# Observation of Multimode Strong Coupling of Cold Atoms to a 30-m Long Optical Resonator

## TL;DR

This paper demonstrates multimode strong coupling between a small atomic ensemble and a long fiber resonator, revealing new regimes of light-matter interaction with potential for quantum non-linearities.

## Contribution

It reports the first observation of multimode strong coupling in a 30-meter fiber resonator with cold atoms, exceeding the free spectral range.

## Key findings

- Coupling strength exceeds free spectral range
- Atoms couple to multiple longitudinal modes
- System shows evidence of strong quantum non-linearities

## Abstract

We report on the observation of multimode strong coupling of a small ensemble of atoms interacting with the field of a 30-m long fiber resonator containing a nanofiber section. The collective light--matter coupling strength exceeds the free spectral range and the atoms couple to consecutive longitudinal resonator modes. The measured transmission spectra of the coupled atom-resonator system provide evidence of this regime, realized with a few hundred atoms with an intrinsic single-atom cooperativity of 0.26. These results are the starting point for studies in a new setting of light-matter interaction, with strong quantum non-linearities and a new type of dynamics.