  We report on experiments demonstrating the reversible mapping of heralded
single photons to long lived collective optical atomic excitations stored in a
Pr$^{3+}$:Y$_2$SiO$_5$ crystal. A cavity-enhanced spontaneous down-conversion
source is employed to produce widely non-degenerate narrow-band ($\approx
2\,\mathrm{MHz}$) 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 Pr$^{3+}$ transition. The
signal photons are stored and retrieved using the atomic frequency comb
protocol. We demonstrate storage times up to $4.5\,\mathrm{\mu s}$ 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.
