Continuous-wave measurement of the 1SLamb shift in atomic hydrogen

We have determined the 1S Lamb shift in atomic hydrogen with a precision of 2 parts in ${10}^4$ by measuring the frequency of the 1S-2S transition observed by continuous-wave Doppler-free two-photon spectroscopy. We employ an interferometrically calibrated absorption line in ${}_{}{}^{130}{}_{}{}^{}\mathrm{Te}_2^{}{}_{}{}^{}$ as our reference and obtain a 1S-2S interval of 2 466 061 413.8(1.5) MHz. Choosing a value of the Rydberg constant measured independently by high-resolution spectroscopy of the hydrogen Balmer-β transition, we find the 1S Lamb shift to be 8173.3(1.7) MHz, in good agreement with the theoretical value of 8172.94(9) MHz.