Metastable helium: Atom optics with nano-grenades

In this, the hundredth year since Einstein first postulated the existence of photons, the successful application of wave - particle duality to matter has seen an explosion of activity in the field of atom optics and Bose - Einstein condensation (BEC). This article provides a brief introduction to atom optics, illustrated with applications taken from experiments using helium atoms in long-lived (metastable) excited states. Metastable helium atoms store the greatest amount of energy (∼20 electron volts) in any atomic or molecular system. They behave like nano-hand grenades, making it easy to detect single atoms, opening up promising applications as well as fundamental studies of the quantum statistical properties of atomic systems.