Symmetry Breaking in a Condensate of Light and its Use as a Quantum Sensor

Bose-Einstein condensates (BECs) represent one of the very few manifestations of purely quantum effects on a macroscopic level. Recently, a new type of condensate has been observed—the photon BEC, where light in a dye-filled cavity thermalizes with dye molecules under the influence of an external driving laser, condensing to the lowest-energy mode. 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 polarizable objects such as chiral molecules lifts the degeneracy between cavity modes of different polarizations. Even a tiny imbalance is imprinted on the condensate polarization, 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 symmetry-breaking mechanism that is independent of the external pump and demonstrate that the photon BEC can be used for practical purposes.