The Study Of Dynamical Characteristics And Relevant Issues On Quantum Information Processing In Open Quantum Systems

In most realistic situations,it is always inevitable to interact with surrounding environment for quantum systems which are crucial for the workings of most quantum information processing schemes.Such quantum systems are subjected to various types of quantum decoherence,which has presented a major obstacle to the construction of reliable quantum information devices.A quantum system has to be actually considered as an open quantum system.In the current development of quantum information,the study of relevant issues on open quantum systems has been thought as an important subject.It is necessary in theory to investigate the dynamical behaviors of open quantum systems.It is urgent in reality to investigate some relevant issues on quantum information processing in such systems.For examples,the analysis of affecting factors on the dynamics of an open quantum system is a theoretical basis of describing well some intrinsic effects of environment on the system dynamics and seeking those ways to overcome decoherence;the study of the protection of quantum states encoding quantum information in an open quantum system,which is the central issue in the current studies of quantum teleporation,is very urgent in the real application;to study quantum metrology in open quantum systems is also greatly significant.This thesis makes some studies about the above questions.In chapter 1,we give a brief introduction on open quantum systems and quantum decoherence.Then we review some developments and conclude previous works associating with the relevant studies on open quantum systems.The main content takes into account three parts as follows,the dynamical behavior of open quantum systems;the protection of quantum states and entanglement;quantum precision metrology.In chapter 2,we mainly introduce some relevant knowledge including some basic theories,concepts and laws,which in the content refers to some definition in quantum physics,the dynamical theory of open quantum systems,the theory of quantum measurement and feedback control and the theory of quantum parameter estimation.In chapter 3,we study affecting factors on the dynamics of open quantum systems based a quantum collision model we propose.Especially,we analyze the relations between these factors and non-Markovianity or memory effect in environment.These factors to be considered include the initial system-environment correlations,environment states and these couplings between system and environment.In chapter 4,we propose a scheme to protect quantum states against amplitude damping noise.Our scheme consists of quantum weak measurement,quantum feedback-forward and its reversal operations,by which we can protect well quantum states against decoherence.The main idea of our scheme is that according to different results from weak measurement before noise channel,we need to make a corresponding feedback so that the protected quantum state can be transformed into a frozen state almost free to the decoherence channel.After noise channel we recover the original quantum state by making some appropriate and reversal operations.Based on the scheme we achieve respectively the exact and optimal protection for an unknown quantum,two non-orthogonal states and entanglement states of two quantum bits.In chapter 5,we investigate the optimal parameter estimation with respect to the temperature of bath and analyze several features about parameter estimation of open quantum systems.We focus on the optimal estimation when the system lies in a non-steady state,for example the optimal measurement time and the optimal probe states.We explore the rule following which the optimal Fisher information and the optimal measurement time are varying with the number of probe particles.In chapter 6,we make a comprehensive summary for this thesis and suggest some topics for further research.