Two-photon frequency comb spectroscopy of atomic hydrogen

Testing physics using the hydrogen atom Discrepancy between the proton radius determined from hydrogen and muonic hydrogen spectroscopy data, the so-called “proton radius puzzle,” has been a focus of the physics community for more than a decade now. Using two-photon ultraviolet frequency comb spectroscopy below 1 kilohertz, Grinin et al. report a high-precision measurement of the 1S-3S transition frequency in atomic hydrogen (see the Perspective by Ubachs). Combining this measurement with the data for the 1S-2S transition, the authors obtained the Rydberg constant with improved accuracy and an independent value for the proton charge radius that favors the data from muonic hydrogen. However, the present frequency value differs from the one obtained previously using a different spectroscopic technique, leaving the puzzle still unresolved. Science , this issue p. 1061 ; see also p. 1033