  We measure current fluctuations of mesoscopic devices in the quantum regime,
when the frequency is of the order of or higher than the applied voltage or
temperature. Detection is designed to probe separately the absorption and
emission contributions of current fluctuations, i.e. the positive and negative
frequencies of the Fourier transformed nonsymmetrized noise correlator. It
relies on measuring the quasiparticles photon assisted tunneling current across
a superconductor-insulator-superconductor junction (the detector junction)
caused by the excess current fluctuations generated by quasiparticles tunneling
across a Josephson junction (the source junction). We demonstrate unambiguously
that the negative and positive frequency parts of the nonsymmetrized 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.
