  Nonclassical states of light play a central role in many quantum information
protocols. Their quantum features have been exploited to improve the readout of
information from digital memories, modelled as arrays of microscopic beam
splitters [S. Pirandola, Phys. Rev. Lett. 106, 090504 (2011)]. In this model of
"quantum reading", a nonclassical source of light with Einstein-Podolski-Rosen
correlations has been proven to retrieve more information than any classical
source. In particular, the quantum-classical comparison has been performed
under a global energy constraint, i.e., by fixing the mean total number of
photons irradiated over each memory cell. In this paper we provide an
alternative analysis which is based on a local energy constraint, meaning that
we fix the mean number of photons per signal mode irradiated over the memory
cell. Under this assumption, we investigate the critical number of signal modes
after which a nonclassical source of light is able to beat any classical source
irradiating the same number of signals.
