Precision Spectroscopy of Hydrogen and Femtosecond Laser Frequency Combs

The simple hydrogen atom has inspired many advances in quantum electrodynamics and experimental techniques. As the latest spin off, femtosecond optical frequency comb synthesizers have revolutionized the way optical frequencies are measured, and provide a reliable clock mechanism for optical atomic clocks. Precision spectroscopy of the hydrogen 1S-2S two-photon resonance has reached an accuracy of 1.4 parts in 10¹⁴, and considerable future improvements are envisioned. Such laboratory experiments are setting new limits for possible slow variations of the electromagnetic and strong interaction. The frequency comb technique has recently been extended into the extreme ultraviolet and may allow similar measurements on hydrogen like ions that can be held in a trap.