Precise measurement of the Lamb shift and fine structure of the 2S-2Ptransition in triplet helium

The Lamb shift and fine structure of the $2^3$S-$2^3$P transition in helium has been precisely measured using laser excitation of an atomic beam. The Lamb shift is 5311.27(7) MHz, agreeing with the best previous measurement but a factor of 40 more precise. The 13 parts per ${10}^6$ precision makes this Lamb shift one of the most precisely known in atomic physics. The results for the fine structure intervals have precisions equal to or exceeding the best previous rf measurements but are not in good agreement with them. Our results provide timely benchmarks for the testing of atomic theory and QED in helium.