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, $hbaromegasimhbaromega_0gg k_BT$ at very low temperature T=35mK and high frequency $omega_0/2pi=6.2$ 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.
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