Precision Measurement of the Hydrogen 1S-2S Frequency via a 920-km Fiber Link

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 an improved detection method we obtain f(1S-2S) = 2466 061 413 187 018 (11) Hz with a relative uncertainty of 4.5 x 10(-15), confirming our previous measurement obtained with a local cesium clock [C. G. Parthey et al., Phys. Rev. Lett. 107, 203001 (2011)]. Combining these results with older measurements, we constrain the linear combinations of Lorentz boost symmetry violation parameters c((TX)) = (3.1 +/- 1.9) x 10(-11) and 0.92c((TY)) + 0.40c((TZ)) = (2.6 +/- 5.3) x 10(-11) in the standard model extension framework [D. Colladay, V. A. Kostelecky, Phys. Rev. D. 58, 116002 (1998)].