  Multi-photon emitters are a sought-after resource in quantum photonics.
Nonlinear interactions between a multi-level atomic system and a coherent drive
can lead to resonant two-photon emission, but harvesting light from this
process has remained a challenge due to the small oscillator strengths
involved. Here we present a study of two-photon resonance fluorescence at
microwave frequencies, using a superconducting, ladder-type artificial atom, a
transmon, strongly coupled to a waveguide. We drive the two-photon transition
between the ground and second-excited state at increasingly high powers and
observe a resonance fluorescence peak whose intensity becomes comparable to
single-photon emission until it splits into a Mollow-like triplet. We measure
photon correlations of frequency-filtered spectral lines and find that while
emission at the fundamental frequency stays antibunched, the resonance
fluorescence peak at the two-photon transition is superbunched. Our results
provide a route towards the realization of multi-photon sources in the
microwave domain.
