A Bose-Einstein condensate of photons

As massless and chargeless particles of integer spin, photons are the simplest of bosons. Moreover, Satyendra Nath Bose had photons in mind in 1924 when he proposed a new way of counting indistinguishable particles, work that led to Albert Einstein’s prediction of the state of matter known as a Bose–Einstein condensate (BEC): When a collection of bosons becomes cold enough, a macroscopic fraction of them congregate into a single quantum state. But until now, a BEC of photons has never been observed. Blackbody photons, when cooled in a cavity, simply disappear into the walls. And, unlike atoms, photons don’t usually interact with each other, which prevents them from reaching thermal equilibrium apart from a blackbody. Martin Weitz and colleagues from the University of Bonn have now overcome those obstacles to a photon BEC. By confining laser light within an optical cavity filled with dye, they create the conditions for light to equilibrate as a gas of conserved particles. The cavity mirrors’ reflectivity ensures that photons live long enough to scatter among dye molecules, which exchange energy with the photons by repeatedly absorbing and reemitting them. To make a BEC, Weitz and company increase the number of scattered photons in the cavity by dialing up the laser power. Above a critical density, the photon energy spectra exhibit an emerging Bose peak at the cavity’s lowest-energy mode, alongside a broad thermal distribution of uncondensed photons. Shown here, beside the spectra, are spatial profiles of the light emission, below the onset of condensation (top) and above it (bottom). (J. Klaers et al., Nature 468, 545, 2010 .)—R. Mark Wilson