Extending frequency metrology into the extreme-ultraviolet range with highly charged ions*

Extending frequency metrology beyond the optical range will enhance the performance of optical clocks, which have become a key tool for novel fundamental physics studies using atomic systems. Their outstanding resolution, reproducibility, and accuracy make them in principle capable of sensing effects of all Standard Model interactions on the frequency of electronic transitions, such as those due to, e. g., a variation of the fine-structure constant. For disentangling the underlying source of any measurable perturbation of the electronic wave function is thereby crucial to carry out isotopic studies. By changing the neutron number as well as the overlap of the electronic wave function with that of the nucleus in a well-defined way, as in the generalized King-plot method [1], different contributions from Standard Model interactions can be potentially distinguished from hypothetical New Physics effects [2]. Isoelectronic and isonuclear sequences of highly charged ions (HCI) offer a plethora of possibilities in this regard [3], since they possess many different types of exceptionally long-lived metastable states up to x-ray energies, and photoionization coupling the bound electronic system to the continuum is excluded when the charge state is high enough. The recent demonstration of an optical clock based on HCI [4] shows how an extension of frequency metrology beyond the optical range is possible, and the benefits from such an approach. For this purpose, we are preparing an experiment combining an extreme-ultraviolet frequency comb based on high-harmonic-generation [5] with a superconducting radio-frequency trap [6].

[1] Berengut, J. C., et al., Rev. Res. 2, 043444 (2020)

[2] Rehbehn, N.-H., et al., Phys. Rev. A 103, L040801 (2021)

[3] Kozlov, M. G., et al., Rev. Mod. Phys. 90, 045005 (2018)

[4] King, S.A., et al., Nature 611, 43 (2022)

[5] Nauta, J., et al., Opt. Express 29, 2624 (2021)

[6] Stark J., et al., Rev. Sci. Instrum. 92, 083203 (2021)