Quantum Noise in Linear Amplifiers Revisited

This paper presents a model for radiation as a photon gas. For each frequency, the photon distribution is a mixture of Bose-Einstein and Poisson distributions, respectively for thermal noise and the useful signal. Poisson (shot) noise contributes with noise at all frequencies. When applied to the operation of linear amplifiers and attenuators, the model gives a natural interpretation to the noise figure in terms of the uncertainty in the amplification process itself.