We present a measurement of the 1S-2S transition frequency in atomic hydrogen by two-photon spectroscopy
yielding <i>f</i><sub>1S-2S</sub> = 2 466 061 413 187 035 (10) Hz corresponding to a fractional frequency uncertainty of 4.2×10<sup>-15</sup>.
The result presents a more than three times improvement on the previous best measurement.
We purpose a new method for 1s-2s spectroscopy of atomic hydrogen. Our method is based on a new detection
scheme, utilizing a three-photon ionization to obtain a signal correspondent to the excitation of atoms into 2s
state. We report about assembling and test of a proton detection system for this measurement. To measure a
second order Doppler shift we purpose a delayed detection scheme. Using this method we plan to reduce the
uncertainty of the absolute frequency measurement to the 10<sup>-15</sup> domain.