  We have observed photon-assisted Cooper-pair tunneling in an atomic-scale
Josephson junction formed between a superconducting Nb tip and a
superconducting Nb sample in a scanning tunneling microscope (STM) at 30 mK.
High-resolution tunneling spectroscopy data show a zero-bias conduction peak
and other sharp sub-gap peaks from coupling of the STM junction to resonances
in the electromagnetic environment. The sub-gap peaks respond to incident
microwave radiation by splitting into multiple peaks with the position and
height depending on the frequency and amplitude of the microwaves. The
inter-peak spacing shows that the charge carriers are Cooper pairs, rather than
quasiparticles, and the power dependence reveals that the current originates
from photon-assisted phase-incoherent tunneling of pairs, rather than the more
conventional phase-coherent tunneling of pairs that yields Shapiro steps.
