Preparation-free coherent storage based on optical frequency comb echo

Quantum memories are essential elements for future quantum networks. Numerous protocols for building quantum memories have been proposed and demonstrated in experiments, such as controlled reversible inhomogeneous broadening, atomic frequency comb, electromagnetically induced transparency and more. However, they require complicated preparation procedures and have short duty cycles. Here, we propose a preparation-free quantum memory protocol based on optical frequency comb echo. Optical frequency combs can be created by electro-optical modulation. Photon echoes are coherent emissions when the atomic ensemble rephase, which have been used extensively for quantum storage. Quantum frequency mixers enable conversion of multiple frequency-bin states into one frequency-bin single-photon-level state. Based on these techniques, we experimentally demonstrate the protocol with an erbium doped YSO crystal. Erbium ions have optical transitions at telecom wavelength, which are suitable for long distance communications in fibers. Our study can enable a robust and efficient method to build telecom quantum optical memories.