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

Both atomic hydrogen and deuterium can be used to determine physical constants and to test Quantum Electrodynamics. Combining the 1S-2S with additional transition frequency measurements in each isotope separately, one can extract the deuteron radius independent of the proton radius and vice versa [1]. We recently measured the 2S-6P transition in hydrogen, and here report on the status of the ongoing analysis. We also performed a preliminary measurement of the 2S-6P transition in deuterium, where no recent result is available and a discrepancy with muonic deuterium persists [2]. Compared to hydrogen, precision spectroscopy of the same transition in deuterium is complicated by simultaneous excitation of hyperfine transitions, possibly leading to unresolved quantum interference [3]. Since these effects depend on laser polarization, we developed an active fiber-based retroreflector with a polarization monitor [4]. Furthermore, we find that in our case the unresolved quantum interference is suppressed, making a 2S-6P deuterium measurement with similar precision as for hydrogen feasible.

[1] R. Pohl et al., Metrologia 54 (2017). [2] R. Pohl et al., Science 353  (2016). [3] Th. Udem et al., Ann. Phys. 531 (2019). [4] V. Wirthl et al., Opt. Express 29 (2021).