Precise experimental test of calculated two-electron Lamb shifts in helium

We have measured the wave number of the ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ $2^1$S-$3^1$P transition by Doppler-free spectroscopy in a metastable helium beam. The result, 19 931.924 794(45) ${\mathrm{cm}}^{\mathrm{-}1}$, represents an order-of-magnitude improvement over previous measurements and provides a sensitive test of two-electron QED effects. Our result shows an unexpectedly large deviation from theoretical prediction. We derive improved term values for a number of low-lying helium levels. All levels with n=2 show significant deviations from theory that are correlated with the size of calculted two-electron contributions to the Lamb shifts.