量子存储保护及快信号调控诱导绝热性(英文)

任何量子存储和信息处理器时时刻刻都受困于外界环境的干扰或因为其他系统关联而引起的量子态外泄.为了应对这些问题,人们提出了多种量子调控方案.这其中一大类方案由快信号调制或者基于激光脉冲序列的动力学解耦构成,旨在压制量子存储器件的退相干,并且阻止系统状态的外泄或扩散.本文回顾了一些应用此类量子调控方案的工作.人们可以保护处于非马尔可夫耗散噪声中的量子存储器件;或者在免于外泄的前提下,控制系统以进行绝热演化.通过分析各类动力学方程,包括二阶微扰主方程、基于非微扰论的量子态扩散方程、以及由PQ分解得到的单变量主方程,发现快速信号调制的效果与所施加脉冲序列的形态关系微弱.脉冲序列只需能够有效地将系统本征频率调至高于环境截断频率,系统量子态退相干就可以再被压制住.量子调控的效果其实与调控脉冲序列的具体细节无关.人们可以使用任何形态的激光,包括理想脉冲、矩形波脉冲、随机脉冲,甚至噪声型脉冲.     

二能级系统中粒子布居几率振荡的布洛赫矢量分析

考虑系统粒子的弛豫,利用密度矩阵运动方程,研究了线性啁啾脉冲作用下二能级系统的粒子布居几率随时间的演化情况,并在布洛赫矢量模型表象下,通过布洛赫矢量的动态演化过程分析了粒子相干布居转移过程中布居振荡的物理机理.结果表明,粒子数振荡反映的是二能级原子系统中粒子布居转移、色散和吸收三者之间的动态转化过程.     

量子微腔中暗态原子粒子数转移的绝热条件

为了明确量子微腔中粒子数转移的绝热条件，本文利用准绝热近似方法详细地讨论了量子微腔中单模驻波场与简并三能级原子相互作用的动力学演化，定量地研究了实现粒子数经由暗态在两激发态间绝热转化的条件     

三体量子系统的量子导引方案

复合量子系统的量子导引(Quantum Steering),又称量子操控,是指通过某个子系统上的测量来导引(操控)另一系统的状态.它是介于量子纠缠与Bell非局域性之间的一种非对称量子特性.正是这一特点使得量子导引能在许多方面发挥重要作用,因而备受学者的广泛关注.两体系统中的量子导引已有许多研究,而多体量子系统比两体系统拥有更加复杂的结构,其中的导引方案更加多样化.本文提出三体量子系统中的两种量子导引方案,称为"单边设备独立导引方案"与"双边设备独立导引方案".根据局域隐态的可分性与局域隐变量的独立性,每个导引方案又包括两个导引情景.在描述物理思想的基础上,提出了三体量子态不可导引与可导引的数学定义,给出了量子态不可导引的充要条件及充分条件.     

基于系综金刚石氮空位的色心量子调控系统设计

为了实现量子的自旋调控和精密测量,将金刚石作为自旋载体材料,设计了基于系综金刚石氮空位(nitrogen-vacancy,NV)色心量子调控系统。通过利用金刚石独特的自旋三重态容易被初始化、操控和读出,基于Lab VIEW软件,设计编写了脉冲序列发生模块并搭建了共聚焦系统,调控了系综NV色心的自旋量子态。结果表明:该系统可以调控系综NV色心的自旋量子态;实现了NV色心拉比振荡实验的测量;拉比振荡周期为100 ns。可见该系统结构设计简单,为下一步延长退相干时间,提高系统灵敏度打下基础。     

基于系综金刚石氮空位色心量子调控系统设计

为了实现量子的自旋调控和精密测量,将金刚石作为自旋载体材料,设计了基于系综金刚石氮空位（Nitrogen-vacancy, NV）色心量子调控系统。通过利用金刚石独特的自旋三重态容易被初始化,操控和读出,基于LabVIEW软件,设计编写了脉冲序列发生模块并搭建了共聚焦系统,调控了系综NV色心的自旋量子态。结果表明：该系统可以调控系综NV色心的自旋量子态;实现了NV色心拉比振荡实验的测量;拉比振荡周期为100ns。可见该系统结构设计简单,为下一步延长退相干时间,提高系统灵敏度打下基础。     

整形啁啾脉冲光场作用下的量子相干控制与量子全息

在一级微扰理论近似下,研究了整形啁啾脉冲光场与二能级原子系统作用过程中的量子相干控制问题.利用惠更斯-菲涅耳原理解释了整形脉冲光场作用下量子态波函数的演化过程类似于时域＂光栅衍射＂效应;利用菲涅耳波带片方法研究了布居几率的增强效应.基于利用整形脉冲实现的量子相干控制和量子全息思想在理论上提出并论证了测量波函数的一种新方法.     

耦合谐振子的量子绝热捷径设计

量子绝热捷径技术旨在加快量子绝热慢过程, 已经被广泛用于原子的冷却、转移等量子信息处理过程. 研究耦合谐振子模型的量子绝热捷径设计及其热机应用, 基于耦合谐振子模型的量子不变量, 首先得到 Ermakov 方程, 然后反设计耦合谐振子频率, 最终加快绝热过程而不产生末态激发. 本工作为耦合谐振子的量子态操控及超绝热量子热机提供了新思路.     

相位阻尼对量子传输保真度的影响

研究了在相位阻尼作用下一个二能级原子与两个不同光场相互作用的系统中的量子态的保真度.讨论了相位阻尼和失谐对量子传输的最大保真度的影响,并且获得了通过该信道进行量子传输的最大保真度.此外,还分析了通过该信道进行量子传输的平均保真度.     

利用3-Λ型五能级原子介质操控纠缠态

通过分析3-Λ型五能级原子系统的暗态极化子,讨论三模量子场与原子介质的信息转换过程以及三个信号场之间的信息转换过程.研究结果表明,利用3-Λ型五能级原子系统,可以存储三模量子态,从理论上解释了三模量子信息在介质中的"写入"和"读出"过程.另外利用3-Λ型五能级原子系统还可以操控某些纠缠态,如实现纠缠态的生成、存储、读出、转换等.     

波函数时域衍射的布洛赫矢量模型

 为了揭示波函数时域衍射的物理本质,采用密度矩阵的矢量模型方法,研究线性啁啾脉冲光场作用下二能级系统波函数的时域衍射效应;讨论光场啁啾因子对时域衍射效应的调制作用;分析不同啁啾因子作用时,量子系统粒子布居转移和时域衍射效应的动态演化过程。在布洛赫矢量表象下,对时域衍射效应的物理作用机制给出了清晰的描述。     

Optimal quantum measurement of finite-dimensional systems and coherent anti-Stokes Raman spectroscopy

Quantum measurement is a fundamental problem in quantum control theory and experiments.It can obtain unknown information of quantum systems,and can also change state of the systems inevitably.Both the outcome and back action could be used to control quantum systems.This paper presents recent research progress about optimal control of state transformation in finite-dimensional quantum systems by back action of non-selective quantum measurement,and optimal control of signal and background of CARS (coherent anti-Stokes Raman spectroscopy) by phase shaping technique.In measurement sequence control of finite-dimensional quantum systems,the necessary condition for critical points of the underlying state transformation objective is found to be a highly symmetric form as a chain of equalities,and analytical and numerical solutions in several cases are explored.In the CARS control,it is found that the maximal resonant signal and minimal background at a specific frequency can be achieved by shaping the probe pulse only while keeping pump and Stokes pulses in transform limited forms (TLFs).An arctan-type phase function is obtained for the probe pulse to simultaneously enhance the resonant signal and suppress the background.For broadband background elimination,we find that the optimal phase shaping scheme of probe pulse is quasi-time-delay while keeping the pump and Stokes pulses in TLFs.These conclusions could help design control strategies of quantum devices.     

用微波-超导量子比特系统实现一位通用量子逻辑门

该文研究了二能级射频超导量子干涉仪量子比特与经典微波脉冲的相互作用.研究结果表明,当微波场的角频率接近于系统二能级|0〉|1〉跃迁频率时,即ω1-ω0≈ωm,可以得到一位通用量子逻辑门.通过调节控制微波场的角频率ωm,幅度Vm和脉冲作用时间t,可以得到不同的一位量子逻辑门.     

Stabilized quantum coherence and remote state preparation in structured environments

The radiation decay of a two-level atom could be inhibited within structured environments even under longtime evolution. We investigate the stabilized quantum coherence of composite systems undergoing local dissipation and exploit it further as a resource for remote state preparation. We focus on outputs of quantum states with solely quantum discord (i.e., without entanglement) and demonstrate that they could be resulted from various initial states providing specific spectral structure of the reservoir. In detail, we elaborate the behavior of stabilized quantum discord and the corresponding fidelity for remote state preparation in connection with structural spectra of Ohmic class reservoir and of photonic band gap mediums.     

频率啁啾的激光场中钾原子的激发与态囚禁

采用含时多态展开方法,对频率啁啾的激光场中的里德堡钾原子的布居数相干迁移特性进行了计算研究.结果表明:在合适的激光参数下,可以实现布居数在两个量子态之间的完全迁移和量子态的囚禁.     

Coherent properties of a two-level system based on a quantum-dot photodiode

Present-day information technology is based mainly on incoherent processes in conventional semiconductor devices. To realize concepts for future quantum information technologies, which are based on coherent phenomena, a new type of 'hardware' is required. Semiconductor quantum dots are promising candidates for the basic device units for quantum information processing. One approach is to exploit optical excitations (excitons) in quantum dots. It has already been demonstrated that coherent manipulation between two excitonic energy levels--via so-called Rabi oscillations--can be achieved in single quantum dots by applying electromagnetic fields. Here we make use of this effect by placing an InGaAs quantum dot in a photodiode, which essentially connects it to an electric circuit. We demonstrate that coherent optical excitations in the quantum-dot two-level system can be converted into deterministic photocurrents. For optical excitation with so-called pi-pulses, which completely invert the two-level system, the current is given by I = fe, where f is the repetition frequency of the experiment and e is the elementary charge. We find that this device can function as an optically triggered single-electron turnstile.     

双模光场与原子共振拉曼相互作用中的纠缠特性

在二能级原子静止和运动的情况下,研究了原子与双模压缩真空态量子化腔场共振拉曼过程中的纠缠特性,利用量子化熵和量子相对熵分别讨论了原子与双模光场之间以及双模光场两模之间的纠缠性质．研究发现:通过适当改变场模参量和原子初态可以控制纠缠的时间和纠缠强度演化．     

Laser cooling and real-time measurement of the nuclear spin environment of a solid-state qubit

Control over quantum dynamics of open systems is one of the central challenges in quantum science and engineering. Coherent optical techniques, such as coherent population trapping involving dark resonances, are widely used to control quantum states of isolated atoms and ions. In conjunction with spontaneous emission, they allow for laser cooling of atomic motion, preparation and manipulation of atomic states, and rapid quantum optical measurements that are essential for applications in metrology. Here we show that these techniques can be applied to monitor and control individual atom-like impurities, and their local environment, in the solid state. Using all-optical manipulation of the electronic spin of an individual nitrogen-vacancy colour centre in diamond, we demonstrate optical cooling, real-time measurement and conditional preparation of its nuclear spin environment by post-selection. These methods offer potential applications ranging from all-optical nanomagnetometry to quantum feedback control of solid-state qubits, and may lead to new approaches for quantum information storage and processing.