Physics

Keeping Light Around

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Science  18 Jun 2010:
Vol. 328, Issue 5985, pp. 1457
DOI: 10.1126/science.328.5985.1457-b

Bose-Einstein condensation (BEC) is a phenomenon in which an equilibrium population of bosons (particles of integer spin) slump down to their lowest energy state under conditions of low temperature and high density. The bosons that undergo this transition include cold gases of 87Rb, superconducting electron pairs, and interacting He atoms in their superfluid phase. However, the most ubiquitous bosons, photons, are predicted not to exhibit BEC because their number is usually not conserved and their chemical potential, consequently, vanishes at all temperatures. Klaers et al. now realize a two-dimensional thermalized gas of photons confined in a parabolic potential, with a conserved particle number and a quadratic dispersion—a system equivalent to a gas of massive atoms and expected to undergo BEC under favorable conditions. They achieve this scenario by trapping photons in a curved-mirror optical microcavity filled with dye molecules. The photons are absorbed and emitted repeatedly by the dye molecules, which are pumped by a laser; the pumping ensures that the number of excited dye molecules is constant, in turn ensuring that the photon number is conserved too. The thermalization of the photon gas at the dye solution temperature is confirmed through comparison of measured spectra with theoretical predictions.

Nat. Phys. 6, 10.1038/nphys1680 (2010).

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