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Can a Bose–Einstein condensate form at room temperature? If it’s made of atoms, not by a long shot. To coax a gas of bosonic atoms to pile up in their quantum ground state, researchers must cool the gas to within a few millionths of a degree of absolute zero. And they must keep it isolated in an otherwise ultrahigh vacuum.

More accessible condensates can be made by replacing the atoms with polaritons: quasiparticles of light and matter that form when photons trapped in an optical cavity couple to electronic excitations in a solid. Quantum condensation of polaritons sets in at a much higher temperature, and polariton systems are more easily integrated with semiconductor devices—both factors that pave the way for potential technological applications. (See the article by David Snoke and Jonathan Keeling, Physics Today, October 2017, page 54.)

But polaritons differ from atoms because of their short lifetime. Photons leak out of even the best cavities after a few picoseconds, so a polariton condensate must be continually refreshed with new photons to replace the ones that are lost. The condensate, therefore, never truly reaches thermal equilibrium; at best, it reaches a steady state.

Now Jacqueline Bloch (University of Paris–Saclay), Léonie Canet (Grenoble Alpes University), and colleagues have shown experimentally that because of its nonequilibrium nature, a polariton condensate behaves in an observably different way from its equilibrium counterparts. The behavior follows the form of the Kardar-Parisi-Zhang (KPZ) equation, derived in 1986 by Mehran Kardar, Giorgio Parisi (recipient of a share of the Nobel Prize in Physics; see Physics Today, December 2021, page 17), and Yi-Cheng Zhang to describe a wide variety of nonequilibrium systems, including raging wildfires and delicate frost crystals growing on a window.

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