  Quantum information theory determines the maximum rates at which information
can be transmitted through physical systems described by quantum mechanics.
Here we consider the communication protocol known as quantum reading. Quantum
reading is a protocol for retrieving the information stored in a digital memory
by using a quantum probe, e.g., shining quantum states of light to read an
optical memory. In a variety of situations using a quantum probe enhances the
performances of the reading protocol in terms of fidelity, data density and
energy dissipation. Here we review and characterize the quantum reading
capacity of a memory model, defined as the maximum rate of reliable reading. We
show that, like other quantities in quantum information theory, the quantum
reading capacity is super-additive. Moreover, we determine conditions under
which the use of an entangled ancilla improves the performance of quantum
reading.
