Precision measurement of the hydrogen and deuterium 1 S ground state Lamb shift

We report on new precision measurements of the 1S ground state Lamb shift in hydrogen and deuterium, based on a direct frequency comparison of the 1S-2S and 2S-4S two-photon resonances. By observing the 2S-4S transition via blue Balmer-{β} fluorescence we are now surpassing the accuracy of the 2S Lamb shift. Our results of 8172.86(6) MHz for hydrogen and 8184.00(8) MHz for deuterium are not in perfect agreement with the theoretical predictions of 8173.12(6) and 8184.13(6) MHz, respectively. We also derive values for the Rydberg constant and the electron-proton mass ratio.