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

Abstract

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 problemswhich could not be addressed before. Of particular interest is the frequency range ofthe order of or higher than the applied voltage or temperature characteristicenergy scales. In this regime quantum effects are expected to show up with inparticular a fundamental asymmetry between emission and absorption noise whichis undetectable at low frequency. This asymmetry can only be probed using aquantum detector which enables one to probe separately the absorption and emissioncontributions of fluctuations, i.e. respectively the positive and negative frequenciesof the Fourier transformed non-symmetrized noise correlator. We show that asuperconductor–insulator–superconductor tunnel junction constitutes just such a goodquantum noise detector when measuring the photon assisted tunneling current ofquasiparticles. It was in particular possible to detect the asymmetric excess currentfluctuations generated by the tunneling of quasiparticles across a Josephson junctionpolarized in the vicinity of the superconducting gap. This experiment demonstratesunambiguously that the negative and positive frequency parts of the non-symmetrizednoise correlator are separately detected and that the excess current fluctuations of a voltagebiased Josephson junction present a strong asymmetry between emission and absorption,related to the superconducting gap in the density of states. We discuss other physicalsituations where this asymmetry of noise can be in principle detected using a similarsetup.