Preparation Of Entangled State Based On Technologies Of Cavity Quantum Electrodynamics

It is well known that quantum entanglement lies in the heart of the quantum information science. It is also considered as an important resource for quantum com-munication and various computational tasks. Much attention has been focused on production and manipulation of entangled states, which promoted the development of experimental technique and led to the realization of entangled states in a variety of physical systems. Cavity quantum electrodynamics (QED) system is considered as one of the most ideal physical systems to realize quantum communication and quan-tum computing. The cavity QED system could combine the photons with atoms organically, which provides a technical foundation for the physical implementation of quantum information processing (QIP) and the establishment of optic quantum network. Recently, various schemes are presented for generating entangled states via the cavity QED techniques by domestic and foreign scientists. However, a quantum system used in QIP inevitably interacts with the surrounding environment, which re-mains a major obstacle to experimental realizations of the QIP. Therefore, it is very meaningful to suppress decoherence effect, meanwhile, implement the preparation of entangled state.In this dissertation, we first summarize the basic principles and methods of cavity QED system. Then we investigate the entanglement preparation based on some mature technologies which could effectively suppress the decoherence effect. The main research contents as following:Based on quantum Zeno dynamics, we propose two schemes to generate three-qubit decoherence-free state for atoms trapped in a cavity. Compared with the existing schemes, the operations in our schemes are much simpler, since no mea-surement, post selection and auxiliary bits are needed during the whole process. We investigate the influence of various decoherence processes such as spontaneous emission and photon loss on the fidelity of the entangled state. The quantum Zeno dynamics and the adiabatic passage can make our schemes robust against the de-coherence induced by cavity decay and spontaneous emission. We can generate the three-qubit decoherence-free state with a high fidelity.Based on quantum-jump-based feedback, we propose a scheme to generate steady four-atom decoherence-free state via four atoms with the Raman level con-figuration interacting with a single-mode vacuum cavity field. The scheme needs a detector to monitor the output of photons from the cavity mode. The effects of inef-ficiencies of detection and the atomic spontaneous emissions have been discussed in detail. Although the spontaneous emission still plays a negative role in the proposed system, we can improve the feedback control to reduce its effect. Furthermore, the scheme meets the condition of a strongly dissipative cavity easily and has a simpli-fied feedback control, so our scheme is more feasible for the current experimental techniques.Moreover, a scheme is presented for generating steady three-(four-) dimen-sional entangled states for two atoms trapped in a strongly dissipative single-mode (doublemode) cavity via quantum-jump-based feedback. Unlike the previous unitary dynamics schemes, the cavity decay is no longer undesirable, but plays an integral part in the schemes. With the help of quantum-jump-based feedback, any initial states can be used to generate steady three-and four-dimensional entangled states. Moreover, our scheme is insensitive to moderate fluctuations of experimental param-eters and detection inefficiencies without atomic decay since the system can always reach the target state. Nevertheless, the atomic decay still plays a negative role in the current scheme. Finally, we should note that the scheme can be generalized to realize N-dimensional entanglement for two atoms.All of these schemes are nearly deterministic and feasible with present experi-mental techniques, so they have very important value for the research of quantum information processing based on cavity quantum electrodynamics system and can provide reliable theory basis for the experiment implementation of optical quantum information and quantum computation.