An optical atomic clock based on a highly charged ion

Optical atomic clocks are the most accurate measurement devices ever constructed and have found many applications in fundamental science and technology^1-3. The use of highly charged ions (HCI) as a new class of references for highest-accuracy clocks and precision tests of fundamental physics^4-11 has long been motivated by their extreme atomic properties and reduced sensitivity to perturbations from external electric and magnetic fields compared with singly charged ions or neutral atoms. Here we present the realization of this new class of clocks, based on an optical magnetic-dipole transition in Ar¹³⁺. Its comprehensively evaluated systematic frequency uncertainty of 2.2 × 10^-17 is comparable with that of many optical clocks in operation. From clock comparisons, we improve by eight and nine orders of magnitude on the uncertainties for the absolute transition frequency¹² and isotope shift (⁴⁰Ar versus ³⁶Ar) (ref. ¹³), respectively. These measurements allow us to investigate the largely unexplored quantum electrodynamic (QED) nuclear recoil, presented as part of improved calculations of the isotope shift, which reduce the uncertainty of previous theory¹⁴ by a factor of three. This work establishes forbidden optical transitions in HCI as references for cutting-edge optical clocks and future high-sensitivity searches for physics beyond the standard model.