Pop goes the atom

Quantum Evaporation occurs when a beam of phonons (little pulses of sound from a warm filament) inside a pool of superfluid helium-4 is aimed at the liquid surface from below. As in the photoelectric effect -- where light ejects electrons from a surface -- the phonons pop helium atoms out of the liquid. By measuring the momentum of the phonons and the evaporated atoms, scientists recently determined that the atoms originally had zero momentum parallel to the surface, proving, for the first time, that the atoms were part of a Bose-Einstein condensate, in which they fall into a single-quantum state (Nature , Vol 391, No 1).