Backward retrieval in optical quantum memory controlled by an external   field
  A scheme for backward retrieval in optical quantum memories in which information is stored in collective states of an extended resonant atomic ensemble is developed such that phase conjugation can be implemented by application of an external nonuniform electric (magnetic) field without use of coherent exciting pulses. The possibilities of realizing such a scheme using resonant solid-state materials are discussed. 
Alexey Kalachev, Stefan Kroll