Highly efficient optical quantum memory with long coherence time in cold atoms

Optical quantum memory is an essential element for long-distance quantum communication and photonic quantum computation protocols. The practical implementation of such protocols requires an efficient quantum memory with a long coherence time. Beating the no-cloning limit, for example, requires efficiencies above 50%. An ideal optical fiber loop has a loss of 50% in 100 μs, and until now no universal quantum memory has beaten this time efficiency limit. Here, we report results of a gradient echo memory experiment in a cold atomic ensemble with a 1∕e coherence time up to 1 ms and maximum efficiency up to 87%_ 2%for short storage times. Our experimental data demonstrate greater than 50% efficiency for storage times up to 0.6 ms. Quantum storage ability is verified beyond the ideal fiber limit using heterodyne tomography of small coherent states.