  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 $k_BT/h$ 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.
