Noise Dynamics in the Quantum Regime

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 $\tilde{f}$ generating correlations between current fluctuating at any frequency $f$ and at frequency $\pm$ $\tilde{f}-f$. We report the measurement of such a correlation in the fully quantum regime, i.e. when both frequencies are much greater than $k_{B}T\mathrm{/}h$ 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.