Bose-Einstein Condensation of Photons in a Four-Site Lattice Potential

Bose-Einstein condensation can be observed with e.g. ultracold atomic gases, polaritons, and since about a decade ago also with low-dimensional photon gases [1]. Cold atomic gases in lattice potentials are usually prepared following the creation of a condensate by transferring it into the periodic potential, while in recent work with photon gases direct condensation into a coherently split state of light has been realized [2]. Here we report on experimental work directed at realizing thermalized photon gases in periodic potentials of increased complexibility, i.e. beyond a double well.