The Effect Of Quantum Vacuum Fluctuations On Quantum Metrology And The Spontaneous Excitation And Geometric Phase Of Circularly Moving Atom

Every quantum system is inevitably in interaction with the environment. Different environment will lead to different behaviors of evolutions of the quantum systems. One environment which no system can be isolated from is the vacuum that ?uctuates all the time in quantum sense. The curvature of the space-time and the existence of the boundary will modify the distribution of the ?eld, which will lead to different behaviors of the evolutions of the coupled quantum systems.In this dissertation, on the one hand, we will use a two-level atom as the simpli?ed model of quantum system to study the quantum thermal effects caused by the non-inertial motion of the atom. We will study the spontaneous excitation and geometric phase of a circularly moving atom coupled to electromagnetic ?eld in comparison with the linear acceleration case and the scalar ?eld case. On the other hand, we will analyze the effects of vacuum ?uctuations on quantum metrology without and with boundary. We have obtained the following results:1. We study, using the formalism proposed by Dalibard, Dupont-Roc and Cohen-Tannoudji, the contributions of the vacuum ?uctuation and radiation reaction to the rate of change of the mean atomic energy for a circularly accelerated multilevel atom coupled to vacuum electromagnetic ?elds in the ultrarelativistic limit. We ?nd that the balance between vacuum ?uctuation and radiation reaction is broken, which causes spontaneous excitations of accelerated ground state atoms in vacuum. Unlike for a circularly accelerated atom coupled to vacuum scalar?elds, the contribution of radiation reaction is also affected by acceleration, and this term takes the same form as that of a linearly accelerated atom coupled to vac-uum electromagnetic ?elds. For the contribution of vacuum ?uctuations, we ?nd that in contrast to the linear acceleration case, terms proportional to the Planckian factor are replaced by those proportional to a non-Planck exponential term,and this indicates that the radiation perceived by a circularly orbiting observer is no longer thermal as is in the linear acceleration case. However, for an ensemble of two-level atoms, an effective temperature can be de?ned in terms of the atomic transition rates, which is found to be dependent on the transition frequency of the atom. Speci?cally, we calculate the effective temperature as a function of the transition frequency and ?nd that in contrast to the case of circularly accelerated atoms coupled to the scalar ?eld, the effective temperature in the current case is always larger than the Unruh temperature.2. We study, in the framework of open quantum systems, the time evolution of a circularly accelerated two-level atom coupled in the multipolar scheme to a bath of ?uctuating vacuum electromagnetic ?elds. We ?nd that both the spontaneous transition velocity and the geometric phase for a circularly accelerated atom do not exhibit a clear sign of thermal radiation characterized by the Planckian factor in contrast to the linear acceleration case. The spontaneous transition rates and effective temperature of the atom are examined in detail in the ultrarelativistic limit and are shown to be always larger than those in the linear acceleration case with the same proper acceleration. Unlike the effective temperature, the geometric phase is dependent on the initial atomic states. We show that when the polar angle in Bloch sphere, θ, that characterizes the initial state of the atom equals π/2, the geometric phases acquired due to circular and linear acceleration are the same.However, for a generic state with an arbitrary θ, the phase will be in general different, and then we demonstrate in the ultrarelativistic limit that the geometric phase acquired for the atom in circular motion is always larger than that in linearacceleration with same proper acceleration for θ ∈(0,π2) ∪(π2, π).3. We study the dynamics of the quantum Fisher information for the atomic parameter estimation for a static polarizable two-level atom coupled in the multipolar scheme to a bath of ?uctuating vacuum electromagnetic ?elds without and with the presence of a re?ecting boundary. When we estimate the parameters of initial atomic state, we ?nd that, in the case without a boundary, the electromagnetic vacuum ?uctuations always cause the quantum Fisher information of the initial parameters and thus the precision limit of parameter estimation to decrease.However, with the presence of a boundary, the quantum Fisher information becomes position and atomic polarization dependent, and thus the precision of the initial parameter estimation may be decreased, enhanced or remain unchanged as compared to the case without a boundary depending on the position and polarization. When the atom is extremely close to the boundary and is transversely polarizable, the quantum Fisher information may even be shielded from the in?uence of the vacuum ?uctuations and remains constant with time as if it were a closed system. For the estimation of the atomic frequency, there exist a maximum quantum Fisher information and optimal measurement time, which can also both be enhanced or decreased as compared to the case without a boundary.