  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.
