Quantum Storage of Heralded Single Photons in a Praseodymium Doped Crystal

We report on experiments demonstrating the reversible mapping of heralded single photons to long lived collective optical atomic excitations stored in a Pr³⁺:Y₂SiO₅ crystal. A cavity-enhanced spontaneous down-conversion source is employed to produce widely non-degenerate narrow-band $approx 2 MHz$ photon-pairs. The idler photons, whose frequency is compatible with telecommunication optical fibers, are used to herald the creation of the signal photons, compatible with the Pr³⁺ transition. The signal photons are stored and retrieved using the atomic frequency comb protocol. We demonstrate storage times up to 4.5 μs while preserving non-classical correlations between the heralding and the retrieved photon. This is more than 20 times longer than in previous realizations in solid state devices, and implemented in a system ideally suited for the extension to spin-wave storage.