Quadripartite Entanglement Generated From A Double A-type Atomic System

Quantum entanglement is an important resource in the field of quantum information science, which has more advantages for implementing the information processing, computation and transmission than the classical communication. The quantum communication by using quantum entanglement provides a new way for developing more rapid and secure communication of quantum information. Moreover, the preparation of multipartite entanglement is the key technology for establishing quantum information network and quantum computing. The theoretical and experimental research works have thus attracted interests for scientists.It was shown that the four-wave parametric interaction based on atomic coherence can be used to generate quantum entanglement beam with the wavelength matching the atomic node, the progress is of great significant for developing the multi-node quantum information network. Therefore, the preparation of multipartite entanglement via interaction between atoms and lights in atomic system is particularly important. Motivated by this, we discussed a theoretical scheme to generate the entanglement of four light fields in a three-level double-A atomic system. The effective Hamiltonian presented in this thesis provides a simple way to analyze the dependence of four mode entanglements on the interaction time, the detuning and strength of pump field, the strength of coupling force between light and atoms. These results show that the supposed system can be easily realized in experiment.The main research works of the thesis are:1. The fundamental theory for the interaction between atoms and field, the quantum entanglement including the concept, classification, criteria, and its applications were briefly reviewed, meanwhile, the entanglement preparation in the atomic system was also summarized.2. We discussed a theoretical scheme to generate the entanglement of four light fields in a three-level double-A atomic system driven by two counter-propagating far-detuned pump beams. Using the second-order perturbation theory, an effective Hamiltonian for this system was derived under the phase-matching condition, it revealed that the three-order nonlinear interaction is an important mechanism of generating four-mode entanglement.3. The quantum properties of the generated four-mode entanglement were discussed by using the inseparability criterion. Therefore, the dependence of entanglement on the detuning and rabi frequency of pump beams, atomic ground state splitting, the ratio of rabi frequency of two counter-propagating pump fields, and the coupling constant of light fields with the atomic system were discussed. The results for obtaining the best degree of entanglement were analyzed with the concrete parameters of Rb and Cs atomic system.