Optimizing the efficiency of a quantum memory based on rephased amplified spontaneous emission

We studied the recall efficiency as a function of optical depth of rephased amplified spontaneous emission (RASE), a protocol for generating entangled light. The experiments were performed on the H43→D21 transition in the rare-earth doped crystal Pr3+:Y2SiO5, using a four-level echo sequence between four hyperfine levels to rephase the emission. The efficiency of RASE was observed to increase from 3% to 14% as the optical depth was reduced from 2.0 to 0.8. This is a significant improvement over the previously reported nonclassical result but is well short of the predicted efficiency. We discuss the possible mechanisms limiting the protocol's performance, and suggest ways to overcome these limits.