  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. Here we observe Bose-Einstein condensation of photons into
the hybridized ground state of a four-site ring potential with coherent tunnel
couplings. In our experiment, the periodically-closed ring lattice superimposed
by a weak harmonic trap for photons is realized inside a spatially structured
dye-filled microcavity. Photons thermalize 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.
