High‐Resolution Hydrogen Spectroscopy and The Proton Radius Puzzle

High resolution spectroscopy of the hydrogen atom takes on particular importance in the new SI, as it allows to accurately determine fundamental constants, such as the Rydberg constant and the proton charge radius. Recently, the second most precisely measured transition frequency in hydrogen, $1S?3S$, is obtained by our group. In the context of the Proton Radius Puzzle, this result calls for further investigation.