Dynamics of Quantum Noise in a Tunnel Junction under ac Excitation

We report the first measurement of the \emph{dynamical response} of shot noise (measured at frequency $\omega$) of a tunnel junction to an ac excitation at frequency ${\omega }_0$. The experiment is performed in the quantum regime, $\mathrm{\hslash }\omega \sim \mathrm{\hslash }{\omega }_0\gg k_{B}T$ at very low temperature T=35mK and high frequency ${\omega }_0\mathrm{/}2\pi =6.2$ GHz. We observe that the noise responds in phase with the excitation, but not adiabatically. The results are in very good agreement with a prediction based on a new current-current correlator.