Two Photon Direct Frequency Comb Spectroscopy of the 1S-3S Transition in Hydrogen

The energy levels of hydrogen-like atoms can be both calculated and measured very precisely. Precision spectroscopy of two transitions at the current level of accuracy allows the determination of the Rydberg constant and the proton charge radius. Comparison with additional transitions serves as a consistency check for the theory of quantum electrodynamics. The recent discrepancy in these consistency checks is known as the proton radius puzzle.

Here we present the latest measurement of the 1S-3S transition in hydrogen, using two photon direct frequency comb spectroscopy. This is the first time this spectroscopy technique is applied in hydrogen. The obtained result ( f_1S-3S = 2,922,743,278,665.79(72) kHz) supports the value of the proton charge radius obtained from muoic Hydrogen.

We also give an outlook on the next anticipated measurements, current problems and recent improvements of the experiment.