with L. Rondoni (Politecnico Torino), M. Bonaldi (CNR-INFN Trento), L. Conti (INFN Padova)

In recent times several studies, both theoretical and experimental, have been devoted to the possibility of analyzing signals from sources operating at effective temperatures lower than environment temperature, obtainable through a feedback system which operates as an external driving. Such feedback induces, for example in the case of the gravitational wave detector AURIGA, some extra dissipation in the source-current system. Hence the above mentioned effective cooling effect. A useful theoretical viewpoint is that of regarding the measuring system as out of equilibrium, and one in which the feedback acts in the concurrent role of external driving and of observable current. In the simplest case, which is outlined here, the source of the signal (a current) can be thought of as as purely thermal in origin. Here we analyze different types of feedback, without concurrently altering the nature of the interplay between the thermal source and the measuring tool, which is well described by a second order Langevin equation. Among many, in particular we shall show that one of the results that are readily available is the determination of the power spectra of the currents generated, as well as their differences depending on the different (and only apparently equivalent) feedback settings, thus showing weaker or stronger deviations from the Lorentzain spectra expected at equilibrium. Finally, the strongly-nonequilibrium global properties that characterize the current generated in the measuring apparatus will be briefly discussed in relation to other popular approaches in statistical physics.

http://www.rarenoise.lnl.infn.it