Experiments Reveal 'Critical Behavior' Of Light

Research results fill a key gap in the physics of phase transitions

Quantum gases of photons – the particles of light – exhibit “critical behavior” close to the phase transition to a Bose-Einstein condensate, where thermodynamic quantities become extremely large and eventually diverge at the transition point. While this phenomenon had so far not been observed for light, experimental evidence has now been obtained by a research team from the universities of Bonn and Heidelberg, and Universidad Nacional Autónoma de México. For this, the team used a nearly non-interacting, two-dimensional photon gas and measured its spatial correlations just before the condensate formed. The results indicate that photon gases form a distinct, universal class of systems in nature, which differs from condensates of interacting particles, says project leader Prof. Dr. Julian Schmitt (formerly in Bonn, now in Heidelberg).

In the current study, the light was trapped in an optical microresonator filled with a dye solution. For this, a resonator mirror was structured by means of laser writing at the nanometer scale to realize a box-shaped container – a mirrored box in which the photons were stored. Inside, the photons reached thermal equilibrium through interaction with dye molecules and were effectively cooled down to the onset of the condensed quantum phase. In this range of temperatures, a camera was used to measure the angular distribution of the light emitted from the mirror box. The researchers then determined the correlation length and its critical exponent. A Bose-Einstein condensate is an extremely cold state of matter that can be observed not only with light but also with atomic gases. “However, it is only with light that the condensation happens essentially free of interactions, since photons reach equilibrium through contact with dye molecules and, unlike atoms, not through collisions,” Prof. Schmitt notes.

The scientist explains that the phenomenon of “critical behavior” is known for many substances. For example, water, too, can suddenly appear cloudy at its critical point due to the formation of extended density fluctuations, although it was perfectly clear before. Observing or measuring such behavior for photons, however, was never possible in the past. The study shows that photon gases must be seen as a distinct, universal class of physical systems in nature. “Our research results fill a key gap in the physics of phase transitions and have the potential to open exciting new research avenues, including on the physics of systems far from equilibrium, as well as on future applications,” underlines the experimental physicist, who first worked at the University of Bonn and since 2025 has pursued research on optical quantum systems at the Kirchhoff Institute for Physics of Heidelberg University.

Besides those from Heidelberg, researchers also participated from the University of Bonn’s Institute of Applied Physics and the Instituto de Física and the Universidad Nacional Autónoma de México (UNAM). The studies were part of the STRUCTURES Cluster of Excellence at Heidelberg University and the Collaborative Research Centre/Transregio OSCAR, which is also cosponsored by the University of Bonn. They were funded by the German Research Foundation, the European Union and the UNAM. The findings appeared in “Science Advances”.