An ultra-high optical depth cold atomic ensemble for quantum memories

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Quantum memories for light lie at the heart of long-distance provably-secure communication. Demand for a functioning quantum memory, with high efficiency and coherence times approaching a millisecond, is therefore at a premium. Here we report on work towards this goal, with the development of a 87Rb magneto-optical trap with a peak optical depth of 1000 for the D2 f = 2 → F′ = 3 transition using spatial and temporal dark spots. With this purpose-built cold atomic ensemble we implemented the gradient echo memory (GEM) scheme on the D1 line. Our data shows a memory efficiency of 80 ± 2% and coherence times up to 195 μs.