  The fluctuation dissipation theorem (FDT) is the basis for a microscopic
description of the interaction between electromagnetic radiation and matter.By
assuming the electromagnetic radiation in thermal equilibrium and the
interaction in the linear response regime, the theorem interrelates the
spontaneous fluctuations of microscopic variables with the kinetic coefficients
that are responsible for energy dissipation.In the quantum form provided by
Callen and Welton in their pioneer paper of 1951 for the case of conductors,
electrical noise detected at the terminals of a conductor was given in terms of
the spectral density of voltage fluctuations, $S_V({\omega})$, and was related
to the real part of its impedance, $Re[Z({\omega})]$, by a simple relation.The
drawbacks of this relation concern with: (I) the appearance of a zero point
contribution which implies a divergence of the spectrum at increasing
frequencies; (ii) the lack of detailing the appropriate equivalent-circuit of
the impedance, (iii) the neglect of the Casimir effect associated with the
quantum interaction between zero-point energy and boundaries of the considered
physical system; (iv) the lack of identification of the microscopic noise
sources beyond the temperature model. These drawbacks do not allow to validate
the relation with experiments. By revisiting the FDT within a brief historical
survey, we shed new light on the existing drawbacks by providing further
properties of the theorem, focusing on the electrical noise of a two-terminal
sample under equilibrium conditions. Accordingly, we will discuss the duality
and reciprocity properties of the theorem, its applications to the ballistic
transport regime, to the case of vacuum and to the case of a photon gas.
