Observation of Quantum Oscillations in the Photo-assisted Shot Noise of a Tunnel Junction

We report measurements of the low frequency current fluctuations of a tunnel junction placed at very low temperature biased by a time-dependent voltage $V(t)=V(1+\cos 2\pi\nu t)$. We observe that the excess noise generated by the ac excitation exhibits quantum oscillations as a function of the dc bias, with a period given by $h\nu/e$ with $e$ the charge of a single electron. This is a direct consequence of the quantum nature of electricity in a normal conductor.