We explore photon coincidence counting statistics in the ultrastrong-coupling
regime where the atom-cavity coupling rate becomes comparable to the cavity
resonance frequency. In thisregime usual normal order correlation functions
fail to describe the output photon statistics. By expressing the electric-field
operator in the cavity-emitter dressed basis we are able to propose correlation
functions that are valid for arbitrary degrees of light-matter interaction. Our
results show that the standard photon blockade scenario is significantly
modified for ultrastrong coupling. We observe parametric processes even for
two-level emitters and temporal oscillations of intensity correlation functions
at a frequency given by the ultrastrong photon emitter coupling. These effects
can be traced back to the presence of two-photon cascade decays induced by
counter-rotating interaction terms.
We investigate a chain of superconducting stripline resonators, each
interacting with a transmon qubit, that are capacitively coupled in a row. We
show that the dynamics of this systemcan be described by a Bose-Hubbard
Hamiltonian with attractive interactions for polaritons, superpositions of
photons and qubit excitations. This setup we envisage constitutes one of the
first platforms where all technological components that are needed to
experimentally study chains of strongly interacting polaritons have already
been realized. By driving the first stripline resonator with a microwave source
and detecting the output field of the last stripline resonator one can
spectroscopically probe properties of the system in the driven dissipative
regime. We calculate the stationary polariton density and density-density
correlations $g^{(2)}$ for the last cavity which can be measured via the output
field. Our results display a transition from a coherent to a quantum field as
the ratio of on site interactions to driving strength is increased.