Gradient echo memory in an ultra-high optical depth cold atomic ensemble
  Quantum memories are an integral component of quantum repeaters - devices that will allow the extension of quantum key distribution to communication ranges beyond that permissible by passive transmission. A quantum memory for this application needs to be highly efficient and have coherence times approaching a millisecond. Here we report on work towards this goal, with the development of a $^{87}$Rb magneto-optical trap with a peak optical depth of 1000 for the D2 $F=2 \rightarrow F'=3$ transition using spatial and temporal dark spots. With this purpose-built cold atomic ensemble to implement the gradient echo memory (GEM) scheme. Our data shows a memory efficiency of $80\pm 2$% and coherence times up to 195 $\mu$s, which is a factor of four greater than previous GEM experiments implemented in warm vapour cells. 
B. M. Sparkes, J. Bernu, M. Hosseini, J. Geng, Q. Glorieux, P. A.   Altin, P. K. Lam, N. P. Robins, B. C. Buchler
quant-ph physics.atom-ph