Precise optical Lamb shift measurements in atomic hydrogen

The 1S ground-state Lamb shift in atomic hydrogen has been measured to an accuracy of 1.3 parts in 105 by directly comparing the optical frequencies of the 1S-2S and the 2S-4S,4D two-photon transitions. The result, 8172.82(11) MHz, agrees with the theoretical prediction of 8172.94(9) MHz and rivals measurements of the 2S Lamb shift as a test of QED for a bound system. A comparison of the 2S-4S and 2S-4D intervals yields a 4S Lamb shift of 131.66(4) MHz