Thermalisation 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. In this work, we observe Bose-Einstein condensation of photons into the hybridised ground state of a coupled four-site ring potential.
In our experiment, a periodically closed ring lattice superimposed by a weak harmonic trap for photons is realised inside a spatially structured dye-filled microcavity. To produce the desired potential landscape for light, we use an iterative surface delamination technique that allows us to modulate the surface of optical mirrors permanently without adding another material or reducing the mirror reflectivity. In the assembled dye-filled microcavity, the photons thermalise to room temperature, and above a critical photon number macroscopically occupy the symmetric linear combination of the site eigenstates with zero phase winding, which constitutes the ground state of the system. The mutual phase coherence of photons at different lattice sites is verified by optical interferometry. For the future, lattices of interacting photons induced, e.g., by effective Kerr nonlinearities offer prospects for the preparation of entangled many-body ground states using the demonstrated thermal equilibrium process.