  We address the problem of quantum reading of optical memories, namely the
retrieving of classical information stored in the optical properties of a media
with minimum energy. We present optimal strategies for ambiguous and
unambiguous quantum reading of unitary optical memories, namely when one's task
is to minimize the probability of errors in the retrieved information and when
perfect retrieving of information is achieved probabilistically, respectively.
A comparison of the optimal strategy with coherent probes and homodyne
detection shows that the former saves orders of magnitude of energy when
achieving the same performances. Experimental proposals for quantum reading
which are feasible with present quantum optical technology are reported.
