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 designs trap 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 a 45-nK Bose gas of very weakly interacting rubidium-87 atoms. The gas became diluted, not because the team expanded it but because atoms were naturally ejected via collisions with the background gas in the trap’s vacuum chamber. The rate of exodus is independent of energy, so just as for free expansion, the dilution does not change the energy per particle in the system. In time, 80% of the initial Rb atoms exited, and the temperature dropped to 23 nK, in agreement with theory. (T. F. Schmidutz et al., Phys. Rev. Lett., in press.)—Steven K. Blau