We report on an absolute frequency measurement of the 1S-2S two-photon transition frequency in atomic hydrogen. In our experiment, we have directly linked the transition frequency to a cesium atomic clock. A careful analysis of the spectroscopic line-shape by means of a theoretical model allowed the determination of the line center to about 1/100 of the linewidth, yielding a value for the 1S-2S transition frequency of 2 466 061 413 187 103(46) Hz.

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