# A generalization of the quantum Rabi model: exact solution and spectral structure

## TL;DR

This paper presents an exact solution to a generalized quantum Rabi model with an added Stark-like term, revealing unique spectral features in ultrastrong coupling regimes.

## Contribution

It extends the quantum Rabi model by including a Stark-like coupling and provides an exact analytical solution using Braak's method.

## Key findings

- Avoided level crossings for same parity states
- Rapid onset of near-degenerate levels with increasing coupling
- Spectral features absent in the original model

## Abstract

We consider a generalization of the quantum Rabi model where the two-level system and the single-mode cavity oscillator are coupled by an additional Stark-like term. By adapting a method recently introduced by Braak [Phys. Rev. Lett. {\bf 107}, 100401 (2011)], we solve the model exactly. The low-lying spectrum in the experimentally relevant ultrastrong and deep strong regimes of the Rabi coupling is found to exhibit two striking features absent from the original quantum Rabi model: avoided level crossings for states of the same parity and an anomalously rapid onset of two-fold near-degenerate levels as the Rabi coupling increases.