Nature: A clock that uses quantum mechanics to measure time also provides an alternative way to define the standard measurement of mass. Holger Müller of the University of California, Berkeley, created two matter waves from a cloud of ultracold cesium atoms. One half was given a set of precisely calibrated momentum kicks with a laser, after which it was recombined with the other, unaffected half. At 3 × 1025 Hz, the matter waves’ characteristic frequency is far too high to be measured directly. However, recombining the two halves creates an interference pattern whose frequency can be determined accurately with a laser. What’s more, because the frequency depends on the mass of an atom, the laser measurement allows for a direct calculation of a single atom’s mass. In principle, the exact number of atoms in 1 kg of different elements can therefore be determined. The current standard for mass is a 1-kg block of metal, whose mass is slowly changing because of microscopic contamination. The new single-atom clock joins two other options that are vying to replace the current definition of the mass standard.