(2011):

ABSTRACT:
The readout of a classical memory can be modelled as a problem of quantum
channel discrimination, where a decoder retrieves information by
distinguishing the different quantum channels encoded in each cell
of the memory (Pirandola 2011 Phys. Rev. Lett. **106**, 090504). In
the case of optical memories, such as CDs and DVDs, this discrimination
involves lossy bosonic channels and can be remarkably boosted by the
use of nonclassical light (quantum reading). Here we generalize these
concepts by extending the model of memory from single-cell to
multi-cell encoding. In general, information is stored in a block of
cells by using a channel-codeword, i.e. a sequence of channels chosen
according to a classical code. Correspondingly, the readout of data is
realized by a process of 'parallel' channel discrimination, where the
entire block of cells is probed simultaneously and decoded via an optimal
collective measurement. In the limit of a large block we define the
quantum reading capacity of the memory, quantifying the maximum number
of readable bits per cell. This notion of capacity is nontrivial when
we suitably constrain the physical resources of the decoder. For
optical memories (encoding bosonic channels), such a constraint is
energetic and corresponds to fixing the mean total number of photons
per cell. In this case, we are able to prove a separation between the
quantum reading capacity and the maximum information rate achievable
by classical transmitters, i.e. arbitrary classical mixtures of
coherent states. In fact, we can easily construct nonclassical
transmitters that are able to outperform any classical transmitter,
thus showing that the advantages of quantum reading persist in the
optimal multi-cell scenario.