Quantum Decoherence And Preparation Of Entangled States

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.