Gaussian beam-caused imperfect rephasing in photon echo-based quantum memories (original) (raw)

A double rephasing photon echo scheme inherently satisfies no-population inversion condition for quantum memory applications, while the resultant absorptive coherence prohibits echo radiation out of the medium. To solve this absorptive echo problem, a controlled coherence conversion (CCC) has been proposed and demonstrated for the collective atom phase control converting an absorptive echo into an emissive one. The recent observations of absorptive photon echoes in double rephasing schemes, however, seemingly contradict the CCC theory. Here the rephasing process of individual ensemble atoms is investigated by using typical Gaussian light pulses to prove that the absorptive echo observations are due to imperfect rephasing-caused coherence leakage. Although the imperfect rephasing-cuased photon echoes has nothing to do with fidelity, the lower retrieval efficiency limits quantum memory applications due to potential eavesdropping caused by device-dependent loopholes or limited quantum ...