High resolution spectroscopy of 1S-3S transition in hydrogen

The aim of our experiment is to improve the spectroscopy of atomic hydrogen in order to determine the charge radius of the proton. A new continuous laser source at 205 nm allows us to reach a statistical uncertainty of 2.2 kHz on the measurement of the 1S-3S transition frequency. This uncertainty could be sufficient to clarify the proton radius puzzle, however there remain systematic effects that are currently under study.