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Glauber coherent states of laser light are already similar to Bose-Einstein condensates, but now there is a new kind.

Experiments reveal a Bose–Einstein condensate of photons
R. Mark Wilson
Physics Today, Feb. 2011

February 2011, page 10

Key to the achievement is the confinement of photons and molecules in an optical cavity long enough for them to reach thermal equilibrium.

American Institute of Physics
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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 of bosons. They’re also omnipresent. One need only switch on a light bulb for a cheap and ready supply. Moreover, Satyendra Nath Bose had photons in mind in 1924 when he first proposed a new way of counting indistinguishable particles, work that led to Einstein’s prediction the same year.

Yet, as Einstein surely knew, blackbody photons—those in thermal equilibrium with walls of a cavity—simply do not go through the phase transition. Unlike atoms, whose number is strictly conserved as the temperature is varied, photons are easily created and annihilated. As the photons are cooled in a cavity, they simply diminish in number by disappearing into its walls. Indeed, the blackbody spectrum is precisely that of a critical Bose gas, stubbornly on the verge of condensation, with the maximum possible number of uncondensed photons residing in the cavity at any given temperature.

Also unlike atoms, photons do not usually interact with each other. That’s not an issue in systems such as a laser, whose coherent light is achieved under conditions far from equilibrium. But their lack of interaction complicates how the photons might reach thermal equilibrium among themselves, a key prerequisite to achieve a BEC.

Martin Weitz and colleagues at the University of Bonn have now overcome both obstacles using a simple and elegant approach: By confining laser light within a thin cavity filled with dye at room temperature and bounded by two concave mirrors, they create the conditions required for light to thermally equilibrate as a gas of conserved particles rather than as ordinary blackbody radiation.1