Can a Bose–Einstein condensate exist 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.

More accessible condensates can be made by replacing the atoms with polaritons: light–matter quasiparticles that form when photons in an optical cavity couple to electronic excitations in a solid. Because the quasiparticles have such low effective masses, their quantum effects set in much more easily.

Polariton condensates form at standard cryogenic temperatures—around 4 K, the temperature of liquid helium—and under some circumstances, even at room temperature.1 The warmer conditions make it easier to study the physics of quantum condensation and to pursue potential technological applications (see the article by David Snoke and Jonathan Keeling, Physics Today, October 2017, page...

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