Implementation of a Two-Qubit C-NOT Quantum Gate in a System of Two Double Quantum Dots,a Microcavity,and a Laser

We study the possibility of implementing a two-qubit controlled-NOT gate operation in a structure consisting of two semiconductor double quantum dots placed in a high-Q optical microcavity and governed by a resonant laser field. The effect of relaxation processes on the dynamics of the two-electron system is discussed. The dissipation rates allowing quantum error correction algorithms to be used are determined. The existing additional excitation channel (laser pulse) is shown to weaken the effect of the nonideality of the cavity on the evolution of the states. Optimal electron coupling coefficients in the quantum dots with control fields are fitted, in which the controlled qubit is switched with the highest probability and the gate implementation takes several hundreds of picoseconds.     

Photon properties of light in semiconductor microcavities

Properties of atom-like emitters in cavities are successfully described by cavity quantum electrodynamics(cavity-QED).In this work,we focus on the issue of the steady-state and spectral properties of the light emitted by a driven microcavity containing a quantum well (QW) with the excitonic interactions using simulation of fully quantum-mechanical treatment.The system is coherently pumped with laser,and it is found that depending on the relative values of pumping rate of stimulated emission,either one or two peaks close to the excitation energy of the QW or to the natural frequency of the cavity are shown in the emission spectrum.Furthermore,the nonclassical proprieties of the emitted photon have been investigated.This excitonic system presents several dynamical and statistical similarities to the atomic system,in particular for the bad-cavity and good-cavity limits.The results show that the photon emission can be significantly amplified due to the coupling strength between a single emitter and radiation field in the microcavity,and it is concluded that the present semiconductor microcavity system may serve as a QW laser with low threshold.     

利用非Bell基测量实现三粒子W态的隐形传送

研究非Bell基测量的未知三粒子W态的隐形传态方案.先利用原子和腔场的共振相互作用制备未知三原子W态.接着,利用两原子同时在强经典驱动场驱动下和单模腔场发生相互作用,产生态的演化,来制备EPR对量子信道.最后利用和制备量子信道同样的方法来实现未知三原子W态的隐形传送.在传送过程中,可忽略腔场热作用和腔延迟作用的影响.传送成功的总几率为1.     

Quantum operations on charge qubits with the electrostatic control in semiconductor cavities

We consider one- and two-qubit operations on charge qubits that represent double quantum dots with one electron in each of them. The dots are formed inside a high-Q semiconductor cavity (disk, toroidal, or spherical) in the antinodes of one of its optical eigenmodes; the frequencies of the transitions between the ground (logic) and excited (auxiliary) states of the discrete electron spectrum in quantum dots are close to the frequency of this mode. The precise tuning of the transition frequency is performed by applying an electric potential on the qubit control gate. Within the model of qubits coherently interacting with a cavity quantum field, a few methods for controlling their states are developed. In particular, we propose different variants for implementing two-qubit CNOT and CZ gates and generating qubit entangled states. By the example of a micro-disk cavity, we calculate the operating characteristics that ensure a high rate of quantum gate implementation.     

含介质的边界振动的微腔中薛定谔猫态的产生

采用全量子理论，考虑了边界振动的含克尔介质的微腔（振动边界视为频率为 的量子谐振子）与单模辐射场构成的系统，给出了系统的时间演化算符及其变换，得到了系统的态函数随时间的演化关系.讨论了当振动边界回到初态时的腔场态，得到了许多与已有文献不同的薛定谔猫态，这对于研究猫态的性质和应用是十分有意义的.     

双原子非简并拉曼过程中原子与腔场的动力学

本文研究两原子与双模量子腔场的拉曼相互作用，分析原子反转度和腔场平均光子数的时间演化，考察腔场初态、初场强度以及原子间偶极相互作用的影响，并与单原子情况进行了比较，得到了一些新的有意义的结果。     

V-grooved quantum wires as prototypes of 1D-systems: Single particle properties and correlation effects

We present a combined theoretical and experimental study of the electronic and excitonic properties of GaAs-based V-grooved quantum wires as well as their dependence on external magnetic fields. We propose a numerical approach which allows us to compute electron and hole states, as well as their mutual correlation effects for realistic wire geometries. Our predictions for the optical spectra and their dependence on magnetic field are compared with several spectroscopical observations on samples grown by MBE on holographic patterns. The agreement is excellent, thus demonstrating the one-dimensional nature of the ground state exciton and the excitonic nature of the first-subband transitions in the presence of a magnetic field.     

腔QED系统中三体纠缠的转移和动力学特性研究

通过归纳法得到共振条件下k 个独立Jaynes-Cummings (J-C)模型系统 波函数概率幅的递推关系,给出了任意初态下波函数的解析解。在此基础上重点 研究了共振条件下三个独立J-C模型系统中的纠缠动力学特性。结果表明,系统 原子间的纠缠与子系统腔场光子数有关系,光子数越多,原子间纠缠第一次死亡 的时间越快。同时发现三个子系统的腔场处于真空态时,原子间的纠缠与腔场间 的纠缠随时间的演化呈现周期性的行为,并且原子间纠缠达到最小值0时,腔场 间纠缠达到最大值1,实现了原子与腔场之间纠缠的相互转移。     

Controlled exciton spontaneous emission inoptical-microcavity-embedded quantum wells

A theory for polaritons and intrinsic spontaneous emission by excitons in quantum wells embedded in planar optical microcavities is presented. Excitons in ideal quantum wells are extended states, and the resulting spatial coherence leads to spontaneous-emission characteristics different from those for point dipoles in a planar cavity. It is pointed out that the quality factor Q of a planar cavity is typically strongly dependent upon the in-plane wave vector k&oarr; _∥ of excitation; in particular, we demonstrate that both strongly enhanced emission (low Q) as well as strongly inhibited emission accompanied by vacuum-field Rabi oscillations (high Q) occur in the same cavity at different k&oarr;_∥. Free-and localized-exciton radiative decay and radiation dynamics associated with confined and radiation modes of the optical field are considered, as well as the temperature dependence of the emission as measured in time-resolved photoluminescence spectroscopy. Other results obtained are a two-dimensional form of the longitudinal-transverse splitting for and dispersion of exciton polaritons in the presence of a cavity     

Perfectly and imperfectly controlled quantum operations on a charge qubit

A theoretical investigation is presented into the coherent dynamics of a charge qubit in the form of two tunnel-coupled quantum dots containing a single electron, with the logic states represented by electron orbitals localized at the quantum dots. Analytical expressions are derived for the evolution of a one-or two-qubit system in an applied field. The system and field parameters are evaluated in terms of performing basic one-qubit operations. The possibility is explored for implementing the CNOT operation in a two-qubit system driven by error-free or error-prone control pulses.     

一个特殊五粒子纠缠态的制备方案

实现不同的量子信息过程通常需要不同的纠缠态,一个特殊的五粒子纠缠态已被证明可用于量子隐形传态、量子态共享以及量子密集编码等多个量子信息过程.基于腔量子电动力学,提出此特殊五粒子纠缠态的制备方案.选择原子和腔场处于一定的初始态,让多个原子在腔场中发生相互作用,选择相互作用时间,通过经典的幺正变换实现将五个原子制备于特殊的纠缠态.整个制备过程中原子和腔场之间没有能量的交换,无需进行任何量子测量,这使得该方案在实验上更易实现.     

Modulation optical spectroscopy of excitons in structures with GaAs multiple quantum wells separated by tunneling-nontransparent barriers

Contactless optical electroreflectance measurements at different temperatures are used to study exciton states in a structure involving a periodic system of 36 GaAs quantum wells separated by tunneling-nontransparent AlGaAs barriers with thickness 104 nm. In the structure, the width of 32 of the quantum wells is 15 nm, while the width of the remaining four quantum wells, numbered 5, 14, 23, and 32, is 20 nm. The periodicity of the structure corresponds to the Bragg interference condition at the excitonic frequency in quantum wells at the angle of incidence of light ～43°. From the quantitative analysis of the shape of the contactless electroreflectance line, the parameters of the exciton ground states and excited states are determined for both types of quantum wells. It is established that, for the system of four 20-nm-wide quantum wells separated by a distance of 830 nm, the size-quantization energy in the ground state is 8.4 ± 0.1 meV, and the parameter of broadening of the excitonic peak is 1.8 ± 0.1 meV at 17 K and increases with temperature up to 2.0 ± 0.1 meV at 80 K. For the system of 32 wells with the width 15 nm, the quantum confinement energy in the ground state is 14.9 ± 0.1 meV, and the parameter of broadening of the excitonic peak is 2.2 ± 0.1 and 2.6 ± 0.1 meV at 17 and 80 K, respectively. The possible causes of radiative and nonradiative broadening of exciton states in the systems are discussed.     

V型无反转激光的非绝热线宽

应用c数量子朗之万方法在激光场的低阶近似下讨论了强相干驱动场下V型三能级系统无反转激光的非绝热线宽,证明了在强驱动场下,不管激光腔是“好腔”还是“坏腔”,系统的非绝热效应消,了系统均可作绝热近似。     

Polarization states of a single-mode (microchip)Nd3+:YAG laser. I. Theory

We present a microscopic mathematical model for the polarization states of a single-frequency Nd³⁺:YAG laser. It is a plane wave, mean field, vector model carried to all orders in the laser field. The crystal is assumed to be optically pumped longitudinally with a laser of specified polarization. For D₂ site symmetry and an odd number of electrons, we establish the phase relationships between the components of the electric dipole matrix elements between the Kramers states. These relationships are central in determining the site-specific coupling between both, the pump and laser fields to the Nd ³⁺ ions. The laser cavity is assumed to be linear and quasi-isotropic. The residual optical anisotropies are included using a round-trip Jones matrix formalism     

激光场与两能级原子相互作用的全量子理论分析

本文用全量子理论研究激光场与两个原子的相互作用。在不同的初始条件下,得到了光子数和原子状态各自随时间的变化关系。光子数很大时,全量子理论结果与半经典理论结果是一致的。     

激光脉冲驱动下双腔耦合系统的动力学特性

在外加激光控制下,研究了含有两能级量子比特的双腔耦合系统的动力学行为.基于系统的相互作用哈密顿量,得出了系统量子态激发概率的解析解.通过数值模拟讨论了3种特定情况下系统的动力学特性:外加脉冲与腔场发生近共振、比特与腔模大失谐条件下,腔模的最大激发概率小于0.07,腔模处于非激发状态,比特的激发概率出现瞬时衰减;比特与腔模强耦合条件下,腔模的激发概率近似0.5,腔模处于半激发状态,比特的激发概率出现高频振荡;腔场之间大跳跃条件下,腔模的最大激发概率小于0.07,腔模处于非激发状态,比特的激发概率出现高频振荡.     

未知原子态经纠缠腔场信道的隐形传态

为了有效地进行原子态的量子隐形传输,提出了一个使用纠缠腔场的原子量子比特态的量子隐形传输新方案,利用位于高品质光腔中的三能级原子与腔场进行大失谐的相互作用来实现,并讨论了光腔的耗散对量子隐形传输的影响。结果表明,该方案不需要辅助原子就能将一个原子态直接传输到另一个原子上,并且在不考虑光腔损耗的情况下传输效率达到100%;对于小的衰减,对任意态的传输,其保真度都接近1.0。该研究对量子通讯的发展是有帮助的。     

连续波DF激光器光腔温度和相对粒子数范围估算

通过讨论连续波DF激光器谱线小信号增益随光腔温度和相对粒子数变化，指出随光腔温度和相对粒子数增加，谱线小信号增益最大值转动量子数和短波方向零小信号增益转动量子数向高转动态移动，且与一定范围的光腔温度和相对粒子数对应。整数零小信号增益转动量子数较难取得，结合实测光谱给出了替代条件:谱线短波方向相对强度最小截止转动量子数JL小信号增益大于零和假定转动量子数JL-1谱线小信号增益小于零。根据DF化学激光器实测光谱三个谱带所表征光腔温度范围相等，估算出某连续波DF激光器光腔温度范围为305～368 K，1P、2P和3P谱带相对粒子数范围分别为:1.77～2.09、1.65～2.05和1.19～1.72。     

Molecular orbital topology and optical properties of galliumarsenide clusters

The oscillator strength is expressed in terms of a density matrix formulation for optical absorption and is calculated for a GaAs cluster by using the self-consistent-field Xα-scattered wave cluster molecular orbital method. The discrete value of oscillator strength due to optical excitation defined by the selection rule leads to a model of the fundamental nature of the excitonic absorption behavior in GaAs bulk material and quantum well structure. Calculated results of band-to-band transition and exciton states are found to agree well with absorption spectra published in literature. The implication of the excitonic states for the nonlinear-optical behavior observed in GaAs quantum well structures is discussed. The presence of excitonic states may cause a `saturation' phenomenon which will lead to the optical nonlinearity     

Dynamics of quantum discord for a two-qubit system

We study the dynamics of quantum discord of a two-qubit system coupled to a common structured reservoir at zero temperature. The conditions to maximize reservoir-induced quantum discord for the two-qubit system wiht an initially factorized state are derived. In particular, when the two qubits are placed in a lossy cavity, high values of quantum discord can be obtained in the dispersive regime, even in the bad-cavity limit. Finally, we show that under certain conditions, the quantum discord dynamics exhibits quantum beats.