Several types of quantummemory protocols have been presented over the last ten years,
including photon echoes, off-resonant Raman scattering, ultraslow light-based quantum mapping
processes and resonant Raman optical echoes. These quantum optical memory protocols are limited
by a storage time on a scale as short as milliseconds, determined by the spin phase decay time of the
storage medium. For applications of long-distance quantum communications, a quantum repeater
composed of quantum entanglement swapping and quantum memory must be used. Achieving longer
storage times in quantum memory therefore brings a definite advantage to applications of quantum
repeaters for long-distance quantum communications.
Here, we propose a quantum optical data storage protocol to extend the storage time by several orders
of magnitude beyond the conventional limitation of the order of milliseconds.
The present ultralong quantum optical storage technique is achieved by introducing an optical locking
method to the resonant Raman optical echo protocol.