HYDROGEN 1S - 2S · Precision laser spectroscopy of the 1S-2S optical clock transition in atomic hydrogen

Final Activity Report Summary - HYDROGEN 1S-2S (Precision Laser Spectroscopy of the 1S-2S Optical Clock Transition in Atomic Hydrogen)

Precision optical spectroscopy has become increasingly popular during the recent years thanks to a revolutionary ‘optical frequency comb’ method which was pioneered by year 2005 Nobel Prize laureates and allowed to count the frequency of light with great precision. The ability to count the frequency of light led to active development in the field of optical atomic clocks that might replace current microwave atomic time standards in the future. All optical atomic clocks require a very stable laser source. During the Marie Curie scholarship the researcher succeeded to build some of the world’s most stable diode lasers, with spectral linewidth of only 0.5 Hz at the optical frequency of 3e14 Hz, at 972 nm wavelength, and small long-term drift equal to only 50 mHz/s. This narrow linewidth was actually limited by the thermal noise of the high-finesse Fabry-Perot resonator which was used for laser stabilisation. It was possible to achieve this fundamental limit thanks to a vibration compensating mounting of the Fabry-Perot resonator and an improved temperature stabilisation at the optimal thermal expansion temperature of the resonator. The researcher subsequently consulted other groups on building similar stable lasers. During the scholarship a solid state laser system was finalised, including low-noise diode laser oscillator, tapered amplifier and two frequency doubling stages allowing to produce 20 mW of laser radiation at 243 nm that was necessary for two-photon precision spectroscopy of the 1S-2S transition in atomic hydrogen. For the improved precision measurement session of the hydrogen 2S state hyperfine splitting in 2008 it was very useful to have two laser systems available simultaneously, i.e. both the new diode laser system and the old dye laser system which allowed to troubleshoot laser instabilities on the short time scales that were not visible by optical frequency comb measurements. The precision of the measurements was almost three times improved in comparison to the previous value, published in 2004, thus leading to a new value of 2S hyperfine splitting of 177 556 840(5) Hz. This was an important result for testing quantum electrodynamics (QED) theory with ever higher accuracy.

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