  When light originating from a laser diode driven by non-fluctuating
electrical currents is incident on a photo-detector, the photo-current does not
fluctuate much. Precisely, this means that the variance of the number of
photo-electrons counted over a large time interval is much smaller that the
average number of photo-electrons. At non-zero Fourier frequency $\Omega$ the
photo-current power spectrum is of the form $\Omega^2/(1+\Omega^2)$ and thus
vanishes as $\Omega\to 0$, a conclusion equivalent to the one given above. The
purpose of this paper is to show that results such as the one just cited may be
derived from a (semi-classical) theory in which neither the optical field nor
the electron wave-function are quantized. We first observe that almost any
medium may be described by a circuit and distinguish (possibly non-linear)
conservative elements such as pure capacitances, and conductances that
represent the atom-field coupling. The theory rests on the non-relativistic
approximation. Nyquist noise sources (in which the Planck term $\hbar\omega/2$
is being restored) are associated with positive or negative conductances, and
the law of average-energy conservation is enforced. We consider mainly
second-order correlations in stationary linearized regimes.
