Observation of “Photon-Assisted” Shot Noise in a Phase-Coherent Conductor

We observe a novel signature of coherent transport in the nonequilibrium current fluctuations of a diffusive metallic conductor shorter than the electron phase-breaking length. Under illumination with monochromatic microwaves, the shot noise develops features at voltages corresponding to the photon energies, $V=nh\nu /e$, which are oscillatory functions of the microwave power, while the conductance and the $I-V$ curve are unaffected. The observed effect bears a strong resemblance to photon-assisted tunneling, although this marks the first demonstration in a linear system, and in a system without an explicit tunnel barrier.