  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.
