Observation of Photon-Assisted Noise in a Diffusive Normal Metal–Superconductor Junction

We report measurements of nonequilibrium noise in a diffusive normal metal–superconductor (N-S) junction in the presence of both dc bias and high-frequency ac excitation. We find that the shot noise of a diffusive N-S junction is doubled compared to a normal diffusive conductor. Under ac excitation of frequency $\nu$ the shot noise develops features at bias voltages $|V|=h\nu /\left(2e\right)$, which bear all the hallmarks of a photon-assisted process. Observation of these features provides clear evidence that the effective charge of the current carriers is $2e$, due to Andreev reflection.