  We demonstrate efficient and reversible mapping of a light field onto a
thulium-doped crystal using an atomic frequency comb (AFC). Thanks to an
accurate spectral preparation of the sample, we reach an efficiency of 9%. Our
interpretation of the data is based on an original spectral analysis of the
AFC. By independently measuring the absorption spectrum, we show that the
efficiency is both limited by the available optical thickness and the
preparation procedure at large absorption depth for a given bandwidth. The
experiment is repeated with less than one photon per pulse and single photon
counting detectors. We clearly observe that the AFC protocol is compatible with
the noise level required for weak quantum field storage.
