  Photon antibunching in the light scattered by single quantum emitters is one
of the hallmarks of quantum optics, providing an unequivocal demonstration of
the quantized nature of the electromagnetic field. Antibunching can be
intuitively understood by the need for a two-level system lying in its lower
state after emitting a photon to be re-excited into the upper one before a
second emission can take place. Here we show that such a picture breaks down in
the ultrastrong light-matter coupling regime, when the coupling strength
becomes comparable to the bare emitter frequency. Specialising to the cases of
both a natural and a superconducting artificial atom, we thus show that a
single emitter coupled to a photonic resonator can emit bunched light. The
result presented herein is a clear evidence of how the ultrastrong coupling
regime is able to change the nature of individual atoms.
