We show that a spontaneous release of virtual photon pairs can occur in a quantum optical system in the ultrastrong coupling regime. In this regime, which is attracting interest both in semiconductor and superconducting systems, the light-matter coupling rate {\Omega}R becomes comparable to the bare resonance frequency of photons {\omega}0. In contrast to the dynamical Casimir effect and other pair creation mechanisms, this phenomenon does not require external forces or time dependent parameters in the Hamiltonian.
@article{arxiv.1210.2367,
title = {Spontaneous Conversion from Virtual to Real Photons in the Ultrastrong Coupling Regime},
author = {R. Stassi and A. Ridolfo and O. Di Stefano and M. J. Hartmann and S. Savasta},
journal= {arXiv preprint arXiv:1210.2367},
year = {2015}
}