Properties And Regulation Of Quantum Entanglement And Coherence Under Different Environmental Model

Quantum coherence and entanglement are essential physical resources for achieving some quantum tasks.However,all realistic quantum systems cannot be isolated,but would be affected by the external environment.The inevitable interaction between the system and the environment will lead to decoherence,which makes the initial entanglement and coherence of a particular quantum system for quantum tasks easy to be destroyed.Even before the completion of the quantum task,the entanglement and coherence of the quantum system have completely disappeared.Therefore,it is of great practical significance to investigate how to improve the steady-state coherence and entanglement of an open quantum system.Recent studies demonstrated that the non-Markovianity in open quantum dynamics can induce the revival of quantum coherence and quantum entanglement,which injects great vitality into the development of many fields with quantum coherence and quantum entanglement as resources.In addition,non-Markovianity has important applications in quantum communication,quantum information processing,channel discrimination,protection of quantum correlation,and so on.Therefore,it is of great significance to study how to enhance and manipulate the non-Markovianity in dynamics.This thesis mainly focuses on these two aspects,including the following parts:1.By means of quantum collision models,we have studied the entanglement dynamics of two-qubit and three-qubit systems.In the collision model we consider,the open system composed of two qubits interacts indirectly with the environment(or called reservoir)through an auxiliary qubit,while the reservoir is composed of a large number of qubits in the same state with the same frequency.In this work,we considered two cases of equilibrium thermal reservoir and non-thermal reservoir.The effects of the collision strength between two adjacent reservoir qubits,environmental temperature,and coherence in the environment on the dynamics of entanglement have been studied.Then,we further explored the conditions for the generation of the steady-state entanglement.Our results show that in the long-time limit,whether the steady-state entanglement can occur depends on the initial state of the system,the temperature of the environment,and the interaction strengths between the system qubits,but is independent of the collision strength between the reservoir qubits.In particular,we find that the coherence in the environment contributes to the generation of entanglement and can significantly improve the steady-state entanglement of the studied system.2.The steady-state properties of an open quantum system are investigated via the collision model approach of system-reservoir interaction.In our collision model,the system of interest consists of two coupled qubits,each of which interacts with its own independent thermal reservoir.Each thermal reservoir is modeled as a set of clusters of qubits(or linear harmonic oscillators).First,the steady-state entanglement of the system is studied.We show that collective interaction between the system and the elements(qubits or linear harmonic oscillators)in the clusters is beneficial to the generation and enhancement of the steady-state entanglement.And increasing the size of the clusters forming the low-temperature thermal reservoir is more conducive to the improvement of steady-state entanglement.Remarkably,we show that the steady-state entanglement can be greatly improved by choosing the suitable size of the clusters forming the thermal reservoirs.We also study the effect of the size of the cluster on the steady-state coherence.The numerical results show that for the qubit clusters,whether the steady-state coherence of the system can be enhanced by increasing the size of clusters depends on the coupling strength between the two system qubits and the coupling strength between the system and the thermal reservoirs.While for the case of the harmonic oscillator clusters,in addition to the coupling strengths,whether the steady-state coherence can be enhanced also depends on the temperature of the thermal reservoirs.3.A quantum system composed of two coupled qubits is considered,in which each qubit is weakly coupled with its own bosonic thermal reservoir.In our model,we considered the composite system-reservoir interaction(i.e.,the interaction Hamiltonian between the system and the thermal reservoirs consists of two parts:the parallel components and orthogonal components).By constructing the global quantum master equation without secular approximation,we study the effects of the parallel components in the interaction Hamiltonian between the system and the heat reservoirs on the steadystate coherence,steady-state entanglement,and steady-state heat flow.We show that in the non-equilibrium case,the steady-state coherence and steady-state entanglement of the system can be significantly improved by introducing parallel components into the interaction Hamiltonian between the system and the heat reservoirs.Increasing the parallel component of the interaction between the system and the low-temperature thermal reservoir is more conducive to the improvement of the steady-state coherence and entanglement of the system.We also find that increasing the parallel components in the interaction Hamiltonian can enlarge the temperature(or temperature difference)region where the steady-state concurrence maintains a nonzero value.The study of steady-state heat current shows that the parallel component in the interaction Hamiltonian can amplify the steady-state heat current in the system,but does not change the direction of the heat current.4.Employing collision models,we investigate the effects of the entanglement and coherence embedded in the environment on the non-Markovianity in open quantum dynamics.In this work,we present two collision models.One is that the system(a qubit)directly interacts with the environment,while the other is that the system qubit indirectly interacts with the environment through two auxiliary qubits.In both models,the environment is simulated by a large number of qubit clusters,and each qubit cluster is composed of two correlated qubits.We show that in both models,it is feasible to enhance the non-Markovianity in dynamics using the entanglement or coherence embedded in the environment.On one hand,the successive transitions between nonMarkovian and Markovian regimes for the system dynamics can be achieved by manipulating the initial entangled state of the environment qubits.In particular,we find that the initial state of the auxiliary qubits can also affect the non-Markovianity of the system in the second model.There exists an optimal combination of the initial environmental state and the initial state of auxiliary qubits,which can maximize the non-Markovianity.On the other hand,we find that the effect of the coherence in the environment on the non-Markovianity is closely related to the temperature.To improve the non-Markovianity,we can choose the appropriate parameters(e.g.,the phase of the coherence)according to the temperature of the reservoir.We also find that the initial coherence of auxiliary qubits can also improve the non-Markovianity.In terms of improving the non-Markovianity,compared with the case where the environment qubits contain coherence,adding coherence to the initial state of auxiliary qubits can achieve better results.