Toward XUV frequency comb spectroscopy of the 1 S–2 S transition in Jorge Moreno, Fabian Schmid, Johannes Weitenberg, Savely G. Karshenboim, Theodor W. Hänsch, Thomas Udem and Akira OzawaEur. Phys. J. D, 77 4 (2023) 67DOI: https://doi.org/10.1140/epjd/s10053-023-00645-1