Optical Frequency Measurement is Getting a Lot More Precise

At the Max Planck Institute for Quantum Optics in the Munich suburb of Garching, Theodor Hansch and colleagues have measured the ultraviolet transition frequency between the 1S and 2S states of atomic hydrogen to be $2.466 061 413 187 34(84)×1015 Hz.$ With an uncertainty of only 3 parts in $1013,$ this result exceeds the accuracy of the best previous measurement of the 1S‐2S transition by two orders of magnitude. It is, in fact, the most accurate measurement to date of any frequency in the visible or ultraviolet. It's so accurate that simply repeating the measurement a year from now would provide a better and more direct verification (or falsification) of the constancy of the fine‐structure constant over cosmological time than any astrophysical data we have.