Amplified Light Storage With High Fidelity Based On Electromagnetically Induced Transparency In Rubidium Atomic Vapor

Quantum memory is essential for quantum information processing,including quantum communication and quantum computation. Quantum memory with high storage efficiency and storage fidelity and long coherence time is a keys for the successful operation of long-distance quantum communication and quantum information processing. Because of its propagation speed is about3*108m/s in natural medium, the photon is regarded as an ideal carrier for the information transmission. On the other hand, the atomic medium is regarded as an ideal medium for storing optical quantum information because of its low decoherence rate between the ground states. Several storage devices based on different mechanisms, such as EIT and time-space reversing method, have been proposed and demonstrated experimentally.In this paper, for an typical three-level atomic scheme, we calculate the fidelity and storage efficiency for the retrieved signal with and without Doppler broadening. Because of Doppler broadening, a FWM process is possible in many EIT-based quantum memory. This idler field results in the noise and the lower storage efficiency and fidelity. It is resulted from the extra photons generated directly from the vacuum due to FWM gain and the finite population which the FWM adds to the excited state, leading to dephasing of the dipoles due to spontaneous emission. In the practical application, the high storage efficiency and the high fidelity are necessary for light storage in hot atoms. We need to find a way to improve the storage efficiency.We propose an N-type scheme to suppress the FWM effect and then to improve the storage efficiency by introducing a pump field beyond three-level Lambda-type configuration. The pump field provides a Raman gain for the probepulse so as to compensate the probe loss before it is stored in the medium.Furthermore, the delay time of the slow light is decreased and even slow light for the probe pulse could be converted to fast light by the action of the added pump field. Thus the waveform distortion of the probe pulse is reduced and its fidelity during the storage process could be maintained. The numerical results show that the storage efficiency is significantly improved while its fidelity keeps a rather high level.As known, the storage efficiency and the fidelity are governed by the optical depth OD and storage time. The previous studies predict that the storage efficiency of EIT-based memory could increase significantly with the optical depth. And the longer storage time results in an exponential reduction of the retrieved signal pulse energy due to the spin wave decoherence during the storage process. So with the increasing storage time the efficiency decreases gradually. In order to achieve a higher quality of quantum storage, its characters, such as high storage efficiency and long coherence time, become the hot topic to be studied. In this paper, we calculate the fidelity and storage efficiency as a function of the storage time and the optical depth, respectively. We find that the storage efficiency could increase even over unity with the increasing optical depth and decrease with the prolonging storage time. These results build a good theoretical basis for further research work.