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]. In this poster, we report on our current efforts to synchronize and overlap counter-propagating XUV pulses at the trapped He$^+$ ions, which is essential for Doppler-free and recoil-free spectroscopy. For this, we plan to employ Raman spectroscopy in Be$^+$, using counter-propagating pulses generated from our high-power infrared frequency comb. Additionally, we explore a counterintuitive carrier revival effect [3], where long ion chains allow the excitation of strong carrier transitions, even when the strong recoil from XUV photons is not cancelled and the Lamb–Dicke regime is not reached for a single ion.
[1] T. Udem, Nature Phys 14, 632 (2018)
[2] J. Moreno et al., Eur. Phys. J. D 77, 67 (2023)
[3] F. Egli et al., in preparation (2026)