Formal Theory of Quantum Fluctuations from a Driven State

A formal apparatus is developed for dealing with quantum fluctuations in a driven (nonequilibrium) state. Phenomenological equations for the decay of the system from an arbitrary state to the driven state are presumed known. By assuming that at some initial time the system-reservoir density matrix can be factored, we establish that the spectrum of fluctuations is given by the Fourier transform of the solution of the phenomenological equations, i.e., the regression of fluctuations obey the "macroscopic" equations of motion even in the nonequilibrium case.