A Bose–Einstein condensate (BEC) is the remarkable state of matter that spontaneously emerges when a system of bosons becomes cold enough that a significant fraction of them condenses into a single quantum state to minimize the system’s free energy. Particles in that state then act collectively as a coherent wave. The phase transition for an atomic gas was predicted by Albert Einstein in 1924 and experimentally confirmed with the discovery of superfluid helium-4 in 1938. Once the techniques to trap and cool atoms to nanokelvin temperatures were developed more than half a century later, it was finally observed in dilute rubidium clouds in 1995 (see the articles by Wolfgang Ketterle and by Keith Burnett, Mark Edwards, and Charles Clark in PHYSICS TODAY, December 1999, pages 30 and 37).

Atoms aren’t the only option for a BEC. Photons, as massless, chargeless, and structureless particles of integer spin, are the simplest...

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