High-Precision Optical Measurement of the2SHyperfine Interval in Atomic Hydrogen

We have applied an optical method to the measurement of the $2S$ hyperfine interval in atomic hydrogen. The interval has been measured by means of two-photon spectroscopy of the $1S-2S$ transition on a hydrogen atomic beam shielded from external magnetic fields. The measured value of the $2S$ hyperfine interval is equal to 177 556 860(16) Hz and represents the most precise measurement of this interval to date. The theoretical evaluation of the specific combination of $1S$ and $2S$ hyperfine intervals $D_{21}$ is in fair agreement (within $1.4\sigma$) with the value for $D_{21}$ deduced from our measurement.