Storage Optical Information In Ultra-high Optical Depth Cold Ensembles

The achievement of quantum memory for light is of significance, because the processes of light storage will preserve its quantum properties, which is significantly useful to sorts of quantum information systems, such as secure communication and quantum computation. Ideally, one desire to obtain the quantum memroy system with the efficiency approaching unity, with the storage times from milliseconds to seconds, with the bandwidth capble to store a great deal of information simultaneously, and without any noise. During the past several years, var-iously different techniques, such as electromagnetically induced transparency and gradient echo memory have been proposed to achieve these requirements. Ideally, both of them have the potential to store quantum information with the efficiency of unity and with high fidelity quantum memory; however, the storage efficiency exprimentally depends on the optical depth of the slow-light medium. Therefore, the increase of optical depth is of significanly importance to quantum efficiency, which is still a great challenge. On the other hand, it should be pointed out that the nonlinear processes such as four-wave mixing will be more obvious at large optical depth, which may in turn contribute noise to the output, thus diminishing the suitability of EIT as a quantum memory.On this thesis, we focus on the generation of cold atom assemble with ultra-large optical depth up to1000, and the achievement of quantm memory based on electromagnetically induced transparency and gradient echo memory in this cold atomic assemble.Firstly, we investigate the delay of optical pulses using electromagnetically induced trans-parency in a cold atomic ensemble with the optical depth exceeding500. We employ a four energy levels model and conducte the experiments in both Rb85and Rb87independently to identify the influence of four-wave mixing on electromagnetically induced transparency.The theorical calculations are excellent agreement with our experimental results in both isotopes. In Rb85four-wave-mixing contributes to the output; however, in Rb87, the four-wave mix-ing is negligible. We obtained one pulse-width of delay with the efficiency of50%in Rb87. And in Rb85, we got a delay-bandwidth product of3.7at50%efficiency, allowing temporally multimode delay, which we demonstrate by compressing two pulses into the memory medium. Furthermore, based on the slow light effect of electromagnetically induced transparency, the light pulse was delayed by100us, implying that the group velocity of this optical pulse was roughly lowered to120m/s.Secondly, We used cold atomic ensembles to improve the storage time of gradient echo mem-ory. The maximum coherence time of350μs in cold atoms is seven times greater than that of the warm vapour system.The corresponding efficiency is80%, which is the highest efficiency achieved with a cold atomic ensemble so far.Thiedly, We also report the works toward an optical memory for dual-rail qubits based on a cold atomic ensemble. The measured storage efficiency of these two signals were39%and32%, respectively. We made use of the Zeeman-split Raman absorption lines to store two signals pulses in one memory and demonstrate that the relative phase and amplitude between the pulses is preserved. We extend the protocol to make use of the Zee-man sub-levels to store and recall two frequency channels.The multi-mode quantum memory with high storage efficiency, long storage time paves a way to store frequency qubits with a high fidelity, and may provide a basic guide to the potential application of future quantum computation and communication.