Precision measurements of atomic hydrogen (H) have long been successfully used to extract fundamental constants and to test bound-state quantum electrodynamics. Both the Rydberg constant R∞ and the proton root mean square charge radius rp are determined to a large degree by H spectroscopy, requiring the measurement of at least two transition frequencies. With the very precisely measured 1S-2S transition frequency  serving as a corner stone, the current limitation of this extraction is the measurement precision of other H transition frequencies. We have recently measured the 2S-4P transition in H with a relative uncertainty of 4 parts in 1012 , allowing the most accurate determination of R∞ and rp from atomic hydrogen. Moreover, we find good agreement of rp with the much more precise value extracted from spectroscopy of muonic hydrogen .
We are working on a conceptually similar measurement of the 2S-6P transition in H, which has a three times lower line width of 3.9 MHz compared to the 2S- 4P transition. A cryogenic beam of H atoms is optically excited to the initial 2S state and then interacts with a standing wave at 410 nm, driving the one-photon 2S-6P transition. Custom-designed optics are used to avoid any residual running component of this light field, which would otherwise lead to a first-order Doppler shift. As the 6P state has a high sensitivity to stray electric fields, special care was taken to suppress these fields to a sufficiently low level. Furthermore, we have greatly improved the quantum efficiency of our fluorescence detection. We are currently investigating a potential frequency shift caused by the light force acting on the atoms in the standing light wave. Here, I will discuss preliminary results and implications on the determination of the proton radius.
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 A. Beyer, L. Maisenbacher, A. Matveev et al., Science 358, 79 (2017).
 A. Antognini et al., Science 339, 417 (2013).
Host: David DeMille (firstname.lastname@example.org)