  Using on-demand coherence conversion via optical locking in conventional
stimulated (three-pulse) photon echoes, ultralong, ultraefficient photon
storage has been observed in a rare-earth Pr3+ doped Y2SiO5, where storage time
is several orders of magnitude longer and retrieval efficiency is enhanced by
fifty times. Compared with the same method applied to two-pulse photon echoes
for a rephasing halt, use of spectral grating in the stimulated photon echoes
offers a phase-independent coherence conversion between optical and spin
states. Eventually an ultralong spin population decay-dependent photon storage
protocol is achieved by applying optical locking to a spectral grating in
conventional stimulated photon echoes. The present breakthrough in both photon
storage time and retrieval efficiency opens a door to long-distance quantum
communications utilizing quantum repeaters.
