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.Comment: To appear on Phys. Rev. Let
