Speaker
Description
The 1S-2S transition of hydrogenic systems is a benchmark for tests of fundamental physics [1]. The most prominent example is the 1S-2S transition in atomic hydrogen, where impressive relative accuracies have been achieved [2-3]. Nowadays, these fundamental physics tests are hampered by estimates of uncalculated higher-order QED terms and the uncertainties in the fundamental constants required for their calculation [4]. An independent, experimental approach to contribute to and further improve these fundamental physics tests is to measure the 1S-2S transition in He$^{+}$. Because He$^{+}$ has twice the nuclear charge of hydrogen, certain interesting QED contributions are strongly enhanced and can therefore be tested more precisely than in hydrogen. Furthermore, nuclear properties such as the alpha particle charge radius or nuclear polarizability contributions can be probed [4].
We aim to use the Ramsey-comb spectroscopy (RCS) method [5] developed in our lab in order to measure the 1S-2S transition in singly-ionized helium in the extreme ultraviolet (XUV) spectral range and contribute to fundamental tests of QED. RCS uses two amplified and up-converted pulses out of the infinite pulse train of a frequency comb laser to perform a Ramsey-like excitation. The He$^{+}$ spectroscopy scheme is based on two-photon excitation, using one XUV photon at 32 nm (generated through High-Harmonic Generation, the 25th harmonic) and one infrared photon at 790 nm from the fundamental beam. The atomic sample will consist of a He$^{+}$ ion confined in a Paul trap, sympathetically cooled by a Doppler- and Raman-cooled Be$^{+}$ ion, which has a cycling transition at 313 nm that we also use to monitor the Be$^{+}$ ion. The readout scheme for two-photon He$^{+}$ excitation is based on quantum logic spectroscopy [6], which relies on detecting the recoil of He$^{+}$ upon excitation, transferred to the Be$^{+}$ ion.
Recently we demonstrated that RCS can be combined with HHG [7] leading to a high precision measurement in xenon at 110 nm [8]. The many new components required for the He$^{+}$ experiment, such as a new RCS laser, the ion trap, laser cooling and imaging systems, are approaching completion and we will report on their current status. Using the RCS method we aim to do a first 1S-2S measurement of He$^{+}$ with an accuracy of 1-10 kHz, while an accuracy of better than 50 Hz should be ultimately achievable.
References:
[1] Herrmann et al., Phys. Rev. A 79, 052505 (2009)
[2] Niering et al., Phys. Rev. Lett. 84, 5496 (2000)
[3] de Beauvoir et al., Eur. Phys. J. D 12, 61 (2000)
[4] Krauth et al., PoS(FFK2019) 49, (2019)
[5] Morgenweg et al., Nature Phys. 10, 30–33 (2014)
[6] Schmidt et al., Science 309, 749 (2005)
[7] Dreissen et al., Phys. Rev. Lett. 123, 143001 (2019)
[8] Dreissen et al., Phys. Rev. A 101, 052509 (2020)
