Physics

Electron

Isotope

Dipole

Lamb shift

Engineering

Atomic physics

Ion source

Atomic clock

Magnetic dipole

Nuclear physics

Recoil

Sensitivity (control systems)

Electronic engineering

Highly charged ion

Quantum mechanics

Computational physics

Nature

Optical atomic clocks are the most accurate measurement devices ever constructed and have found many applications in fundamental science and technology1–3. The use of highly charged ions (HCI) as a new class of references for highest-accuracy clocks and precision tests of fundamental physics4–11 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)&nbsp;(ref.&nbsp;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. An optical atomic clock operating on a magnetic-dipole transition in a highly charged argon ion is shown to improve uncertainties for the absolute transition frequency and isotope shift by several orders of magnitude.

An optical atomic clock based on a highly charged ion