Scattering in the ultrastrong regime: nonlinear optics with one photon

TL;DR

This paper investigates how a single photon scatters in an ultrastrong coupling regime with a two-level system, revealing new inelastic scattering channels and nonlinear frequency conversion effects beyond traditional approximations.

Contribution

It introduces a numerical analysis of photon scattering in the ultrastrong coupling regime using Matrix Product States, highlighting new inelastic channels and spectral features.

Findings

Inelastic scattering channel with up to 50% efficiency

Blueshift of reflection resonance due to counter-rotating terms

Observation of Fano resonance from long-lived excited states

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 multi-photon 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 counter-rotating terms are a blueshift of the reflection resonance and a Fano resonance due to long-lived excited states