The existence of the third cumulant
S3 of voltage fluctuations has demonstrated the non-Gaussian aspect of shot noise in electronic transport. Until now, measurements have been performed at low frequency, \textit{i.e.} in the classical regime
ℏω<eV,kBT where voltage fluctuations arise from charge transfer process. We report here the first measurement of
S3 at high frequency, in the quantum regime
ℏω>eV,kBT. In this regime, experiment cannot be seen as a charge counting statistics problem anymore. It raises central questions of the statistics of quantum noise: 1) the electromagnetic environment of the sample has been proven to strongly influence the measurement, through the possible modulation of the noise of the sample. What happens to this mechanism in the quantum regime? 2) For
ℏω>eV, the noise is due to zero point fluctuations and keeps its equilibrium value:
S2=Gℏω with
G the conductance of the sample. Therefore,
S2 is independent of the bias voltage and no photon is emitted by the conductor. Is it possible, as suggested by some theories, that
S3=0 in this regime? With regard to these questions, we give theoretical and experimental answers to the environmental effects showing that they involve dynamics of the quantum noise. Using these results, we investigate the question of the third cumulant of quantum noise in the a tunnel junction.
A time-dependent bias voltage on a tunnel junction generates a time-dependent modulation of its current fluctuations, and in particular of its variance. This translates into an excitation at frequency
f~ generating correlations between current fluctuating at any frequency f and at frequency ±
f~−f. We report the measurement of such a correlation in the fully quantum regime, i.e. when both frequencies are much greater than kBT/h with T the temperature. Such a correlator, usually referred to as the noise susceptibility, is involved in corrections to the measurements of higher-order moments and in the squeezing of noise.
We consider the electromagnetic field generated by a coherent conductor in which electron transport is described quantum mechanically. We obtain an input-output relation linking the quantum current in the conductor to the measured electromagnetic field. This allows us to compute the outcome of measurements on the field in terms of the statistical properties of the current. We moreover show how under ac bias the conductor acts as a tunable medium for the field, allowing for the generation of single- and two-mode squeezing through fermionic reservoir engineering. These results explain the recently observed squeezing using normal tunnel junctions [G. Gasse et al., Phys. Rev. Lett. 111, 136601 (2013); J.-C. Forgues et al., Phys. Rev. Lett. 114, 130403 (2015)].
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.