Detection of quantum noise

We discuss the detection of quantum fluctuations in the light of the relati onship between time-dependent correlators and measurable properties [L. Van Hove, Phys. Rev 95, 249 (1954)]. Considering the interaction between the f luctuating electron system and a resonant circuit or a photon mode, we prov e that zero-point fluctuations (ZPF) are not observable by a passive detect or, 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 t he 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 th e connection between these statements and the recent discussion of whether decoherence can be caused by the ZPF. The distinction is made between decoh erence via making a real excitation in the environment and effects due to i ts polarization by virtual excitations.