Nonclassical Effects Of Open Quantum Systems Under Quantum Feedback Control

Non-classical effects are peculiar quantum effects,which result from interac-tion between quantum systems and are different from classical physics,such as uncertainty relations,squeezing effects,quantum correlations and quantum speed limit.In the theory of quantum optics and quantum information,non-classical ef-fects of quantum systems attract much attention all the time,and they have been applied in various aspects.However,due to the fact that real quantum systems inevitably interact with surrounding environments,this leads to decolierence,and hence destroys the non-classical effects.So,how to restrain decoherence and pro-tect the non-classical effects of open quantum systems is a difficult and hot topic in quantum optics and quantum informatics.With the development of quantum information science and technology,quantum feedback control theories have been widely used.In recent years,people find that quantum feedback control can per-fectly manipulate quantum systems.Then,can the quantum feedback control effectively manipulate the non-classical effects?This thesis makes use of the quan-tum feedback control to study the non-classical effects of quantum systems,and some innovative results are obtained.The primary contents and consequences are as follows:In Chapter 1,a brief introduction of open quantum system theories is given firstly.Then,introducing quantum feedback control theories and the derivation of feedback master equations.Finally,concepts of several typical non-classical effects are elaborated.In Chapter 2.the dynamical properties of the quantum-memory-assisted en-tropic uncertainty for a system comprised of a qubit to be measured and a memory qubit,are investigated.The behaviors of the entropic uncertainty and its lower bound have been explored in three different cases:Only one of the two qubits interacts with an external environment and subjects to quantum-jump-based feed-back control(QJB feedback control),or both of the two qubits independently experience their own environments and local QJB feedback control.The results reveal that the QJB feedback control with an appropriate feedback parameter can reduce the entropic uncertainty and its lower bound,and for the three different scenarios:the reduction of the entropic uncertainty relates to different physical quantities.Besides,the QJB feedback control not only can remarkably decrease the entropic uncertainty,but also can make the entropic uncertainty reach its lower bound where the dynamical map becomes unital.In Chapter 3,the entropy squeezing of a two-level atom coupled to a.dissi-pative cavity has been investigated under two different controls:In the first case QJB feedback is alone applied,whereas in the second case,the combined effect of QJB feedback and classical driving is considered,in which we provide a scheme to generate and protect steady and optimal entropy squeezing of the two-level atom.The results show that the entropy squeezing of atomic polarization components greatly depends on the control of QJB feedback and classical driving.Under the condition of designing proper QJB feedback parameters,the entropy squeezing can be generated and protected.Furthermore,when both QJB feedback and classical driving are simultaneously applied,steady and optimal entropy squeezing of the tvo-level atom can be obtained even though there is initially no entropy squeezing,which is explained by making use of the steady-state solution of the atom.In Chapter 4,quantum correlations of two atoms independently coupled to their own environments under local QJB feedback control are investigated.We analyze the dynamics of geometric measure of quantum discord and entanglemen-t,The results show that the protection of geometric measure of quantum dis-cord and entanglement strongly depends on quantum feedback parameters,initial atomic states and classical driving.Under the condition of designing proper QJB feedback parameters,geometric measure of quantum discord and entanglement can be protected.But,for a specific initial state,the evolution of entanglement is independent of the feedback parameter,while the evolution of geometric mea-sure of quantum discord relies on the chosen feedback parameter.The influence of classical driving on the evolution of quantum correlations is also explored.The results show that under appropriate feedback control,classical driving can protect geometric measure of quantum discord for a period of time in the initial stage,but not for entanglement.In Chapter 5.the quantum speed limit time of a two-level atom under QJB feedback control or homodyne-based feedback control(HB feedback control)has been investigated.The results show that the two different feedback control schemes have different effects on the evolutionary speed.By adjusting the feedback param-eters,the QJB feedback control can speed up atomic evolution from excited state while the HB feedback control cannot influence the limit speed.Besides,the quan-tum speed limit time for the whole dynamical process is also explored.Under the QJB feedback control,the quantum speed limit time exhibits oscillatory behaviors,which means multiple speed-up and speed-down processes during the evolution.While,the HB feedback control can accelerate the speed-up process and improve the speed of the uniform evolution process.In Chapter 6,a conclusion and outlook are presented.