Hydrogen and deuterium are, chemically, quite similar, but without their electrons, a proton is less than half the radius of a deuteron. M Lozada Hidalgo and colleagues of the University of Manchester in the UK used this fact to provide a new and novel way to separate them. Both can diffuse through monolayers of graphene and boron nitride, but deuterons go through much more slowly, allowing a separation factor of about 10 to be achieved at room temperature. The isotope effect is explained by a roughly 60 meV difference in zero-point energies, which affect the activation barriers to pass through the monolayers – a surprising application of the uncertainty principle.