Precision spectroscopy of the 2S-6P transition in atomic hydrogen

Precision spectroscopy of atomic hydrogen (H) is an important way to extract physical constants and test bound-state quantum electrodynamics (QED), one of the building blocks of the Standard Model. Both the Rydberg constant R_∞ and the proton charge radius r_p can be determined by H spectroscopy with high precision, with a comparison of the values of R_∞ and r_p determined from measurements of different transitions constituting a test of QED.

We have previously measured the 2S-4P transition frequency to 4 parts in 10¹² (A. Beyer et al., Science 358, 79 (2017)), finding good agreement in r_p with the spectroscopy of muonic H (A. Antognini et al., Science 339, 417 (2013)). Recently, we have probed the 2S-6P transition, which has a three times lower linewidth compared to the 2S-4P transition. This factor, together with other experimental improvements (V. Wirthl et al., Optics Express 29, 7024 (2021)), allows for a five-fold improvement in fractional precision and in the determination of R_∞ and r_p. Here, we will discuss the ongoing data analysis and present preliminary results.