A controlled ac Stark echo for quantum memories

A quantum memory protocol of controlled ac Stark echoes (CASE) based on a double rephasing photon echo scheme via controlled Rabi flopping is proposed. A double rephasing scheme of photon echoes inherently satisfies the no-population inversion requirement for quantum memories, but the resultant absorptive echo has been a fundamental problem. In the present studies, firstly, the first echo in the double rephasing scheme is dynamically controlled not to affect the second echo by using unbalanced ac Stark shifts. Secondly, a control Rabi pulse coherently manipulates the second echo to be emissive via controlled coherence conversion. Owing to far-off resonance of ac Stark fields, the present protocol can be applied for wavelength-selective dynamic control of quantum memories in real time for erasing, buffering, and multiplexing.