【作者】 李军奇；

【导师】 梁九卿；

【作者基本信息】 山西大学 ， 理论物理， 2008， 博士

【摘要】 近些年来,以量子力学为基础的量子信息学以其独特的性质引起了人们广泛的关注,它显示了经典信息学所无法比拟的优越性,具有潜在的巨大应用价值,甚至会引起划时代的信息产业革命。量子信息科学中用量子态表示信息,由于量子态满足相干叠加原理,从而导致纠缠态的存在。为了利用纠缠,纠缠态的制备成为量子信息学中重要的研究内容,现今人们在实验上已经实现了一些重要的纠缠态,并取得了许多令人瞩目的成就,但是对多粒子纠缠的制备问题还处于不断的努力探索之中。在实际的量子系统中,环境的影响是不可避免的。环境导致的量子退相干以及由此引发的退纠缠效应是实现通用量子计算机的绊脚石。因此,确切知道量子退相干是如何破坏物体的相干性,对量子信息科学发展至关重要。现今,量子退相干成为理论界和实验界的研究热点之一。本文着重研究了量子退相干及多粒子纠缠态的制备问题,并进行了一些讨论,主要内容如下:1、用短时近似方法研究了约瑟夫森电荷量子比特中的量子Hadamard门的退相干问题,我们发现最大偏离算子范数和先前文献有相同的表达式,并进一步说明了无论系统的初始态是什么,都能够在DiVincenzo低退相干的要求下,完成Hadamard门操作,从而证实了约瑟夫森装置是一个很好的量子计算装置。2、在一般的自旋-玻色模型中,我们研究了混态纠缠的时间演化行为,并讨论了现今引起很大关注的纠缠猝死现象。结果表明,纠缠衰减不但依赖于温度而且依赖于环境和系统间的耦合强度,且混态纠缠衰减因子和纯态有相同的形式。3、借助于主方程方法,我们讨论了EPR系统中与各自腔场相互作用的两粒子纠缠动力学问题,发现环境导致的纠缠猝死现象与系统的初始态以及平均光子数有关。通过对纠缠度量和系统能量转移的时间演化分析,我们得出最大纠缠度对应着EPR系统的最小能量,且它们之间的时间变化既不是一对一,也不是同步的关系,甚至在没有初始纠缠的情况下,仍有能量转移现象发生的结论。这从侧面反映出了,用能量变化来表征纠缠猝死现象至少在我们的模型中是行不通的。4、通过分析腔场驱动下开放系统中的两能级双量子点的激子纠缠动力学,我们获得以激子-光子耦合g_1,2、声子布局数N_1,2、黄昆-里斯因子λ_1,2为变量的纠缠度量—concurrence的函数表达式,发现纠缠猝死现象可以通过这些相关的参数来调控,从中说明了抑制纠缠猝死的条件。5、基于经典外场驱动的多原子微波腔量子电动力学系统,我们提出了一种产生多原子纠缠态的理论方案。该方案通过单步操作即可完成,由于采用了大失谐,所以被提议的量子操作对腔场衰减起到了免疫作用,因此可以赢得理想几率的cluster态。更多还原

【Abstract】 The quantum information science basedon quantum theory has attracted a wide attention in recent years.It shows the superiorities that classical information is not to be compared with and has so many great potential applications that it may lead to a revolution in information domain.Quantum information is carried by quantum coherent superposition states,which induce the appearance of entangled states.In order to utilize the entanglement, the generation of entangled states lie at the heart of the quantum information. Now,various schemes of generating entangled states were proposed and some remarkable achievements were made.However,the preparation of multipartite entangled states are still under research.It is well known,however,that the effect of environment on the quantum system,which may deteriorate the coherence and even leads to decoherence and disentanglement and becomes a major obstacle towards the realization of a universal quantum computer,must be taken into account in practical applications.So,it is important to understand the effect of decoherence on the development of the quantum information and much attention has been paid to the research about decoherence either in theories or in experiments, recently.In our paper,we mainly investigate the decoherence,disentanglement and the generation schemes of multipartite entangled states in quantum computation.The results are following:First,we investigate the short-time decoherence of a quantum Hadamard gate with the Josephson-junction device（JJD）.It is shown that the maximum norm of the deviation operator has the same expression as in the previous literature.We demonstrate that the elementary gate operation can be completed under the low decoherence criterion no matter what the initial state is.Thus,the JJD can be a good candidate of quantum computating devices.Second,we study the time-evolution of the mixed,state entanglement in a general spin-boson model including the entanglement sudden death.It is shown that the decay of entanglement depends on the temperature and coupling strength.Moreover the entanglement decay-factor of mixed states has the same form as that of the pure states.Third,we study the entanglement dynamics of the Einstein,Podolsky and Rosen（EPR） system in which two spatially separated particles are coupled with their own cavity field.The environment-induced entanglement sudden death depends on the parameters of an initial entangled state and a mean photon number as well.We,moreover,investigate the time-evolution behaviour for both concurrence and energy transfer between the EPR particles and the environment,from which it is found that the maximal concurrence indeed corresponds to the minimal energy of the EPR system. However,their time variations are neither in one-to-one correspondence nor in step.Particularly when the state of two particles becomes disentangled,the time variation of energy transfer still exists.This observation suggests that the concurrence seems not directly related to the energy transfer at least for the model at hand.Then,the entanglement dynamics of excitons in the two-level double-quantum-dot driven by cavity fields is investigated with the particular attention on the effect ofphonon bath which makes an open two-qubit system of the quantum-dot and cavity-field.The time-evolution of the concurrence, which is the entanglement measure of mixed states for the open system,is obtained explicitly as a parameter function of exciton-photon couplings g_1,2, the phonon-populations N_1,2 and the Huang-Rhys factorsλ_1,2.The entanglement sudden death（ESD） is found and is seen to be manipulated by the parameters g_1,2,N_1,2 andλ_1,2 as well.We moreover demonstrate the condition to suppress the ESD sufficiently.Finally,we propose a potential scheme to generate cluster states with one-step operation based on cavity quantum electrodynamics in a multiatom and microwave cavity system with an additional driven classical field.The proposed quantum operation avoids cavity-field induced decay duo to large detuning and may achieve the cluster states with ideal success probability.更多还原