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Precise determination of transition frequencies of simple atomic systems are required for a number of fundamental applications

such as tests of quantum electrodynamics (QED), the determination of fundamental constants and nuclear charge radii.

The sharpest transition in atomic hydrogen occurs between the metastable 2S state and the 1S ground state with a natural line width of only 1.3 Hz. Its transition frequency

has been measured with almost 15 digits accuracy using an optical frequency comb and a cesium atomic clock as a reference[1].

A measurement of the Lamb shift in muonic hydrogen is in significant contradiction to the hydrogen data if QED calculations are assumed to be correct[2].

In order to shed light on this discrepancy the transition frequency of one of the broader lines in atomic hydrogen has to be measured with very good accuracy

[3,4].

References [1] C. G. Parthey et al., Phys. Rev. Lett. 107, 203001 (2011).

[2] A. Antognini et al., Science 339, 417, (2013).

[3] A. Beyer et al., Science 358, 79 (2017).

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