title: Scattering in the ultrastrong regime: nonlinear optics with one photon.
abstract: The scattering of a flying photon by a two-level system ultrastrongly coupled to a one-dimensional photonic waveguide is studied numerically. The photonic medium is modeled as an array of coupled cavities and the whole system is analyzed beyond the rotating wave approximation using matrix product states. It is found that the scattering is strongly influenced by the single- and multiphoton dressed bound states present in the system. In the ultrastrong coupling regime a new channel for inelastic scattering appears, where an incident photon deposits energy into the qubit, exciting a photon-bound state, and escaping with a lower frequency. This single-photon nonlinear frequency conversion process can reach up to 50% efficiency. Other remarkable features in the scattering induced by counterrotating terms are a blueshift of the reflection resonance and a Fano resonance due to long-lived excited states.
authors.affiliation: Instituto de Ciencia de Materiales de Aragon and Departamento de Fisica de la Materia Condensada, CSIC-Universidad de Zaragoza, E-50012 Zaragoza, Spain.
authors.forename: E
authors.initials: E
authors.lastname: Sanchez-Burillo
authors.affiliation: Instituto de Ciencia de Materiales de Aragon and Departamento de Fisica de la Materia Condensada, CSIC-Universidad de Zaragoza, E-50012 Zaragoza, Spain and Fundacion ARAID, Paseo Maria Agustin 36, E-50004 Zaragoza, Spain.
authors.forename: D
authors.initials: D
authors.lastname: Zueco
authors.affiliation: Instituto de Fisica Fundamental, IFF-CSIC, Calle Serrano 113b, E-28006 Madrid, Spain.
authors.forename: J J
authors.initials: JJ
authors.lastname: Garcia-Ripoll
authors.affiliation: Instituto de Ciencia de Materiales de Aragon and Departamento de Fisica de la Materia Condensada, CSIC-Universidad de Zaragoza, E-50012 Zaragoza, Spain.
authors.forename: L
authors.initials: L
authors.lastname: Martin-Moreno