  We study a superconducting single-electron transistor (SSET) which is coupled
to a LC-oscillator via the phase difference across one of the Josephson
junctions. This leads to a strongly anharmonic coupling between the SSET and
the oscillator. The coupling can oscillate with the number of photons which
makes this system very similar to the single-atom injection maser. However, the
advantage of a design based on superconducting circuits is the strong coupling
and existence of standard methods to measure the radiation field in the
oscillator. This makes it possible to study many effects that have been
predicted for the single-atom injection maser in a circuit quantum
electrodynamics setup.
