Phys.org  September 2, 2024 Thermalization of radiation by contact to matter is a well-known concept, but the application of thermodynamic methods to complex quantum states of light remains a challenge. Researchers in Germany observed Bose-Einstein condensation of photons into the hybridized ground state of a coupled four-site ring potential. They realized the periodically closed ring lattice superimposed by a weak harmonic trap for photons inside a spatially structured dye-filled microcavity. Photons thermalized to room temperature, and above a critical photon number macroscopically occupied the symmetric linear combination of the site eigenstates with zero phase winding, which constituted the ground state […]