# Symmetry-breaking in a condensate of light and its use as a quantum sensor

## TL;DR

This paper explores how symmetry-breaking in photon Bose-Einstein condensates can be harnessed as a highly sensitive quantum sensor for detecting chiral molecules, surpassing existing circular dichroism techniques.

## Contribution

It introduces a novel symmetry-breaking mechanism in photon BECs that enables practical quantum sensing applications independent of external pumping.

## Key findings

- Symmetry-breaking lifts degeneracy between cavity modes of different polarizations.
- Tiny imbalances are amplified in the condensate polarization via a 'winner takes it all' effect.
- Sensor sensitivity exceeds that of current circular dichroism methods.

## Abstract

Bose-Einstein condensates (BECs) represent one of the very few manifestations of purely quantum effects on a macroscopic level. The vast majority of BECs achieved in the lab to date consist of bosonic atoms, which macroscopically populate the ground state once a threshold temperature has been reached. Recently, a new type of condensate was observed - the photon BEC, where light in a dye-filled cavity thermalises with dye molecules under the influence of an external driving laser, condensing to the lowest-energy mode. However, the precise relationship between the photon BEC and symmetry-breaking phenomena has not yet been investigated. Here we consider medium-induced symmetry breaking in a photon BEC and show that it can be used as a quantum sensor. The introduction of polarisable objects such as chiral molecules lifts the degeneracy between cavity modes of different polarisations. Even a tiny imbalance is imprinted on the condensate polarisation in a 'winner takes it all' effect. When used as a sensor for enantiomeric excess, the predicted sensitivity exceeds that of contemporary methods based on circular dichroism. Our results introduce a new symmetry-breaking mechanism that is independent of the external pump, and demonstrate that the photon BEC can be used for practical purposes.