  Photon echo-based quantum memories demonstrated in rare-earth doped solids
over the last decade have solved the major constraint of population inversion
in conventional photon echoes by using collective atom phase controls. Both
atomic frequency comb and gradient echoes have also made a breakthrough in
higher echo efficiency, where conventional photon echo efficiency remains at a
few per cent. Here we review, analyze, and discuss the collective atom phase
control applied to conventional photon echoes for quantum memory applications
to clarify fundamental physics of coherent transients in a three-level system
specifically for optical-spin coherence conversion in a controlled double
rephasing echoes. Some critical misunderstandings in various protocols are also
analyzed, and corrected for near unity echo efficiency under no population
inversion.
