Filters For Quantum Memory In Atomic Ensemble

Quantum computation, quantum communication, quantum crypto-graphy as three major research areas of quantum information have become important topics in academic circles because of its attractive application aspects, and quantum communication causes widely attention due to its unique safety and high efficiency. However, because of the exponential growing communication error rate with the increase of the distance, long-distance quantum communication seems to be very difficult. In classical communication, people set up repeater stations at a particular position in channel to magnify signal and restore the original waveform, so as to solve the problem of signal attenuation. By using this method, quantum repeater can be used to realize long-distance quantum communication.Quantum repeater was proposed firstly by Briegel et al in1998. Quantum memory is a core technology of quantum repeater; its main function is the realization of storage and release of quantum information by means of coherence characteristic of quantum states. Photon becomes an ideal information carrier in quantum memory due to its fast spread and insusceptible to outside environment; atomic ensemble is the ideal storage medium of quantum information due to collective excitation effect and controllability.So far, quantum storage has realized based on atomic ensemble utilizing many ways such as EIT effect, far off-resonant two-photon Raman process and so on. However, an important problem in quantum storage based on the interaction of light with multi-level atom is the release of one particular light pulse. For example, in order to detect weak signal lights in the quantum storage based on far off-resonant two-photon Raman process, the strong write/read pulses and fluorescence lights which is produced by write/read pulses which have GHz frequency detuning with weak signal lasers must be separated or filtered from signal lasers, filters play the crucial role in the whole experiment. The thin-film interference filter’s bandwidth is difficult to be narrowed (～100GHz), cesium atomic filter based on optical pumping, confocal F-P cavity and the temperature-stabilized bulk etalon filter for the detection of weak signal pulse are used.This paper introduces several filters for quantum memory in atomic ensemble, which is mainly divided into following parts:1. The introduction of background knowledge about some usual optical filters which consist of atomic optical filter, grating diffraction filter, interference filter as well as the applications of optical filters in quantum storage.2. In order to filter one of two laser beams with～GHz frequency difference and detect another one successfully, we introduce theoretical model and experimental principle of atomic filter based on optical pumping, the transmission rate of probe lights as a function of pumping laser intensity and temperature of cesium atoms are discussed. Then we make a comparison of experiment results with hollow pumping laser beam scheme and solid pumping laser beam scheme, which aims to avoid the influences on the weak probe laser beams due to the reflected pumping laser beam.3. We design a temperature-stabilized, tunable resonant transmission frequency bulk etalon filter utilizing the multi-beam interference principle which has wide applications in quantum storage based on atomic ensemble. Transmission rate and the resonance transmission frequency detuning versus the temperature of etalon are researched in experiments and theory. However, its distinction ratio is still insufficient for the detection of single-photon-level optical pulse. In order to obtain higher distinction ratio and narrower band, we try to use a combination of atomic filter and Etalon filter. According to the property of tunable resonant transmission frequency of Etalon, we extend it to laser frequency stabilization system. 4. The applications of atomic filter and temperature-stabilized bulk etalon filter in quantum storage based on EIT and a far off-resonant two-photon Raman process.