  We present a circuit-QED scheme which allows to reach the ultrastrong
coupling regime of a nondipolar interaction between a single qubit and a
quantum resonator. We show that the system Hamiltonian is well approximated by
a two-photon quantum Rabi model and propose a simple scattering experiment to
probe its fundamental properties. In particular, we identify a driving scheme
that reveals the change in selection rules characterizing the breakdown of the
rotating-wave approximation and the transition from strong to ultrastrong
two-photon interactions. Finally, we show that a frequency crowding in a narrow
spectral region is observable in the output fluoresce spectrum as the coupling
strength approaches the collapse point, paving the way to the direct
observation of the onset of the spectral collapse in a solid-state device.
