The search for quantum chaos: From celestial mechanics to the helium atom

The classical problem of three bodies interacting under their mutual gravitational force has long been known to exhibit a mixture of regular and chaotic dynamics. Three bodies interacting under the influence of their mutual electric force should exhibit the same dynamical behavior, because the gravitational force and the electric force both obey the same inverse-square power law. However, an atomic-scale three-body electrical system—the helium atom—is also governed by quantum mechanics. The question is how the underlying chaotic classical behavior of the three-body electrical problem manifests in a quantum system. Or, how large does an atom have to be to show classical behavior? This question is addressed by experiments performed using an ultrabright beam of photons from the Advanced Light Source to study doubly excited autoionizing states of the helium atom.