Out of equilibrium noise in electronic devices: from the classical to the quantum regime

Whereas electrical noise has been extensively studied at low frequency in various systems, going from macroscopic to mesoscopic scales, and is now relatively well understood, investigation of high frequency noise is much more recent and raises new physical problems which could not be addressed before. Of particular interest is the frequency range of the order of or higher than the applied voltage or temperature characteristic energy scales. In this regime quantum effects are expected to show up with in particular a fundamental asymmetry between emission and absorption noise which is undetectable at low frequency. This asymmetry can only be probed using a quantum detector which enables one to probe separately the absorption and emission contributions of fluctuations, i.e. respectively the positive and negative frequencies of the Fourier transformed non-symmetrized noise correlator. We show that a superconductor-insulator superconductor tunnel junction constitutes just such a good quantum noise detector when measuring the photon assisted tunneling current of quasiparticles. It was in particular possible to detect the asymmetric excess current fluctuations generated by the tunneling of quasiparticles across a Josephson junction polarized in the vicinity of the superconducting gap. This experiment demonstrates unambiguously that the negative and positive frequency parts of the non-symmetrized noise correlator are separately detected and that the excess current fluctuations of a voltage biased Josephson junction present a strong asymmetry between emission and absorption, related to the superconducting gap in the density of states. We discuss other physical situations where this asymmetry of noise can be in principle detected using a similar setup.