Controlling Spin Qubits in Quantum Dots

We review progress on the spintronics proposal for quantum computing where the quantum bits (qubits) are implemented with electron spins. We calculate the exchange interaction of coupled quantum dots and present experiments, where the exchange coupling is measured via transport. Then, experiments on single spins on dots are described, where long spin relaxation times, on the order of a millisecond, are observed. We consider spin-orbit interaction as sources of spin decoherence and find theoretically that also long decoherence times are expected. Further, we describe the concept of spin filtering using quantum dots and show data of successful experiments. We also show an implementation of a read out scheme for spin qubits and define how qubits can be measured with high precision. Then, we propose new experiments, where the spin decoherence time and the Rabi oscillations of single electrons can be measured via charge transport through quantum dots. Finally, all these achievements have promising applications both in conventional and quantum information processing. PACS: 03.67.Lx, 03.67.Mn, 73.23.Hk, 85.35.Be     

From Dirac to Diffusion: Decoherence in Quantum Lattice Gases

We describe a model for the interaction of the internal (spin) degree of freedom of a quantum lattice-gas particle with an environmental bath. We impose the constraints that the particle-bath interaction be fixed, while the state of the bath is random, and that the effect of the particle-bath interaction be parity invariant. The condition of parity invariance defines a subgroup of the unitary group of actions on the spin degree of freedom and the bath. We derive a general constraint on the Lie algebra of the unitary group which defines this subgroup, and hence guarantees parity invariance of the particle-bath interaction. We show that generalizing the quantum lattice gas in this way produces a model having both classical and quantum discrete random walks as different limits. We present preliminary simulation results illustrating the intermediate behavior in the presence of weak quantum noise     

Quantum Correlations in a Generalized Spin Star System

The problem of detecting quantum signatures in the correlations formed in dynamical evolution of quantum bipartite systems receives a lot of attention in current literature. Generally speaking, the occurrence of correlations between two observables of a system does not necessarily reflect nonclassical behaviour. In this paper, the exact dynamics of a pair of uncoupled spins 1/2 interacting with a common spin 1/2 bath is investigated. Starting from a separable initial condition, the ability of the system to develop purely quantum correlations is brought to light. Physical interpretation of the concurrence function as well as a suggestion on how to measure it are given.     

Mesoscopic Continuous and Discrete Channels for Quantum Information Transfer

We study the possibility of realizing perfect quantum state transfer in mesoscopic devices. We discuss the case of the Fano-Anderson model extended to two impurities in a single excitation regime. For a channel with an infinite number of degrees of freedom, we obtain coherent behaviour in the case of strong coupling or in weak coupling off-resonance. For a finite number of degrees of freedom, coherent behaviour is associated to weak coupling and resonance conditions.     

Entanglement,Quantum Phase Transitions,and Density Matrix Renormalization

We investigate the role of entanglement in quantum phase transitions, and show that the success of the density matrix renormalization group (DMRG) in understanding such phase transitions is due to the way it preserves entanglement under renormalization. We provide a reinterpretation of the DMRG in terms of the language and tools of quantum information science which allows us to rederive the DMRG in a physically transparent way. Motivated by our reinterpretation we suggest a modification of the DMRG which manifestly takes account of the entanglement in a quantum system. This modified renormalization scheme is shown, in certain special cases, to preserve more entanglement in a quantum system than traditional numerical renormalization methods. PACS: 03.65.Ud, 73.43.Nq, 05.10.-a     

借助Lyapunov方法的量子系统平衡态的布居控制

This paper studies the population control problem associated with the equilibrium states of mixed-state quantum systems by using a Lyapunov function with degrees of freedom. The control laws are designed by ensuring the monotonicity of the Lyapunov function; main results on the largest invariant set in the sense of LaSalle are given; and the strict expression of any state in the largest invariant set is normally deduced in the framework of Bloch vectors. By analyzing the obtained largest invariant set and the Lyapunov function itself, this paper also discusses the determination problem of the degrees of freedom. Numerical simulation experiments on a three-level system show the validity of research results.     

A model-theoretic interpretation of environment-induced superselection

The question of what constitutes a ‘system’ is foundational to quantum measurement theory. Environment-induced superselection or ‘einselection’ has been proposed as an observer-independent mechanism by which apparently classical systems ‘emerge’ from physical interactions between degrees of freedom described completely quantum mechanically. It is shown here that einselection can only generate classical systems if the ‘environment’ is assumed a priori to be classical; einselection therefore does not provide an observer-independent mechanism by which classicality can emerge from quantum dynamics. Einselection is then reformulated in terms of positive operator-valued measures acting on a global quantum state. It is shown that this reformulation enables a natural interpretation of apparently classical systems as virtual machines that requires no assumptions beyond those of classical computer science.     

卫星光通信精确跟踪控制系统的参数化综合优化设计

精确跟踪对准控制系统在卫星光通信中起着至关重要的作用.我国已完成的墨子号量子科学实验卫星,是基于经典随动系统理论设计的跟踪与瞄准系统,并在实践中取得了圆满效果.面向未来更远距离的空间通信应用,对跟踪与瞄准系统提出了更高的精度要求,传统的控制方法很难满足.为此本文提出了精确瞄准系统的一种参数化设计方法,抛弃了传统方法的精、粗系统分别设计的思想,对两级子系统进行整体设计,充分地利用了系统中的设计自由度.通过综合优化这些设计自由度,实现了系统对阶跃干扰的解耦和复杂干扰的抑制、不敏感极点配置和控制增益极小化等各项设计要求,从而显著地提高了对准精度.仿真结果表明,对准精度由原来的微弧度量级提高到了纳弧度量级.     

Cascade cavity realization for a class of complex transfer functions arising in coherent quantum feedback control

This paper presents a realization algorithm for a class of complex transfer function matrices corresponding to physically realizable linear quantum systems. The aim of the realization algorithm is to enable a coherent quantum feedback controller, which has been synthesized using methods such as quantum H^∞ control or quantum LQG control, to be constructed using optical components such as cavities and phase-shifters. The class of linear quantum systems under consideration are passive linear quantum systems which can be described purely in terms of annihilation operators. The proposed algorithm enables a complex transfer function matrix to be realized as a pure cascade connection involving only cavities and phase-shifters.     

介观电容耦合电路的量子纠缠效应

应用介观电子线路制备量子态并实现相应的信息传输与控制是量子信息学的重要课题。利用介观耦合电路给出了制备双模压缩真空态的方案,研究了模间纠缠度与压缩幅度参数的关系。结果显示:模间纠缠度和2个回路的电感比值有关,与耗散无关,且存在一个极大值。     

量子隐形传态的通用线路

EPR态作为最基本的量子纠缠态,在量子隐形传态中起着重要作用.研究适应任意类型EPR通道的单量子比特隐形传送通用线路,并推广到任意N比特量子隐形传送通用线路.首先设计出4种EPR态,分别作为量子通道的单比特量子隐形传态,通过分析EPR量子通道与量子操作门之间的关系,设计一种单比特通用线路;然后,设计两比特的标准量子隐形传态线路,并用Mathematica进行仿真验证线路的正确性,再把它推广到N比特量子隐形传送线路;最后,将单量子比特通用线路与N比特量子隐形传送线路进行融合,最终设计出任意N比特量子隐形传送通用线路.N粒子量子比特通用线路通过信息接受者进行带参数的幺正变换,其中,参数由制备出的EPR对类型确定,解决了因EPR制备中心出错导致的信息传送失败问题.     

From n-qubit multi-particle quantum teleportation modelling to n-qudit contextuality based quantum teleportation and beyond

This paper further generalizes the modelling framework for contextuality based quantum teleportation to n-dimensional quantum states, or n-qudits (quantum odd-prime based units) which holds considerable promise for even higher mathematical abstraction. The proposed generalization extends the two states in the qubit model, to n states and this n could tend towards infinity. We are heading towards point-sized loops or fine-grained particles of nature, which have been rejected out of hand by string theorists. It is not physical reality when measured from the sense of Planck’s length of 10^?35 metres, but finer-grained particles than Planck’s length 10^?35 metres may exist, although, they are not matter anymore. We argue that why stop at degree of freedom of three, but instead pursue quantum odd-prime based units with higher degree n such as 5, 7, 11, 13, 17 and so on till n^th degree of freedom even tending to infinity. String theorists admit its multiple landscapes with higher degrees of freedom which points its uniqueness both for particles finer than 10^?35 metres as well as distances greater than 10¹⁰ light years. Modern research by physicists and mathematicians including Roger Penrose, Max Tegmark, Nick Bostrom and Herbert Bernstein provides scientific basis to this work.     

Design of the annular suspension and pointing system (ASPS) through decoupling and pole placement

This paper describes the decoupling and pole-placement design of the ASPS with continuous-data control and with digital control. The dynamics of the ASPS are represented by a simplified small-angle, small-displacement planar model with four degrees of freedom. The model includes a mount, a gimbal assembly, a pallet with magnetic actuators, and a payload. The pallet has one rotational degree of freedom relative to the mount, and the payload has two translational and one rotational degrees of freedom relative to the pallet. One of the translation degrees of freedom of the payload is not coupled to the other three degrees of freedom of the payload is not coupled to the other three degrees of freedom. The bandwidth requirements of the various degrees of freedom are specified. The continuous-data system with state feedback is designed through decoupling and pole placement. It was found that the digital ASPS cannot be completely decoupled. However, the bandwidth requirements were satisfied by pole placement and a trial-and-error method based on approximate decoupling. The time responses of the designed systems are investigated by computer simulation.     

Robust H∞ controller design for a class of linear quantum systems with time delay

Time delay is frequently encountered in practical quantum feedback control systems with long transmission lines and measurement process. This paper is concerned with measurement‐based feedback H^∞ control for quantum systems with time delays appearing in the feedback loops. A physical model is presented for the quantum time‐delay system described by complex quantum stochastic differential equations. Quantum versions of some fundamental properties, such as dissipativity and stability, are discussed for this model. A numerical procedure is proposed for H^∞ controller synthesis, which can deal with a non‐convex optimization problem arising in the design processes. Copyright © 2016 John Wiley & Sons, Ltd.     

Implicit Lyapunov‐based control strategy for closed quantum systems with dipole and polarizability coupling

In this paper, the state transfer of finite dimensional closed quantum systems with dipole and polarizability coupling in non‐ideal cases is investigated. Two kinds of non‐ideal systems are considered, where the internal Hamiltonian of the system is not strongly regular and not all the eigenvectors of the internal Hamiltonian are directly coupled with the target state. Such systems often exist in practical quantum systems such as the one‐dimensional oscillator and coupled two‐spin system. An implicit Lyapunov‐based control strategy is proposed here with convergence analysis for quantum systems modeled by finite dimensional bilinear Schrödinger equations. Specifically, two kinds of Lyapunov functions are defined via implicit functions, and their existences are guaranteed with the help of a fixed point theorem. Then, the local convergence analysis is investigated with the explicit characterization of the largest invariant set by LaSalle invariance principle. Finally, the performance of the feedback design is illustrated by numerical simulations. Copyright © 2017 John Wiley & Sons, Ltd.     

Entanglement and multiple quantum coherence dynamics in spin clusters

With the purpose to reveal consistency between multiple quantum (MQ) coherences and entanglement, we investigate numerically the dynamics of these phenomena in one-dimensional linear chains and ring of nuclear spins 1/2 coupled by dipole–dipole interactions. As opposed to the calculation of the MQ coherence intensity based on the density matrix describing the spin system as a whole, we consider the “differentiated” intensity related only to the chosen spin pair based on the reduced density matrix. It is shown that the entanglement and the MQ coherence have similar dynamics only for nearest neighbors while we did not obtained any consistency for remote spins.