Vacuum Fluctuations And Entanglement Dynamics And Radiative Properties For Quantum Systems

The vacuum is no longer a completely empty space in quantum sense,since it fluctuates all the time.Thus any real quantum system in the quantum world inevitably couples to the external environment,which is no longer considered as an isolated system.One environment which no system can be isolated from is the vacuum,and different types of fluctuating fields will lead to various dynamical behaviors of the quantum systems.Recently the direct detection of gravitational wave signals from black hole merging systems,on the one hand confirms the prediction based on Einstein's general relativity,on the other hand,stimulates one's more curiosity about what will happen if gravitational waves are quantized.One of the consequences is the quantum fluctuations of spacetime itself,if we accept that the basic quantum principles we are already familiar with apply as well to a quantum theory of gravity.In this dissertation,we study two effects associated with quantum fluctuations of spacetime itself,the spontaneous excitation and quantum entanglement generation of an elementary quantum system,especially in comparison with the vacuum matter field cases.In addition,the vacuum field modes can be modified in the presence of a boundary in a flat spacetime,and it has been demonstrated that this modification in vacuum fluctuations can lead to a lot of novel effects,such as the Casimir effect,the light-cone fluctuations,and the Brownian motion of test particles in an electromagnetic vacuum.Therefore,we next investigate the effects of the existence of a reflecting boundary on quantum entanglement dynamics of a pair of two-level atoms moving with a uniform acceleration.Finally,we concentrate on another expected observable effectdue to quantum-field vacuum fluctuations,the fluctuation-induced interactions between two atoms,known as the Casimir-Polder interactions,and utilize a simpler method,in the framework of open quantum systems,to compute the CasimirPolder interaction between a pair of two-level atoms.The main results come as follows:1.We study the spontaneous excitation of a gravitationally polarizable atom with a uniform acceleration in interaction with a bath of fluctuating quantum gravitational fields in vacuum,and compare the result with that of a static one in a thermal bath of gravitons at the Unruh temperature.We find that in analogy to the matter field cases,under the fluctuations of spacetime itself,transitions to higher-lying excited states from the ground state are possible for both the uniformly accelerated atom in vacuum and the static one in a thermal bath.The appearance of power terms in acceleration in the transition rates suggests that the equivalence between uniform acceleration and thermal field is lost.2.Based on the theory of open quantum systems,we calculate the CasimirPolder interaction of two atoms in an arbitrary quantum state under fourth-order perturbation approximation.We find that when the two-atom state satisfies some certain conditions,the second-order perturbation is the leading order,or the Casimir-Polder interaction is at least fourth-order perturbation effect.3.We find that,in analogy to the scalar field case,a bath of fluctuating quantum vacuum gravitational fields serves as an environment that provides indirect interactions between the two gravitationally polarizable subsystems,which may lead to entanglement generation.However in contrast with the scalar field case,the entanglement generation under fluctuations of spacetime itself is crucially dependent on the polarizations.We also find that it is easier for the two atomsaligned parallel to the reflecting boundary to get entangled.4.In the framework of open quantum systems,we study the entanglement dynamics for two uniformly accelerated two-level atoms in interaction with a bath of fluctuating electromagnetic fields in vacuum with the presence of a reflecting boundary.For the parallel case,the initial entanglement of two transversely polarizable atoms very close to the boundary can be preserved as if it were a closed system,while for two vertically polarizable atoms,the concurrence evolves two times as fast as that in the free space.The birth time of entanglement can be noticeably advanced or postponed for the parallel two-atom system placed close to the boundary,while the maximal concurrence during evolution can be significantly enhanced when the atoms are vertically aligned.