Hyperfine Splittings in Light Atoms
Hyperfine intervals in light hydrogenic atoms and ions are among the most
accurately measured quantities in physics. The theory of QED corrections has
recently advanced to the point that the uncertainty of these corrections for
hydrogenic atoms and ions is probably smaller than 0.1 parts per million (ppm),
which is nevertheless not as accurate as the experiments. The difference of the
experiments and QED theory is the effect of the nuclear charge and magnetic
distributions, and this difference ranges from dozens to hundreds of ppm.