Long-Lived Telecom-Heralded Single-Photon Storage in an Absorptive Spin-Rephased Quantum Memory.

Long-lived storage of single photons under the form of atomic excitations is at the foundation of long-distance entanglement distribution in quantum networks. To mitigate decoherence effects induced by the environment, rephasing of the hyperfine coherences using microwave pulses have been implemented in a variety of single-emitter and ensemble-based solid-state systems. However, the demonstration of storage of single photons in an absorptive quantum memory including such a spin-rephasing mechanism remains elusive. In this Letter, we show nonclassical storage of telecom-heralded single photons in a Pr^{3+}:Y_{2}SiO_{5} rare-earth ion doped crystal quantum memory using the atomic frequency comb spin-wave protocol combined with a XY4 spin-rephasing sequence. Long-lived atomic frequency comb photon echoes are first observed in the classical regime for storage times of up to approximately 3 ms. We then demonstrate nonclassical correlations between heralding photons and stored signal photons generated by a cavity-enhanced parametric photon-pair source for storage times of up to 180  μs and with measured cross-correlation values as high as 4.6(4). Together with the capacity of Pr^{3+}:Y_{2}SiO_{5} quantum memories to support highly efficient and multiplexed storage, this result represents a significant step toward scalable long-distance quantum repeater links.