Accurate Measurement of the2S13−3D13Two-Photon Transition Frequency in Helium: New Determination of the2S13Lamb Shift

We have performed a precise measurement of the ${}_{}{}^3{}_{}{}^{}2S_1^{}{}_{}{}^{}-{}_{}{}^3{}_{}{}^{}3D_1^{}{}_{}{}^{}$ two-photon transition frequency in ${}^4$He at 762 nm. The ${}_{}{}^3{}_{}{}^{}2S_1^{}{}_{}{}^{}-{}_{}{}^3{}_{}{}^{}3D_1^{}{}_{}{}^{}$ frequency is 786 823 850.002(56) MHz, with a relative uncertainty of $7.1\times {10}^{-11\mathrm{}}$. The deduced ${}_{}{}^3{}_{}{}^{}2S_1^{}{}_{}{}^{}$ Lamb shift is 4057.276(60) MHz. This result, the most accurate at the present time, reduces the uncertainty in the ${}_{}{}^3{}_{}{}^{}2S_1^{}{}_{}{}^{}$ Lamb shift by 1 order of magnitude and is 100 times more precise than the theoretical prediction [4062.3(8.0) MHz].