| Quantum entanglement, which can exhibit a nonlocal correlation between quan-tum systems that can not be accounted for classically, is at the heart of quantum information processing and quantum computation. The generation of quantum entangled states has attracted much attention and many physical systems includ-ing cavity quantum electrodynamics (cavity QED), trapped ions, nuclear magnetic resonance and quantum dots, have been suggested to generate entangled states. However, a real quantum system is, in general, influenced by its surrounding envi-ronment and the interactions between them leads to decoherence. This is the main obstacle in quantum information processing. Therefor, it is important to improve entanglement in quantum systems in the presence of decoherence. On the other hand, many efforts have been devoted to the study of the evolution of the joint system formed by two qubits in recent years. Particularly, it is pointed out that the entanglement of an entangled two-qubit system may disappear within a finite time during the dynamics evolution which is referred to as "entanglement sudden death" (ESD). Recently, it is reported that the ESD phenomenon has been observed in the lab for photon pairs and atomic ensembles. One important task of quantum infor-mation processing is the reliable transfer of quantum entanglement from one place to another, spin chains are suitable to perform short distant quantum communication and several schemes have been proposed based on spin chains.In this thesis, we investigate the entanglement dynamics of cavity QED sys-tems and show how to improve and control the entanglement generation, ESD. and ESB in cavity QED. We also discuss the influence of the phase decoherence. next-nearest-neighbor interactions and phase shift on the transfer of quantum states and entanglement in spin chains.First, we show how to control the ESD and ESB in cavity QED in the presence of decoherence. We consider a quantum system consisting of two noninteracting atoms each locally interacting with its own field. The two atoms, which are ini-tially prepared in entangled states, are driven by two classical fields additionally. We calculate the entanglement of two atoms and show that the ESD and ESB may appear in this system. The ESD and ESB time can be controlled by the classical driving fields. In addition, the amount of the entanglement of the two atoms can be significantly increased by applying classical fields. The ESD will disappear if the classical driving fields are strong enough. The entanglement between the atom and the cavity can also be increased by adjusting the classical driving field. We also propose a scheme to implement two-bit quantum phase gates and one-bit unitary gates by using the two-mode two-photon Jaynes-Cummings model. The entangle-ment between the atom and cavity is also investigated in the presence of phase decoherence. It is found that there is stationary entanglement which is sensitive with the detuning.Then, we investigate the entanglement dynamics of three-level atoms in cavi-ties. The relationship between the atomic coherence and the entanglement of two fields of the cavity is discussed. Our results show that the atomic coherence can induce the entanglement between two fields and the amount of entanglement is sen-sitive with the atomic coherence. If the phase decoherence is taken into account. there is stationary state entanglement between two fields. We also study the influ-ence of phase decoherence on the entanglement of the atom and fields. The fields can be in vacuum, coherent or thermal states initially. The entanglement of two three-level atoms and in two separated sets of cavities is also calculated. We find that the atom and the cavity can be disentangled in a finite time(ESD) and then become entangled again.Finally, we propose a scheme to transmit quantum entanglement and states through spin-1/2 chains in the presence of phase decoherence. We investigate the effect of phase decoherence on quantum communication through Heisenberg chain and show how to improve the entanglement and states transfer in the presence of phase decoherence by controlling the phase shift and the next-nearest-neighbor interactions. |
