| Quantum entanglement is a strange phenomenon of quantum mechanics, and it is a research field which always be noted in Physics. Quantum entanglement is the key factor of experimental realization of quantum communication. Quantum state is the carrier of quantum information. Quantum information processing is the information transmission of the encoded quantum state. The preparation of quantum state and the evaluation of the quality of information transmission are very important tasks in this research field. The intensity-dependent coupling two-mode multiphoton Jaynes-Cummings model and the system that consists of a moving two-level atom interacting with a frequency variation field are significative typical quantum systems. This thesis has investigated entanglement evolution, preparation of quantum states and the fidelity of quantum information in the two systems. A series of significant results are obtained.1. The effective Hamiltonian of the two-mode multiphoton Jaynes-Cummings model is given via the intensity-dependent coupling in the rotating wave approximation. Considering strong field, the atom-field entanglement and the entanglement between two modes of coherent field, according to the above model, are investigated using the quantum reduced entropy and quantum relative entropy, respectively. It is showed that properties of these two types of entanglements are considerably relevant to the absorption or emission photon number k, per atomic transition. Different properties of entanglements in the processes of two-photon (k=1) and multiphoton (k>2) are revealed respectively. Given the preparation of entangled states discussed, the EPR states of atom-field irrelevant to time, as well as entangled states between two models of the coherent field are prepared, respectively.2. In the rotating-wave approximation, considering atomic motion and the field frequency varying with time in the form of sine-function at the same time, the evolution of the field quantum entropy in the system that consists of a two-level atom interacting with a single-mode field are studied. In two cases of neglecting atomic motion and considering atomic motion, figures of the time evolutions of the field entropy are plotted respectively using numerical calculations, and the influences of the atomic motion, the field-model structure parameter, amplitude and angular frequency of the field-frequency variation on the field entropy are discussed. The atom-field entangled states, field fock states and atomic high fidelity states are prepared by analytic method and the related system parameters of these quantum states operation are acquired. The results show that the time evolution behavior of the field entropy is modulated by the frequency variation of field, the interaction between the field and atom will weaken with the increase of the amplitude of variation of the field frequency, the period of the field entropy agrees with the period of field-frequency variation; the atomic motion will result in the period of the field entropy doubled; the evolution of the field entropy is related to the parity of field-mode structure parameter; the approximate EPR states of field-atom can be prepared periodically whether the atom moves or not.3. In the rotating-wave approximation, considering atomic motion and the frequency of the field varying with time in the form of sine-function at the same time, the evolution of the fidelity of quantum information in the system that consists of a two-level atom interacting with a single-mode field are studied. The influences of the initial state of the atom, the atomic motion, the field-model structure parameter, amplitude and angular frequency of the field-frequency variation on the fidelity of quantum states are discussed by numerical calculations. The results show that the complete fidelity output of the atomic state can be obtained by choosing appropriate initial atomic state, appropriate atomic velocity and appropriate field-model structure and adjusting field-frequency variation. |
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