  For systems in equilibrium, quantum statistical physics provides a number of
general theorems and relations that are not tied to specific microscopic
models, one example being the fluctuation-dissipation theorem. Much less is
known for nonequilibrium situations. In this work, we discuss certain
properties of the power-spectrum tensor for systems in general steady-states,
i.e. stationary states not necessarily corresponding to equilibrium
configurations. In our analyses, we do not make any direct connection to
specific models for the underlying microscopic dynamics and, therefore, our
results can be applied to a large variety of systems. We also connect the
power-spectrum tensor to other quantities that characterize these systems and,
where appropriate, compare with the equilibrium counterparts. As an
application, we consider the specific problem of quantum friction, where, at
zero temperature, a contactless quantum-electrodynamic drag force acts on a
particle that moves in close proximity to an arrangement of material bodies.
Specifically, we show how the additional information about the system's physics
facilitates the derivation of more precise constraints on the power spectrum
and its functional dependencies.
