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.
 C. G. Parthey et al., Physical Review Letters 107, 203001 (2011).
 A. Beyer, L. Maisenbacher, A. Matveev et al., Science 358, 79 (2017).
 A. Antognini et al., Science 339, 417 (2013).
Host: David DeMille (email@example.com)