First Demonstration of VUV Ramsey-Comb Excitation Based on HHG; Towards 1S-2S He+ Excitation

It is nearly 10 years since a measurement in muonic hydrogen by the CREMA collaboration resulted in a surprisingly small proton charge radius [1]. Since then, an increasing number of high-precision atomic physics experiments have been started (and performed) in order to test bound-state quantum electrodynamics (QED) and contribute to solving the so-called proton radius puzzle (PRP). It is noteworthy that due to the close to 100% correlation between the proton charge radius and the Rydberg constant, the PRP can as well be understood as a Rydberg puzzle. Remarkably, the latest results from laser spectroscopy measurements in atomic hydrogen [2, 3] disagree with each other and do so far not allow further conclusions.