NOISE, ERRORS AND INFORMATION IN QUANTUM AMPLIFICATION

We analyze and compare the characterization of a quantum device in ter ms of noise, transmitted bit-error-rate (BER) and mutual information, showing how the noise description is meaningful only for Gaussian chan nels. After reviewing the description of a quantum communication chann el, 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, refe rred to as the standard quantum limit (SQL). Both noise and BER could be reduced using an ideal amplifier, which is feasible in principle. H owever, just a reduction of noise beyond the SQL does not generally co rrespond to an improvement of the BER or of the mutual information. Th is is the case of a laser amplifier, where saturation can greatly redu ce the noise-figure, although there is no corresponding improvement of the BER. Such mechanism is illustrated on the basis of Monte Carlo si mulations.