Toward XUV frequency comb spectroscopy of the 1 S–2 S transition in He⁺

The energy levels of hydrogen-like atoms and ions are accurately described by bound-state quantum electrodynamics (QED). He⁺ He + ions have a doubly charged nucleus, which enhances the higher-order QED contributions and makes them interesting for precise tests of QED. Systematic effects that currently dominate the uncertainty in hydrogen spectroscopy, such as the second-order Doppler shift and time-of-flight broadening, are largely suppressed by performing spectroscopy on trapped and cooled He⁺ He + ions. Measuring a transition in He⁺ He + will extend the test of QED beyond the long-studied hydrogen. In this article, we describe our progress toward precision spectroscopy of the 1 S–2 S two-photon transition in He⁺ He + . The transition can be excited by radiation at a wavelength of 60.8 nm generated by a high-power infrared frequency comb using high-order harmonic generation (HHG). The He⁺ He + ions are trapped in a Paul trap and sympathetically cooled with laser-cooled Be⁺ Be + ions. Our recently developed signal detection scheme based on secular-scan spectrometry is capable of detecting He⁺ He + excitation with single-event sensitivity. Graphic abstract