An Optical Atomic Clock Based on a Highly Charged Ion

TL;DR

This paper reports the first optical atomic clock based on a highly charged ion, demonstrating high accuracy and enabling new tests of fundamental physics through precise frequency measurements.

Contribution

It introduces a new class of optical clocks using highly charged ions, specifically Ar$^{13+}$, with systematic uncertainties comparable to existing state-of-the-art clocks.

Findings

Achieved a systematic frequency uncertainty of 2.2×10⁻¹⁷.

Improved the accuracy of isotope shift measurements by 8-9 orders of magnitude.

Provided insights into quantum electrodynamic nuclear recoil effects.

Abstract

Optical atomic clocks are the most accurate measurement devices ever constructed and have found many applications in fundamental science and technology. The use of highly charged ions (HCI) as a new class of references for highest accuracy clocks and precision tests of fundamental physics has long been motivated by their extreme atomic properties and reduced sensitivity to perturbations from external electric and magnetic fields compared to singly charged ions or neutral atoms. Here we present the first realisation of this new class of clocks, based on an optical magnetic-dipole transition in Ar$^{13+}$. Its comprehensively evaluated systematic frequency uncertainty of $2.2\times10^{-17}$ is comparable to that of many optical clocks in operation. From clock comparisons we improve by eight and nine orders of magnitude upon the uncertainties for the absolute transition frequency and isotope shift ($^{40}$Ar vs. $^{36}$Ar), respectively. These measurements allow us to probe the largely unexplored quantum electrodynamic nuclear recoil, presented as part of improved calculations of the isotope shift which reduce the uncertainty of previous theory 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.