  Quantum fluctuations in an anharmonic superconducting circuit enable
frequency conversion of individual incoming photons. This effect, linear in the
photon beam intensity, leads to ramifications for the standard input-output
circuit theory. We consider an extreme case of anharmonicity in which photons
scatter off a small set of weak links within a Josephson junction array. We
show that this quantum impurity displays Kondo physics and evaluate the elastic
and inelastic photon scattering cross sections. These cross sections reveal
many-body properties of the Kondo problem that are hard to access in its
traditional fermionic version.
