High-efficiency Coherent Optical Memory based on Electromagnetically Induced Transparency.

Quantum memory is a crucial component in the long-distance quantum communication based on quantum repeaters. To outperform the direct transmission of photons with quantum repeaters, it is crucial to develop quantum memories with high fidelity, high efficiency and a long storage time. Here, we present our work to achieve a storage efficiency of larger than 90{\%} for a coherent memory based on the electromagnetically induced transparency (EIT) scheme in cold atomic media with an optical depth of \textasciitilde 1000. At a storage efficiency of 50{\%}, we also obtain a fractional delay of 1200. At high optical depths, nonlinear optical effects, such as the photon switching and four-wave mixing due to the off-resonant excitation of the EIT control field, may become significant and introduce complications in quantum memory applications. We discuss and present methods to reduce these complications.