# Observing the Spectral Collapse of Two-Photon Interaction Models

## TL;DR

This paper explores the spectral collapse phenomenon in two-photon interaction models within circuit QED, highlighting how strong coupling leads to a continuous spectrum and proposing experimental observation methods.

## Contribution

It introduces the concept of spectral collapse in two-photon models and discusses feasible experimental setups in solid-state systems to observe this phenomenon.

## Key findings

- Spectral collapse occurs at a critical coupling strength.
- Feasible scattering experiments can observe spectral collapse.
- Two-photon interactions exhibit unique spectral features in strong coupling regimes.

## Abstract

Until very recently, two-photon interaction processes have been considered only as arising from second- or higher-order effects in driven systems, and so limited to extremely small coupling strengths. However, a variety of novel physical phenomena emerges in the strong and ultrastrong coupling regimes. Strikingly, for a critical value of the coupling strength the discrete spectrum collapses into a continuous band. In this extended abstract, we discuss recent proposals to implement genuine two-photon interactions in an undriven solid-state system, in the framework of circuit QED. In particular, we review counterintuitive spectral features of two-photon interaction models and we show how the onset of the spectral collapse can be observed in feasible scattering experiments.