# Universal spectral features of ultrastrongly coupled systems

## TL;DR

This paper reveals universal spectral features in ultrastrong light-matter coupling systems, showing symmetry-driven degeneracies and the general emergence of superradiant phases across various quantum optical models.

## Contribution

It analytically proves the origin of spectral degeneracies and extends the understanding of superradiant phases as a universal property in the ultrastrong coupling regime.

## Key findings

- Two-fold degeneracy in the spectrum due to symmetry.
- Superradiant phases are a general feature in the ultrastrong coupling limit.
- Universal spectral properties across different quantum optical models.

## Abstract

We identify universal properties of the low-energy subspace of a wide class of quantum optical models in the ultrastrong coupling limit, where the coupling strength dominates over all other energy scales in the system. We show that the symmetry of the light-matter interaction is at the origin of a two-fold degeneracy in the spectrum. We prove analytically this result for bounded Hamiltonians and extend it to a class of models with unbounded operators. As a consequence, we show that the emergence of superradiant phases previously investigated in the context of critical phenomena, is a general property of the ultrastrong coupling limit. The set of models we consider encompasses different scenarios of possible interplay between critical behaviour and superradiance.