Improved Measurement of the Hydrogen 1S - 2S Transition Frequency
  We have measured the 1S - 2S transition frequency in atomic hydrogen via two photon spectroscopy on a 5.8 K atomic beam. We obtain $f_{1S-2S} = 2 466 061 413 187 035 (10)$ Hz for the hyperfine centroid. This is a fractional frequency uncertainty of $4.2\times 10^{-15}$ improving the previous measure- ment by our own group [M. Fischer et al., Phys. Rev. Lett. 92, 230802 (2004)] by a factor of 3.3. The probe laser frequency was phase coherently linked to the mobile cesium fountain clock FOM via a frequency comb. 
Christian G. Parthey, Arthur Matveev, Janis Alnis, Birgitta Bernhardt,   Axel Beyer, Ronald Holzwarth, Aliaksei Maistrou, Ra...
physics.atom-ph