Dilute a typical gas by allowing it to expand, and its temperature will change. In classical physics, that so-called Joule–Thomson effect disappears in the ideal-gas limit of vanishing interactions. But as shown theoretically in 1937 by Daulat Kothari and B. N. Srivasava, at very low temperatures, for which the rules of quantum mechanics apply, a Bose gas will cool when diluted, even if the gas molecules don’t interact. Now Joule–Thomson cooling of such a gas has finally been observed, by Zoran Hadzibabic and colleagues at the University of Cambridge, thanks to a novel trap. Conventional traps hold gases in harmonic potentials whose confining forces strongly amplify the effect of any interparticle interactions, thereby obscuring the “ideal” quantum Joule–Thomson effect. The researchers created a nearly uniform confining potential by intersecting a tube-like green laser beam with two sheet-like beams, as shown in the figure. They then filled their trap with...

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