Dynamics of quantum noise in a tunnel junction under ac excitation

We report the first measurement of the 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, h omega similar to h omega(0)>>k(B)T at very low temperature T=35 mK and high frequency omega(0)/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.