Abstract
We have measured the frequency of the extremely narrow 1S-2S two-photon transition in atomic hydrogen using a remote cesium fountain clock with the help of a 920 km stabilized optical fiber. With animproved detection method we obtain f1S-2S¼2466 061 413 187 018ð11ÞHz with a relative uncertainty of 4:51015, confirming our previous measurement obtained with a local cesium clock [C. G. Partheyet al., Phys. Rev. Lett.107, 203001 (2011)]. Combining these results with older measurements, we constrain the linear combinations of Lorentz boost symmetry violation parameters cðTXÞ¼ð3:11:9Þ1011 and 0:92cðTYÞþ0:40cðTZÞ¼ð2:65:3Þ1011 in the standard model extension framework [D. Colladay, V. A. Kostelecky ́, Phys. Rev. D.58, 116002 (1998)]