European Conference on Trapped Ions (ECTI)

The energy levels of hydrogen-like atoms can be precisely described by bound-state quantum
electrodynamics (QED). The frequency of the narrow 1s-2s transition of atomic hydrogen has
been measured with a relative uncertainty below ${10}^{-14}$. When combined with other spectroscopic
measurements of hydrogen and hydrogen-like atoms, the Rydberg constant and the proton charge
radius can be determined. Comparing the physical constants extracted from different combina-
tions of measurements serves as a consistency check for the theory. Hydrogen-like He+ ion
is yet another interesting spectroscopy target for testing QED. Due to their charge, He+ ions can
be held near-motionless in the field-free environment of a Paul trap, providing ideal conditions for
high precision measurement. Interesting higher-order QED corrections scale with large exponents
of the nuclear charge, which makes this measurement much more sensitive to these corrections
compared to the hydrogen case. We are currently setting up an experiment to perform precise
spectroscopy of the He+ 1S–2S transition. The main challenge of the experiment is that driv-
ing the 1S–2S transition in He+ requires narrow-band radiation at 61 nm. This lies in the extreme
ultraviolet (XUV) spectral range where no continuous wave laser sources exist. Our approach is
to use two-photon direct frequency comb spectroscopy with an XUV frequency comb. The XUV
comb is generated from an infrared high power frequency comb using intracavity high harmonic
generation. The spectroscopy target will be a small number of He+ ions, which are trapped in a
linear Paul trap and sympathetically cooled by co-trapped Be+ ions. In this talk, we will present
our recent progress in developing XUV frequency comb and Paul-trap for high-precision spec-
troscopy of He+ 1S–2S transition.