  With this work we elaborate on the physics of quantum noise in thermal
equilibrium and in stationary non-equilibrium. Starting out from the celebrated
quantum fluctuation-dissipation theorem we discuss some important consequences
that must hold for open, dissipative quantum systems in thermal equilibrium.
The issue of quantum dissipation is exemplified with the fundamental problem of
a damped harmonic quantum oscillator. The role of quantum fluctuations is
discussed in the context of both, the nonlinear generalized quantum Langevin
equation and the path integral approach. We discuss the consequences of the
time-reversal symmetry for an open dissipative quantum dynamics and,
furthermore, point to a series of subtleties and possible pitfalls. The path
integral methodology is applied to the decay of metastable states assisted by
quantum Brownian noise.
