Speaker
Description
The precise measurement of the 1s–2s transition in hydrogen serves as a cornerstone for testing quantum electrodynamics (QED) in simple atomic systems [1]. Extending such measurements to other hydrogen-like systems such as He$^+$ probes higher-order QED corrections scaling with the atomic number $Z$ and reveals nuclear structure contributions beyond hydrogen. Despite its scientific interest, the 1s–2s transition in He$^+$ has never been measured before due to the demanding requirements of coherent XUV excitation. For this purpose, we have constructed an XUV frequency comb at 60.8 nm based on a cavity enhanced high-harmonic generation system seeded by an ultra-stable high-power infrared frequency comb, and a radio frequency ion trap that stores He$^+$ ion crystals sympathetically cooled with Be$^+$ [2]. We report on our novel non-collinear resonator for the generation of XUV light [3] and on our current efforts to synchronize and overlap counter-propagating XUV pulses at the trapped He$^+$ ions, essential for Doppler-free and recoil-free spectroscopy. Additionally, we explore a carrier-revival effect where strong carrier transitions could be excited in long linear ion chains, even when the strong recoil from XUV photons is not cancelled and the Lamb–Dicke regime is not reached for a single ion [4].
[1] T. Udem Nature Phys 14, 632 (2018)
[2] J. Moreno et al. Eur. Phys. J. D 77, 67 (2023)
[3] S. H. Wissenberg et al. Phys. Rev. Res. 7, 023071 (2025)
[4] F. Egli et al. in preparation (2026)