Preparation Of Entanglement State Based On Quantum Feedback Control

Quantum entanglement occupies an important position in quantum informa-tion science as an important resource for quantum communication and quantum computation. The preparation of entanglement is an important issue in quantum information. The ideal quantum system is closed, so the system is able to keep its quantum coherence. However, the real quantum system is open to outside environ-ment and it will inevitably interact with the environment, thus quantum coherence will be destroyed. The cavity quantum electrodynamics system provides an ap-propriate platform for quantum information processing, but a quantum system will inevitably interact with its surroundings and the information loss can be caused. The typical decoherence factors include cavity decay and atomic spontaneous emission. These decoherence factors make it an obstacle to experimentally generate faithful and reliable entanglement. Recent experimental efforts on real-time monitoring and manipulating individual quantum system indicates that a quantum feedback control may be one possible way to generate entanglement and prevent destruction induced by decoherence. Therefore, it is meaningful to preparation of entanglement in the cavity with the premise that the decoherence is suppressed effectively.In this thesis, we mainly research preparation of steady atom entanglement in a dissipative cavity via quantum feedback control. The main research contents are as follows:A steady three-atom Greenberger-Horne-Zeilinger (GHZ) state is prepared in a dissipative cavity via quantum feedback control. We consider a bimodal cav-ity system in which three atoms are trapped in, two photon detectors are placed outside of the cavity to detect the photon leaked out of the cavity. The quantum feedback control is only applied to a single atom based on quantum-jump detection to improve the fidelity of the target state. The required interaction time need not to be accurately controlled. Thus, the cavity decay plays a key role in obtaining the target state, the fluctuation of the cavity decay rate has almost no influence on the generation of steady entanglement. In addition, the large detuning case is con-sidered so that the excited states are canceled adiabatically in the whole procedure, thus the influence of atomic spontaneous emission is largely suppressed. Through the above process, a nearly perfect GHZ state can be achieved under the condition of a strongly dissipative cavity.A steady three-atom W state is prepared in a dissipative cavity via quantum feedback control. Three atoms are trapped in a single-mode cavity and a photon detector is placed outside of the cavity to detect the photon leaked out of the cavity. The feature of our present scheme is that it can work in the low-Q regime, the cavity decay is helpful for obtaining the intended entanglement and the system is deterministically driven to the intended steady state in the presence of strong cavity decay without accurately controlling evolution time. In addition, the large detuning case is considered so that the excited states are canceled adiabatically in the whole procedure, thus the influence of atomic spontaneous emission is largely suppressed. Through the above process, a nearly perfect W state can be achieved under the condition of a strongly dissipative cavity.The feasible analysis for above schemes show that all of schemes can be realized with present experimental techniques. We hope that our work could offer important reference for the experimental realization of a steady multiqubit GHZ state and W state.