Virtual photons in the ground state of a dissipative system.

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 diagonalisation 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.