© 2015 American Physical Society. ©2015 American Physical Society. A local interaction between photons can be engineered by coupling a nonlinear system to a transmission line. The required transmission line can be conveniently formed from a chain of Josephson junctions. The nonlinearity is generated by side-coupling this chain to a Cooper pair box. We propose to probe the resulting photon-photon interactions via their effect on the current-voltage characteristic of a voltage-biased Josephson junction connected to the transmission line. Considering the Cooper pair box to be in the weakly anharmonic regime, we find that the dc current through the probe junction yields features around the voltages 2eV=n ωs, where ωs is the plasma frequency of the superconducting circuit. The features at n≥2 are a direct signature of the photon-photon interaction in the system.