Towards precision spectroscopy of the 2S-6P transition in atomic deuterium

Similar to atomic hydrogen, atomic deuterium can be used to determine physical constants and to test Quantum Electrodynamics. The isotope shift of the 1S-2S transition links measurements between hydrogen and deuterium through the squared deuteron-proton charge radius difference [1]. A combination of the 1S-2S transition frequency with additional measurements in deuterium determines the deuteron radius independent of the proton radius [2]. However, these determinations are discrepant with results obtained in muonic deuterium [3]. Contrary to hydrogen [4], no recent measurements in deuterium are available. We are working towards a measurement of the 2S-6P transition in deuterium. 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 [5]. Since these effects depend on laser polarization, we developed an active fiber-based retroreflector with a polarization monitor [6]. 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] U. D. Jentschura et al., Phys. Rev. A 83, 042505 (2011).

[2] R. Pohl et al., Metrologia 54, L1 (2017).

[3] R. Pohl et al., Science 353, 669–673 (2016).

[4] A. Grinin et al., Science 370, 1061-1066 (2020).

[5] T. Udem et al., Ann. Phys. 531, 1900044 (2019).

[6] V. Wirthl et al., Opt. Express 29(5), 7024-7048 (2021).