Theoretical Study On Generation Of Multipartite Entangled States

With the coming of information age,the requirements of information processing increase more and more.In order to break the limit of classical physics to bring us more powerful methods of information processing,quantum information science is established.It is based on quantum mechanics,a theory to describe the essential rules of the micro-world.Quantum entanglement is the foundation of quantum information technologies and the prerequisite to realize quantum computation and quantum communication.Due to quantum entanglement,a novel phenomenon in quantum mechanics,it is possible that quantum information technologies may surpass some classical technologies in information processing.So quantum entanglement is also considered as an important resource.The research on quantum entanglement either allows us to understand quantum mechanics more profoundly in order to answer some basic question of quantum mechanics or to utilize it in quantum computation and quantum communication.Therefore,the research on quantum entanglement has significant both theoretical and practical values.This thesis mainly study multipartite quantum entanglement theoretically,and there are five chapters in this thesis in total.Chapter one is a brief introduction of a new inter-discipline, quantum information.In chapter two,we introduce the quantum entanglement,a very important resource in quantum information applications.We discuss the criterion and the measure of the entanglement;review some basic schemes of generation of the entanglement and discuss some basic theories using in these schemes.In chapter three,we mainly study the thermal entanglement in Heisenberg spin chains under rectangle magnetic field.We study the effect of external magnetic field on entanglement,and propose a scheme to improve the critical temperature by adjusting the magnetic field.In chapter four,we mainly propose a scheme of generation three-mode entanglement by using the interactions between four-level cascade atoms and cavity fields.Based on some previous researches,we find a method to generate three-mode entanglement.We also study the effects of some parameters on the entanglement in this system such as the Rabi frequencies of the classical fields and the initial state.In chapter five,we propose a new scheme of generation genuine three-mode entanglement.Due to the introducing of the optical parametric oscillator,we get the genuine three-mode entanglement. We study the contributions to the entanglement in two aspects,the parametric oscillator and the interaction between atoms and cavity.We also analyze the structure of the three-mode entanglement and verify the existence of genuine three-mode entanglement.Finally,we give the measure of the entanglement in every two modes and the global entanglement sharing by all the three modes.