A step toward probing higher-order Feynman diagrams in few-electron highly charged ions

The Lamb shift in medium-heavy H-like and He-like ions has been determined with unprecedented accuracy with a novel x-ray spectroscopy technique. Absolute wavelength measurements of emission lines from trapped highly charged Ar ions thereby achieved uncertainties down to the range of 1.6-2.4 ppm. For the H-like ion Ar17+, the ground state Lamb shift is determined with a 0.4% accuracy, thus excellently confirming quantum electrodynamics (QED) calculations and therefore the hitherto implied suitability of lines from these ionic species as absolute and accurate standards in the x-ray region. Second-order QED terms, i.e. two-electron terms, are also probed for the He-like species Ar16+. Deviations from QED predictions that have recently been claimed are excluded with high significance by the present results.