  A local interaction between photons can be engineered by coupling a nonlinear
system to a transmission line. The required high impedance 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 $2eV=n\hbar\omega_s$, where
$\omega_s$ is the plasma frequency of the superconducting circuit. The features
at $n\ge 2$ are a direct signature of the photon-photon interaction in the
system.
