Precise laser spectroscopy of hydrogen and singly-ionized helium

We have measured the hydrogen 1S Lamb shift to be 8,172,827(51) kHz, the most precise value for any Lamb shift. The result is in reasonable agreement with the predictions of quantum electrodynamics (QED), although its interpretation as a test of QED is limited by the uncertainty in the proton charge radius. A measurement of the He/sup +/(2S) Lamb shift by laser spectroscopy can bypass this limitation and investigate a nine standard deviation discrepancy between theory and experiment for this Lamb shift.