  Much of the novel physics predicted to be observable in the ultrastrong
light-matter coupling regime rests on the hybridisation between states with
different numbers of excitations, leading to a population of virtual photons in
the system's ground state. In this article, exploiting an exact diagonalization
approach, we derive both analytical and numerical results for the population of
virtual photons in presence of arbitrary losses. Specialising our results to
the case of Lorentzian resonances we then show that the virtual photon
population is only quantitatively affected by losses, even when those become
the dominant energy scale. Our results demonstrate most of the
ultrastrong-coupling phenomenology can be observed in loss-dominated systems
which are not even in the standard strong coupling regime. We thus open the
possibility to investigate ultrastrong-coupling physics to platforms that were
previously considered unsuitable due to their large losses.
