Schmid, Fabian
Date: Wednesday, September 14, 2022
Time: 14:00
Place: ETH Zurich, Campus Hönggerberg, HPF G 6
Host: Daniel Kienzler
Towards High Resolution Spectroscopy on Trapped Helium Ions
Fabian Schmid
Max-Planck-Institut für Quantenoptik, Germany
A precision test of a physical theory requires studying it in a system whose properties can be both measured and calculated with very high accuracy. One famous example is the hydrogen atom which can be precisely described by bound-state quantum electrodynamics (QED) and whose transition frequencies can be accurately measured by laser spectroscopy. Two physical constants, the Rydberg constant and the nuclear charge radius, are determined by fitting the theory expression for the energy levels to the experimental data [1]. Comparing the physical constants extracted from different combinations of measurements then serves as a consistency check for the theory itself.
We are currently setting up an experiment to perform spectroscopy on the 1S-2S transition in the hydrogen-like ion He+. By combining the 1S-2S transition frequency with an accurate value of the helium nuclear charge radius measured by muonic helium spectroscopy [2], we will be able to for the first time determine the Rydberg constant in a system other than hydrogen. This value will then be compared with the value obtained from hydrogen spectroscopy [1], serving as a stringent test of the universality of 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 a high precision measurement. Furthermore, interesting higher-order QED corrections scale with large exponents of the nuclear charge which makes this measurement more sensitive to these corrections compared to the hydrogen case [3].
The main challenge of the experiment is that driving the 1S-2S transition in He+ requires narrow-band radiation at 61 nm. This lies in the extreme ultraviolet (XUV) spectral range no cw laser sources exist. Our approach is to use two-photon direct frequency comb spectroscopy [4] with an XUV frequency comb. The comb is generated from an infrared high power frequency comb using intracavity high harmonic generation [5]. 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.
[1] E. Tiesinga et al., Rev. Mod. Phys. 93, 025010 (2021).
[2] J. J. Krauth et al., Nature 589, 527 (2021).
[3] M. Herrmann et al., Phys. Rev. A 79, 052505 (2009).
[4] A. Grinin et al., Science 370, 1061 (2020).
[5] I. Pupeza et al., Nat. Photonics 15, 175 (2021).