Two-photon frequency comb spectroscopy of the 6s-8s transition in cesium

We report a new absolute frequency measurement of the Cs 6s-8s two-photon transition measured using frequency comb spectroscopy. The fractional frequency uncertainty is 5×10^−11, a factor of 6 better than previous results. The comb is derived from a stabilized picosecond laser and referenced to an octave-spanning femtosecond frequency comb. The relative merits of picosecond-based frequency combs are discussed, and it is shown that the AC Stark shift of the transition is determined by the average rather than the much larger peak intensity.