  Quantum memory is important to quantum information processing in many ways: a
synchronization device to match various processes within a quantum computer, an
identity quantum gate that leaves any state unchanged, and a tool to convert
heralded photons to photons-on-demand. In addition to quantum computing,
quantum memory would be instrumental for the implementation of long-distance
quantum communication using quantum repeaters. The importance of this basic
quantum gate is exemplified by the multitude of optical quantum memory
mechanisms being studied: optical delay lines, cavities,
electromagnetically-induced transparency, photon-echo, and off-resonant Faraday
interaction. Here we report on the state-of-the-art in the field of optical
quantum memory, including criteria for successful quantum memory and current
performance levels.
