title: Spontaneous conversion from virtual to real photons in the ultrastrong-coupling regime.
abstract: 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 Ω(R) becomes comparable to the bare resonance frequency of photons ω(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.
authors.affiliation: Dipartimento di Fisica e Scienze della Terra, Università di Messina, Viale F. Stagno d'Alcontres 31, I-98166 Messina, Italy.
authors.forename: R
authors.initials: R
authors.lastname: Stassi
authors.affiliation: Physik Department, Technische Universität München, 85748 Garching, Germany.
authors.forename: A
authors.initials: A
authors.lastname: Ridolfo
authors.affiliation: Dipartimento di Fisica e Scienze della Terra, Università di Messina, Viale F. Stagno d'Alcontres 31, I-98166 Messina, Italy.
authors.forename: O
authors.initials: O
authors.lastname: Di Stefano
authors.affiliation: Physik Department, Technische Universität München, 85748 Garching, Germany.
authors.forename: M J
authors.initials: MJ
authors.lastname: Hartmann
authors.affiliation: Dipartimento di Fisica e Scienze della Terra, Università di Messina, Viale F. Stagno d'Alcontres 31, I-98166 Messina, Italy.
authors.forename: S
authors.initials: S
authors.lastname: Savasta