Precision spectroscopy of atomic hydrogen has motivated advances in nonlinear laser spectroscopy and optical frequency metrology over more than three decades, including the laser frequency comb technique highlighted in the citation for the 2005 Nobel Prize in physics. Past spectroscopic measurements of the ultraviolet 1S-2S two-photon resonance in hydrogen and deuterium in our laboratory have led to new tests of quantum electrodynamic theory, and they have yielded accurate values of the Rydberg constant, the rms charge radius of the proton, and the structure radius of the deuteron. In addition, our measurements were among the first laboratory experiments to set stringent limits to possible slow variations of fundamental constants.

Until a few years ago, the primary challenge in hydrogen spectroscopy has been the precise measurement of the frequency of laser light. Since the advent of the laser frequency comb technique, the challenge has moved to the understanding and control of systematic line shifts, which are particularly serious for very light atoms that can not be laser cooled. We have made major advances towards future improved measurements of the 1S-2S transition frequency.

In particular, we are now routinely achieving sub-Hz line widths with diode-laser based solid state laser systems at 972nm, and we are exploring spectroscopy at lower laser intensities and reduced ac Stark shifts by detecting protons after photo ionization of the metastable 2S atoms rather than Lyman-alpha photons. To verify such advances and to test predicted higher order quantum electrodynamic corrections, we have been performing new precise optical measurements of the hyperfine splitting of the 1S-2S resonance.

A measurement of the 1S-2S frequency with the highest possible precision is of particular interest as a reference for future experiments with anti hydrogen, in order to detect possible small differences between matter and antimatter. We are participating in the ATRAP anti hydrogen spectroscopy collaboration at CERN. For the next absolute frequency measurement, we will rely once more on the transportable cesium fountain microwave clock PHARAO of the Observatoire in Paris. Future experiments will benefit from a long-haul optical fiber link for clock synchronization between our Garching laboratory and the stationary microwave and optical atomic clocks at the PTB in Braunschweig that is now being implemented within a collaboration led by the PTB.

The comparison of the 1S-2S frequency with quantum electrodynamic theory and the determination of the Rydberg constant is presently limited by the large 2% uncertainty of the rms charge radius of the proton, as determined from electron scattering experiments. To overcome this limit, we are working on a complementary experiment to measure the absolute frequency of the 1S-3S two-photon resonance, in order to take advantage of the different scaling of hadronic level shifts with the principal quantum number. This resonance in the deep ultraviolet (205nm) will be directly excited with a frequency comb spectrum, produced with a mode-locked Ti:sapphire picosecond laser and two stages of resonator-enhanced frequency doubling.

The two picosecond doubling cavities that generate 205nm from a 820nm titanium sapphire laser to excite the 1S-3S transition in atomic hydrogen.

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last change September 18 2008