QUANTUM EVAPORATION occurs in a new experiment when a beam of phonons (little pulses of sound issuing from a warm filament) inside a pool of superfluid helium-4 is aimed at the liquid surface from below. In analogy with the photoelectric effect (in which light ejects electrons from a surface), the phonons pop helium atoms up out of the liquid. By measuring the momenta of the phonons and the evaporated atoms, one can determine that the atoms originally had zero momentum parallel to the surface, demonstrating directly (for the first time) that the He-4 atoms had been part of a Bose-Einstein condensate (BEC), in which the atoms fall into a single quantum state. Theories of superfluid He-4 had supposed that the atoms reside in a BEC state, but this had not been experimentally verified until now. The researcher, Adrian Wyatt of the University of Exeter, believes this method can be used to generate beams of coherent helium atoms (an "atom laser" effect). (Nature, 1 January 1998.)