# Theoretical Description of Micromaser in the Ultrastrong-Coupling Regime

## TL;DR

This paper provides a theoretical analysis of a micromaser operating in the ultrastrong-coupling regime, revealing significant shifts in energy, frequency, and phase stability due to counter-rotating effects.

## Contribution

It introduces a detailed theoretical model for ultrastrong-coupling micromasers without relying on common approximations, highlighting new physical effects.

## Key findings

- Electric energy in the resonator declines significantly.
- Microwave oscillation frequency shifts notably.
- Phase fluctuations are suppressed, reducing spectral linewidth.

## Abstract

We theoretically investigate an ultrastrongly-coupled micromaser based on Rydberg atoms interacting with a superconducting LC resonator, where the common rotating-wave approximation and slowly-varying-envelope approximation are no longer applicable. The effect of counter-rotating terms on the masing dynamics is studied in detail. We find that the intraresonator electric energy declines and the microwave oscillation frequency shifts significantly in the regime of ultrastrong coupling. Additionally, the micromaser phase fluctuation is suppressed, resulting in a reduced spectral linewidth.