  Improvements in both theory and frequency metrology of few-electron systems
such as hydrogen and helium have enabled increasingly sensitive tests of
quantum electrodynamics (QED), as well as ever more accurate determinations of
fundamental constants and the size of the nucleus. At the same time advances in
cooling and trapping of neutral atoms have revolutionized the development of
increasingly accurate atomic clocks. Here, we combine these fields to reach the
highest precision on an optical tranistion in the helium atom to date by
employing a Bose-Einstein condensate confined in a magic wavelength optical
dipole trap. The measured transition accurately connects the ortho- and
parastates of helium and constitutes a stringent test of QED theory. In
addition we test polarizability calculations and ultracold scattering
properties of the helium atom. Finally, our measurement probes the size of the
nucleus at a level exceeding the projected accuracy of muonic helium
measurements currently being performed in the context of the proton radius
puzzle.
