Entropy as a measure of the noise extent in a two-level quantum feedback controlled system

By introducing the von Neumann entropy as a measure of the extent of noise, this paper discusses the entropy evolution in a two-level quantum feedback controlled system. The results show that the feedback control can induce the reduction of the degree of noise, and different control schemes exhibit different noise controlling ability, the extent of the reduction also related with the position of the target state on the Bloch sphere. It is shown that the evolution of entropy can provide a real time noise observation and a systematic guideline to make reasonable choice of control strategy.

Quantum speed limit time of a two-level atom under different quantum feedback control

We investigate the quantum speed limit time （QSLT） of a two-level atom under quantum-jump-based feedback control or homodyne-based feedback control. Our results show that the two different feedback control schemes have different influences on the evolutionary speed. By adjusting the feedback parameters, the quantum-jump-based feedback control can induce speedup of the atomic evolution from an excited state, but the homodyne-based feedback control cannot change the evolutionary speed. Additionally, the QSLT for the whole dynamical process is explored. Under the quantum-jump-based feedback control, the QSLT displays oscillatory behaviors, which implies multiple speed-up and speed-down processes during the evolution. While, the homodyne-based feedback control can accelerate the speed-up process and improve the uniform speed in the uniform evolution process.

Markov Quantum Feedback Control Based on Homodyne Measurement of a Two-Qubit System

We consider the system consisting of two qubits collectively damped, with the output being unit-efficiency measured and subsequently fed back to control the system state. Our primary goal in this paper is （i） to solve the feedback-modified master equation, （ii） to demonstrate the ability of feedback control based on the solutions, and （iii） to pick out different steady states by choosing different driving strengths and feedback strengths to counteract the effects of both damping and the measurement back-action on the system. We further investigate some properties of the equilibrium steady state, its distribution probability and entanglement vs. the driving and feedback amplitudes. We find that in our feedback model feedback plays a negative role in producing entanglement.

Quantum feedback networks and control:A brief survey

The purpose of this paper is to provide a brief review of some recent developments in quantum feedback networks and control.A quantum feedback network (QFN) is an interconnected system consisting of open quantum systems linked by free fields and/or direct physical couplings.Basic network constructs,including series connections as well as feedback loops,are discussed.The quantum feedback network theory provides a natural framework for analysis and design.Basic properties such as dissipation,stability,passivity and gain of open quantum systems are discussed.Control system design is also discussed,primarily in the context of open linear quantum stochastic systems.The issue of physical realizability is discussed,and explicit criteria for stability,positive real lemma,and bounded real lemma are presented.Finally for linear quantum systems,coherent H∞ and LQG control are described.

Dissipative preparation of a steady three-dimensional entangled state via quantum-jump-based feedback

A robust and scalable scheme to generate a steady three-dimensional entangled state for a V-type atom and a A- type atom trapped in a strongly dissipative bimodal cavity is proposed by direct feedback control based on quantum-jump detection. The robustness of this scheme reflects in the insensitivity to detection inefficiencies and the strong ability against the parameter fluctuations in the feedback, driving, and coupling strengths. The influence of atomic spontaneous emission can be suppressed by using the local feedback control. The scalability is ensured that N-dimensional entangled states of two atoms can be deterministically generated.

Quantum discord of two-qutrit system under quantum-jump-based feedback control

This paper studies quantum discord of two qutrits coupled to their own environments independently and coupled to the same environment simultaneously under quantum-jump-based feedback control. Our results show that spontaneous emission, quantum feedback parameters, classical driving, initial state, and detection efficiency all affect the evolution of quantum discord in a two-qutrit system. We find that under the condition of designing proper quantum-jump-based feedback parameters, quantum discord can be protected and prepared. In the case where two qutrits are independently coupled to their own environments, classical driving, spontaneous emission, and low detection efficiency have negative effect on the protection of quantum discord. For different initial states, it is found that the evolution of quantum discord under the control of appropriate parameters is similar. In the case where two qutrits are simultaneously coupled to the same environment,the classical driving plays a positive role in the generation of quantum discord, but spontaneous emission and low detection efficiency have negative impact on the generation of quantum discord. Most importantly, we find that the steady discord depends on feedback parameters, classical driving, and detection efficiency, but not on the initial state.

Design of surface emitting distributed feedback quantum cascade laser with single-lobe far-field pattern and high outcoupling efficiency

A 7.8-μm surface emitting second-order distributed feedback quantum cascade laser （DFB QCL） structure with metallized surface grating is studied. The modal property of this structure is described by utilizing coupled-mode theory where the coupling coefficients are derived from exact Floquet-Bloch solutions of infinite periodic structure. Based on this theory, the influence of waveguide structure and grating topography as well as device length on the laser performance is numerically investigated. The optimized surface emitting second-order DFB QCL structure design exhibits a high surface outcoupling efficiency of 22% and a low threshold gain of 10 cm-1. Using a π phase-shift in the centre of the grating, a high-quality single-lobe far-field radiation pattern is obtained.

Development of Surface Grating Distributed Feedback Quantum Cascade Laser for High Output Power and Low Threshold Current Density

We report on the room-temperature cascade laser （QCL） at λ -4.7μm. cw operation of a surface grating Both grating design and material distributed feedback （DFB） quantum optimization are used to decrease the threshold current density and to increase the output power. For a high-reflectivity-coated 13-μm-wide and 4- mm-long laser, high wall-plug efficiency of 6% is obtained at 20℃ from a single facet producing over I W of ew output power. The threshold current density of DFB QCL is as low as 1.13kA/cm^2 at 10℃ and 1.34kA/cm2 at 30℃ in cw mode. Stable single-mode emission with a side-mode suppression ratio of about 30 dB is observed in tile working temperature range of 20-50℃.

Influence of homodyne-based feedback control on the entropic uncertainty in open quantum system

For an open quantum system containing two qubits under homodyne-based feedback control, we investigate the dynamical behaviors of quantum-memory-assisted entropic uncertainty.Moreover, we analyze the influence of feedback modes and coefficients on the entropic uncertainty.Numerical investigations show that the memory qubit should be placed in a non-dissipative channel if the single dissipative channel condition can be chosen, which helps reduce the entropic uncertainty of the system.For the homodyne feedback control F =λσx(or F =λσy), due to different roles of the entangled qubits A and B, when they are subject to feedback control with different feedback coefficients λ, the exchange of feedback coefficients will cause variations of the entropic uncertainty.When the feedback coefficient corresponding to the memory qubit B is larger(λB >λA), the steady value of the entropic uncertainty will be small, which is conducive to enhancing the robustness of the system.However, for the feedback control F =λσz, the difference between the feedback coefficients has no effect on the steady values of the entropic uncertainty.

Optimal parameter estimation of open quantum systems

In quantum information technologies,quantum weak measurement is beneficial for protecting coherence of systems.In order to further improve the protection effect of quantum weak measurement on coherence,we propose an optimization scheme of quantum Fisher information(QFI)protection in an open quantum system by combing no-knowledge quantum feedback control with quantum weak measurement.On the basis of solving the dynamic equations of a stochastic two-level quantum system under feedback control,we compare the effects of different feedback Hamiltonians on QFI and find that via no-knowledge quantum feedback,the observation operatorσx(orσx andσz)can protect QFI for a long time.Namely,no-knowledge quantum feedback can improve the estimation precision of feedback coefficient as well as that of detection coefficient.

Quantum Control Strategy Based on State Distance

Based on Bures distance, a Lyapunov function that represents the distance between a desired state and the actual state of a quantum system is selected. Considering the cases that an initial state is and is not orthogonal to the desired state respectively, we propose a class of control strategies with state feedback that ensures the stability of the closed-loop control system. Especially, the asymptotic stability of the control system is analyzed, deduced and proved in detail. Finally, a simulation experiment on a spin-1/2 particle system is done and the relation between the system state evolution time and control value is analyzed with diffierent parameters . Research results have general theoretical meaning for control of quantum systems.

Dynamic characteristics in an external-cavity multi-quantum-well laser

This paper outlines our studies of bifurcation, quasi-periodic road to chaos and other dynamic characteristics in an external-cavity multi-quantum-well laser with delay optical feedback. The bistable state of the laser is predicted by finding theoretically that the gain shifts abruptly between two values due to the feedback. We make a linear stability analysis of the dynamic behavior of the laser. We predict the stability scenario by using the characteristic equation while we make an approximate analysis of the stability of the equilibrium point and discuss the quantitative criteria of bifurcation. We deduce a formula for the relaxation oscillation frequency and prove theoretically that this formula function relates to the loss of carriers transferring between well regime and barrier regime, the feedback level, the delayed time and the other intrinsic parameters. We demonstrate the dynamic distribution and double relaxation oscillation frequency abruptly changing in periodic states and find the multi-frequency characteristic in a chaotic state. We illustrate a road to chaos from a stable state to quasi-periodic states by increasing the feedback level. The effects of the transfers of carriers and the escaping of carriers on dynamic behavior are analyzed, showing that they are contrary to each other via the bifurcation diagram. Also,we show another road to chaos after bifurcation through changing the linewidth enhancement factor, the photon loss rate and the transfer rate of carriers.

Rubidium-beam microwave clock pumped by distributed feedback diode lasers

A rubidium-beam microwave clock, optically pumped by a distributed feedback diode laser, is experimentally investigated. The clock is composed of a physical package, optical systems, and electric servo loops. The physical package realizes the microwave interrogation of a rubidium-atomic beam. The optical systems, equipped with two 780-nm distributed feedback laser diodes, yield light for pumping and detecting. The servo loops control the frequency of a local oscillator with respect to the microwave spectrum. With the experimental systems, the microwave spectrum, which has an amplitude of 4 n A and a line width of 700 Hz, is obtained. Preliminary tests show that the clock short-term frequency stability is 7 × 10^-11 at 1 s, and 3 × 10^-12 at 1000 s. These experimental results demonstrate the feasibility of the scheme for a manufactured clock.

钬离子掺杂氟氧钇亚微米晶的荧光量子化表征与温度传感特性研究

采用水热合成法和高温煅烧法相结合制备了Ho^(3+)/Yb^(3+)共掺杂三方晶系的YOF亚微米晶体,该晶体具有较强的量子产率和良好的灵敏度。在977 nm激光泵浦下对晶体颗粒的上转换发光性能进行表征,利用荧光光谱测试系统绘制出样品的光谱功率分布。计算了光量子数分布和量子产率等绝对荧光参量。展示了在303~433 K温度范围内的荧光上转换发光性能并计算了灵敏度。测试结果表明,在977 nm激光激发下泵浦功率密度为73 m W/mm~2时,总光通量达到3.35 mlm,Ho^(3+)的绿光和红光的量子产率分别为2.97×10^(-5)和1.40×10^(-5),较高的量子产率确保了足够的荧光强度来实现温度反馈。在303 K时有最大相对灵敏度为0.437%K^(-1),当温度升至433 K时相对灵敏度仍能保持在0.331%K^(-1)。因此Ho^(3+)/Yb^(3+)共掺杂YOF亚微米晶体作为一种高效发光和高温度灵敏度材料为温度传感领域提供了一种潜在的选择。

n比特随机量子系统实时状态估计及其反馈控制

研究了n比特随机量子系统实时状态估计及其反馈控制的问题.对于连续弱测量(Continuous weak measurement, CWM)过程存在高斯噪声的情况,基于在线交替方向乘子法(Online alternating direction multiplier method,OADM)推导出一种适用于n比特随机量子系统的实时量子状态估计算法,即QSE-OADM (Quantum state estimation based on OADM).运用李雅普诺夫方法设计控制律,实现基于实时量子状态估计的反馈控制,并证明所提控制律的收敛性.以2比特随机量子系统为例进行数值仿真实验,通过与基于QST-OADM (Quantum state tomography based on OADM)算法和OPG-ADMM (Online proximal gradient-based alternating direction method of multipliers)算法的量子反馈控制方案的性能对比,验证了所提控制方案的优越性.

结合光电负反馈和模式清洁器的噪声抑制系统

光场噪声的有效抑制是开展精密测量研究的基础,特别是现代量子信息科学对光场噪声提出了散粒噪声极限的要求。本工作研究了一种结合光电负反馈和模式清洁器的噪声抑制系统,在边带模型和郎之万方程的基础上,分析了噪声抑制系统输出光场的噪声谱,讨论了光电负反馈回路的透射率、反馈增益以及光学模式清洁器的腔线宽对噪声抑制水平的影响。结果表明,该系统不仅可以有效抑制高频的强度噪声,使其达到散粒噪声极限,而且能够减小特定频率处的强度噪声。此外,引入带通滤波器能进一步扩展噪声抑制的水平和带宽。研究结果为实验研究提供了直接参考和必要基础。

量子系统控制发展综述

详细论述了量子系统控制理论从提出到现在近二十年的发展过程。首先介绍了量子系统可控性的研究及其进展,并简要分析了简单量子系统的开环控制方法,然后把重点放在学习控制和反馈控制等几种典型闭环控制方法的讨论上,分析了它们各自的优缺点,并根据量子控制的发展趋势,对其应用和发展前景进行了预测,从整体上系统地描述了建立一套完整的量子控制系统所需要的五个基本步骤,最后在此基础上提出了制约量子系统控制发展的几个关键性问题。

基于相干控制的二能级量子系统退相干抑制

对于二能级开放量子系统,研究了利用相干控制抑制退相干效应的问题.首先讨论了二能级开放量子系统在相干控制下的建模问题,将退相干抑制归结为与环境噪声解耦的控制问题.然后,引入开环控制抑制退相干,并证明该控制可使系统状态中的部分分量与环境噪声渐近解耦.最后引入反馈控制,使得系统状态的相应分量可以与环境精确解耦,同时能够避免测量引入的量子噪声的影响.

外腔反馈对量子点激光器输出特性的影响

在对光栅外腔量子点激光器进行理论研究的基础上,分析了外腔反馈对Littrow型光栅外腔量子点激光器输出功率、调谐范围等输出特性的影响,发现器件参数的选择对外腔激光器的性能影响很大。对外腔激光器的输出功率和调谐范围进行了理论计算,并与实验结果进行了对比。计算得到的外腔激光器的输出功率与实验结果符合得很好,忽略了非线性增益相关的增益抑制的单模调谐范围理论计算值稍小于实验结果。

能量天平反馈系统的改进

未来质量单位的定义有可能在很大程度上依赖于普朗克常数h的测定值和不确定度。能量天平方案作为我国提出的解决量子质量基准的电学方法,相比于功率天平方案,在测量普朗克常数h方面有自己独特的优点。着重对能量天平方案中的天平反馈系统进行了研究。通过反馈系统实现了天平的快速平衡,并通过对天平反馈执行器的电路调整,显著改进了天平系统的测量漂移现象和噪声。零位漂移从13 mg降低到7 mg,测量噪声降低到5 mg以下。