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

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 Rb-87 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 mu s.