The Applications Of Cold Atom Ensemble In Quantum Information Processing

The atom-photon interaction is a fundamental issue in quantum optics. Such intreactions will be resonantly enhanced under the condition of Electromagnetically Induced Transparency. It is well known that nonlinear optical processes can be resonantly enhanced with Electromagnetically Induced Transparency (EIT) by simultaneously creating a destructive interference in absorption and a constructive interference in the nonlinear susceptibility. The EIT process in the coherent medium has many imporatant applications. The cold atomic ensembles, isolated from the outside environment, have many advantages for realizing the quantm repeator in a quantum network such as low tempreture, powerful constrollability. Such quantum repeator could be realized based on the nonlinear optical processes. This issue will theoretically and experimentally study several nonlinear optical processes in a cold Rubidium85ensemble, such as four-wave mixing, double four-wave mixing, EIT based light storage and retrieval.Firstly we give the theoretical model of EIT in a classicial Lambda-type three level system.we plot the transmission and dispersion curve under the EIT condition. We also show that the third-order nonlinear suspectibility of the system will be resonantly enhanced under the EIT condition.Then we describe the experimental realization of the cold atom ensemble (Rb85) in a Magneto-optical Trap. Since a large optical depth will help to enhance the photon-atom interaction and increase the nonlinear optical suspectibility, we build up a classical three-dimentional MOT and a two-dimentional MOT, respectively. A Magneto-optical Trap normally cinsists of a ultrahigh vacuum chamber, the laser system, the magnetic field gradient, the monitor and the timing system. We successfully obtain the sigar shaped atomic cloud and optimize the optical depth of the cloud.Based on such a two dimensional atomic cloud, we realize the nondegenerate four-wave mixing process and double four-wave mixing by adding another coupling field. We generate a so-called the bichromatic field, which has a narrow bandwidth and could be used to produce the colour state in the low light level.After that we study the storage and retrieval of the probe field and the generated four-wave mixing (FWM) field under the condition of EIT. When the coupling and pump fields are adiabatically switched off,the generated FWM field and the probe could be simultaneously stored in the collective ground state spin excitations.And they could be retrieved from the ground state coherence by switching on the coupling and pump fields. Such a storage and retrieval process shows damped oscillations in an external uniform magnetic field, which could be explained as the Larmor precession of the phase in the ground state coherence.In summary we study a series of the atom-photon interactions under the conditions of EIT. We believe the cold atomic ensemble has promising prospect in the applications of the quantum communication.