  A high-storage efficiency and long-lived quantum memory for photons is an
essential component in long-distance quantum communication and optical quantum
computation. Here, we report a 78% storage efficiency of light pulses in a cold
atomic medium based on the effect of electromagnetically induced transparency
(EIT). At 50% storage efficiency, we obtain a fractional delay of 74, which is
the best up-to-date record. The classical fidelity of the recalled pulse is
better than 90% and nearly independent of the storage time, as confirmed by the
direct measurement of phase evolution of the output light pulse with a
beat-note interferometer. Such excellent phase coherence between the stored and
recalled light pulses suggests that the current result can be readily applied
to single photon wave packets. Our work significantly advances the technology
of EIT-based optical memory and may find practical applications in
long-distance quantum communication and optical quantum computation.
