Spectroscopy article submitted!


Improvements in both theory and frequency metrology of few- electron systems such as hydrogen and helium have enabled increasingly sen- sitive 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 revolutioni- zed the development of increasingly accurate atomic clocks. Here, we combine these fields to reach the highest precision on an optical transition in the he- lium atom to date by employing a 4He 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, in combination with a similarly accurate measurement in 3He, our measurement will probe their nuclear charge radii at a level exceeding the projected accuracy of muonic helium measurements currently being performed in the context of the proton radius puzzle.