Clocks Based on Highly Charged Ions

Highly charged ions (HCI) have many favorable properties. They offer a high sensitivity to test fundamental physics and for the search of new physics, a simplified atomic structure due to a small number of bound electrons, and a low susceptibility to external perturbing fields [1]. Therefore, HCI are also well-suited for next-generation optical atomic clocks, which can in principle operate at record fractional uncertainties of better than 10-18. However, up to recently HCI were not accessible for such type of instruments.

In this talk, I will review how we overcame all previous obstacles by demonstrating Coulomb crystallization of HCI [2], the implementation of quantum logic spectroscopy [3], and ground-state cooling of weakly-coupled motional modes [4]. With these prerequisites we realized the first optical atomic clock based on an HCI by stabilizing an ultra-stable clock laser to the ground-state fine-structure transition in Ar13+ at 441 nm. By comparing this optical frequency to the one of the electric-octupole transition in 171Yb+, we realized a frequency ratio measurement with a fractional uncertainty of about 1x10-16, limited by statistics. We thereby improved the uncertainty of the absolute transition frequency of Ar13+ by about eight orders of magnitude. The systematic uncertainty was 2.2x10-17, dominated by the time dilation shift uncertainty from excess micromotion. Importantly, this level of excess micromotion can be considerably reduced with a new, carefully manufactured ion trap. All other systematic uncertainties are at or below 10-18, demonstrating the potential of HCI as highly accurate atomic references for time keeping and unprecedented tests of fundamental physics. Furthermore, we compared the transition frequencies of the two isotopes 40Ar13+ and 36Ar13+ in order to determine the isotope shift with an improvement of nine orders of magnitude – resolving the QED nuclear recoil contribution.

The experimental approach is universal and thereby generally unlocks HCI for such precision experiments.

1. M. G. Kozlov et al., Rev. Mod. Phys. 90 (2018)
2. L. Schmöger et al., Science 347 (2015)
3. P. Micke et al., Nature 578 (2020)
4. S. A. King et al., Phys. Rev. X 11 (2021)