Quantum Retire Coherent Problem. Field Interaction With Atoms

In 1935, Einstein, Podolsky and Rosen brought forward the famous EPR correlation. Since then the concept of quantum entanglement state was given birth to. Entanglement is one of the most striking features of quantum mechanics, and plays key pole in quantum information science. So, people pay more attention to the generation and manipulation of the quantum entanglement states. In recent years, quantum information science, which mainly includes quantum computer and quantum communication, has evolved as a new research field. Specially, the significant achievements obtained in experiments extremely promote the progress of the quantum optics and quantum information theoretical research. The preliminary applications of quantum-entangled state in quantum computer and quantum communication extremely promote the progress of the quantum optics and quantum information theoretical research.In quantum information process, because of the inevitable couple between the quantum system and environment, the quantum system loses its quantum characteristics such as nonlocality and entanglement if it is open to the environment. It is important to study how the quantum property such as entanglement, coherence, and nonlocality evolve with time.In this pape, we mainly investigate some quantum optical and quantum informational problems.The main results of this thesis are as follows:1. We investigate the time-evolution of the atom's occupancy in the system of two-mode coherent fields interacting with two atoms via multiphoton process. The results show that the atom's occupancy is zero if the two atoms are initiallyin Bell state . The atom's occupancy is influenced by threefactors, which are the intensity of two dipole-dipole coupled atoms, the coupling constant of light and atoms and the absorbing or emitting photon number per atomic transition, if the two atoms is initially in the other three Bell states. The oscillation frequency of the atom's occupancy accelerates apparently with the increase of the intensity of two dipole-dipole coupled atoms or the absorbing or emitting photon number per atomic transition. The collapse — revivalphenomenon of the atom's occupancy disappear when the intensity of two dipole-dipole coupled atoms is increased to some point.2. We investigate the time evolution of the entanglement in the interacting system of a single two-level atom and a single mode coherence field with phase decoherence. We find that the influence of the detuning on time evolution of the entanglement is obvious without phase decoherence and the quantum state of the field and atom will stay in a most entangled state for a long time when the detuning is proper. The phase decoherence causes the fading of entanglement when the system is resonant and it will fade to zero at last. The entanglement of the system is a fading oscillation at first with phase decoherence. The detuning restrains the phase decoherence with the time evolution. In this case the entanglement of the system will not fade to zero at last. The oscillating amplitude of the entanglement is lessening at first and the entanglement will quickly become a constant value with the increasing of detuning. The system will stay in an entangled state with phase decoherence if the detuning is proper.3. In this paper, the problem of the decoherence of two-state quantum system interacting with a bath through an external controllable driving field is discussed. The time evolution of the off-diagonal elements of the density operator in two- state quantum system ms is obtained. The relation between the decoherence and the external controllable driving field is also obtained. Thus we establish the restriction equation that must be satisfied by the external controllable driving field when the docoherence is to vanish.