Noise, errors and information in quantum amplification

We analyze and compare the characterization of a quantum device in
terms of noise, transmitted bit-error-rate (BER) and mutual
information, showing how the noise description is meaningful only for
Gaussian channels. After reviewing the description of a quantum
communication channel, we study the insertion of an amplifier. We
focus attention on the case of direct detection, where the linear
amplifier has a 3 decibels noise figure, which is usually considered
an unsurpassable limit, referred to as the standard quantum limit
(SQL). Both noise and BER could be reduced using an ideal amplifier,
which is feasible in principle. However, just a reduction of noise
beyond the SQL does not generally correspond to an improvement of the
BER or of the mutual information. This is the case of a laser
amplifier, where saturation can greatly reduce the noise figure,
although there is no corresponding improvement of the BER. Such
mechanism is illustrated on the basis of Monte Carlo simulations.