉效应，发现在强耦合场作用下NFWM频谱呈现AT劈裂，它反映的是两个缀饰态的能级.量子干涉还使NFWM信号被抑制或增强.此方法牵涉到三光子共振，因此可以成为研究原子或分子高激发态的有效光谱学工具. 关键词： 四波混频 量子干涉     

简并能级系统中的量子相干效应与暗态

文章分析了简并二能级原子系统Fg=1→Fe=0中的吸收、色散特性与暗态的对应关系,比较了双Λ型和Λ型能级结构下的量子相干效应.结果表明,双Λ型系统中多个基态相干性会增强原子相干特性.     

通过双Σ型激光方案控制振动态布居转移

通过使用含时量子波包方法,对HF分子该类型的布居转移动力学进行了研究. 提出双Σ型激光控制方案,用于控制布居从|v=16>能级向|v=0>能级进行转移. 该方案的第一步是将布居从|v=16>经过中间能级|v=11>向|v=7>转移,第二步是将布居从|v=7>经过中间能级|v=3>向|v=0>转移. 在每一个步骤中,三个振动能级在两束重叠的激光脉冲作用下形成一个Σ型的布居转移路径. 通过优化激光的强度、频率和延迟时间,得到了最大的布居转移效率. 计算并比较了正序脉冲和反序脉冲两种情况,在这两种情况下,布居都可以超过90%从|v=16>能级向|v=0>能级进行转移.     

探测功率展宽效应引起的电磁诱导透明向电磁诱导吸收的转化

本文对准L型四能级系统中探测功率展宽效应引起的非线性效应进行了理论研究.准L型四能级系统包括三个基态精细结构能级和一个激发态能级,除光学耦合场和探测场分别激励一个基态精细结构能级和激发态能级之间的跃迁外,附加了一个射频驱动场作用于其中一个基态精细结构能级和另一个新的基态精细结构能级之间,并通过与耦合场驱动共同能级建立量子相关性.研究结果表明,在射频驱动场的辅助激励下,探测功率展宽效应不仅可以使EIT的线宽增宽,还能引起吸收曲线中的类色散特性,使EIT最终变化为EIA.     

CdSe/ZnS核-壳结构量子点的非线性光学吸收

利用开孔Z扫描技术研究了吸收峰分别为553 nm和503 nm的两种尺寸CdSe/ZnS核-壳结构量子点溶液的非线性吸收性质.对于532 nm,6 ns激光脉冲,两种材料均表现出饱和吸收向反饱和吸收转化的现象.数值模拟结果表明：当吸收峰波长大于激光波长时,饱和吸收过程由快、慢两种机制组成,分别对应基态载流子被激发至不同的激发态,而强光下的反饱和吸收与快过程相关;当吸收峰波长小于激光波长时,饱和吸收主要由快过程机制引起,强光下的反饱和吸收源自激发态吸收和双光子吸收.我们的研究结果表明半导体量子点是研制光开关和光限制器件的理想候选材料.     

Laser-induced operations with charge qubits in a double-well nanostructure

We present the results of theoretical studies on operations with charge qubits in the system composed of two tunnel-coupled semiconductor quantum dots whose two lowest states (localized in different dots) define the logical qubit states while two excited states (delocalized between the dots) serve for the electron transfer from one dot to another under the influence of the laser pulse. It is shown that in the case of small energy separation between the excited levels, the optimal (from the viewpoint of minimal single-qubit operation time and maximum operation fidelity) strategy is to tune the laser frequency between the excited levels. The pulse parameters for implementation of the quantum NOT operation are determined. Analytical results obtained in the rotating-wave approximation are confirmed by rigorous numerical calculations.     

Transient intraband light absorption by quantum dots: Pump-probe spectroscopy

We have developed a theory of a transient intraband light absorption by semiconductor quantum dots. This absorption plays an important role in the two-pulse pump-probe method, which enables determining the energy relaxation rates of electron-hole excited states. We have considered all possible schemes of this process wherein the carrier frequency of optical pump pulses is close to the resonance with the interband transition of the quantum-dot electronic subsystem, while the carrier frequency of probe pulses is resonant to the intraband transition. For ensembles of identical and size-distributed quantum dots, the probe pulse energy absorption induced by the pump pulse is analyzed in relation to the delay time between the pulses. We have found that, under certain conditions, this dependence can be described by a single, two, or three exponentials. The exponents of the exponentials are proportional to the energy relaxation rates of electron-hole excited states.     

Large index of refraction without absorption via decay-induced coherence in a three-level V system

We study the effects of quantum interference from spontaneous emission on the dispersion-absorption properties in a three-level V-type atomic system with two near-degenerate excited levels. We found that due to the quantum interference between two spontaneous decay channels, large index of refraction without absorption always can be obtained just by choosing proper values of the relative phase between the two applied fields.Received: 27 February 2004, Published online: 26 May 2004PACS: 42.50.Gy Effects of atomic coherence on propagation, absorption, and amplification of light; electromagnetically induced transparency and absorption - 42.50.Hz Strong-field excitation of optical transitions in quantum systems; multiphoton processes; dynamic Stark shift     

Evolution of photoelectron angular distributions in a train of identical,circularly polarized few-cycle laser pulses

Photoelectron angular distribution (PAD) of atoms irradiated by a train of identical, circularly polarized few-cycle laser pulses is studied in the frame of a nonperturbative scattering theory. Our study shows that the PADs vary with the kinetic energy of photoelectron, the carrier-envelope phase, and the pulse duration. We find that along with increasing of the kinetic energy of photoelectron or with decreasing of the pulse duration or the both, the original one maximum of PAD splits into two maxima; the newly produced two maxima evolve to the opposite pole of the symmetric axis, and finally incorporate as a new maximum located in the symmetric axis.Received: 26 February 2004, Published online: 10 August 2004PACS: 32.80.Rm Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states) - 42.50.Hz Strong-field excitation of optical transitions in quantum systems; multiphoton processes; dynamic Stark shift - 42.65.Re Ultrafast processes; optical pulse generation and pulse compression     

Entanglement and Properties of Composite Quantum Systems: A Conceptual and Mathematical Analysis

Various topics concerning the entanglement of composite quantum systems are considered with particular emphasis concerning the strict relations of such a problem with the one of attributing objective properties to the constituents. Most of the paper deals with composite systems in pure states. After a detailed discussion and a precise formal analysis of the case of systems of distinguishable particles, the problems of entanglement and the one of the properties of subsystems of systems of identical particles are thoroughly discussed. This part is the most interesting and new and it focuses in all details various subtle questions which have never been adequately discussed in the literature. Some inappropriate assertions which appeared in recent papers are analyzed. The relations of the main subject of the paper with the nonlocal aspects of quantum mechanics, as well as with the possibility of deriving Bell's inequality are also considered.     

Creation and manipulation of coherences in molecules with a pulsed magnetic field

We demonstrate the application of a pulsed magnetic field for the creation and manipulation of coherences in molecular systems, using quantum beat spectroscopy for the detection of the dynamics of the molecular superposition states. In all cases, the experiments are performed on energy levels in electronically excited states of the (jet-cooled) CS₂ molecule populated by a short laser pulse. In the basic experiment, following excitation of initially degenerate Zeeman sublevels under zero field conditions with suitable laser polarization, quantum beats are generated at the moment the magnetic field is switched on, even when the field is delayed by several excited state lifetimes. By quenching of the field, it is shown that the molecule may be “frozen” in any superposition state of the participating sublevels. Using a combination of static and pulsed fields with different orientations, the molecule can be prepared in a more general state, described by coherences among all Zeeman substrates. This is achieved by choosing an appropriate time delay for the switched field, without any change to the geometrical parameters of the experiment such as laser polarization or detection direction. Numerical simulations of these dynamical coherence phenomena have been performed to support assignment and interpretation of the experimental results. Received: 8 April 1998 / Accepted: 3 June 1998     

Coherent manipulation of electronic States in a double quantum dot

We investigate coherent time evolution of charge states (pseudospin qubit) in a semiconductor double quantum dot. This fully tunable qubit is manipulated with a high-speed voltage pulse that controls the energy and decoherence of the system. Coherent oscillations of the qubit are observed for several combinations of many-body ground and excited states of the quantum dots. Possible decoherence mechanisms in the present device are also discussed.     

Classical Mechanics Is not the ħ, → 0 Limit of Quantum Mechanics

Both the set of quantum states and the set of classical states described by symplectic tomographic probability distributions (tomograms) are studied. It is shown that the sets have a common part but there exist tomograms of classical states which are not admissible in quantum mechanics and, vice versa, there exist tomograms of quantum states which are not admissible in classical mechanics. The role of different transformations of reference frames in the phase space of classical and quantum systems (scaling and rotation) determining the admissibility of tomograms as well as the role of quantum uncertainty relations are elucidated. The union of all admissible tomograms of both quantum and classical states is discussed in the context of interaction of quantum and classical systems. Negative probabilities in classical and quantum mechanics corresponding to tomograms of classical and quantum states are compared with properties of nonpositive and nonnegative density operators, respectively. The role of the semigroup of scaling transforms of the Planck's constant is discussed.