  Quantum storage of light in a collective ensemble of atoms plays an important
role in quantum information processing. Consisting of a quantum repeater
together with quantum entanglement swapping, quantum memory has been
intensively studied recently. Conventional photon echoes have been limited by
extremely low retrieval efficiency and short storage time confined by the
optical phase decay process. Here, we report a storage time-extended near
perfect photon echo protocol using a phase locking method via an auxiliary spin
state, where the phase locking acts as a conditional stopper of the rephasing
process resulting in extension of storage time determined by the spin dephasing
process. We experimentally prove the proposed phase locked photon echo protocol
in a Pr3+ doped Y2SiO5 in a quasi phase conjugate scheme, where the phase
conjugate gives the important benefit of aberration corrections when dealing
with quantum images.
