Detection of quantum noise

We discuss the detection of quantum fluctuations in the light of the relationship between time-dependent correlators and measurable properties [L. Van Hove, Phys. Rev 95, 249 (1954)]. Considering the interaction between the fluctuating electron system and a resonant circuit or a photon mode, we prove that zero-point fluctuations (ZPF) are not observable by a passive detector, corroborating the results of Lesovik and Loosen (Pis'ma Zh. Eksp. Teor. Fit. [JETP Lett. 65, 269 (1997)]). By a passive detector we mean one which is itself effectively in the ground state, and cannot transfer energy to the ZPF whose detection is attempted. We find that the ZPF can, on the other hand, be observed from deexcitation of an active detector. We also make the connection between these statements and the recent discussion of whether decoherence can be caused by the ZPF. The distinction is made between decoherence via making a real excitation in the environment and effects due to its polarization by virtual excitations.