Optical clocks and tests of the equivalence principle

Ekkehard Peik
Physikalisch-Technische Bundesanstalt, Braunschweig, Germany

Stabilizing a laser frequency in resonance with a forbidden transition in laser-cooled trapped atoms or ions has made it possible to realize optical atomic clocks with relative uncertainties in the 10^-18 range. Besides their application in metrology, such clocks can be used in tests of the foundations of physics, like searches for violations of the equivalence principle. The 171Yb+ optical clock that is based on an S-F electric octupole transition possesses a favorable combination of small systematic uncertainty and high sensitivity for such tests because of the strongly relativistic character of the excited state.  In a comparison of two 171Yb+ single-ion clocks over six months we have demonstrated their agreement on the 10^-18 level and have improved limits on violations of local Lorentz invariance for electrons by two orders of magnitude.  A future nuclear clock based on the low-energy (8 eV) nuclear transition in Th-229 will bring further advances in accuracy and opportunities for searches for “new physics” in the nuclear interactions. Spectroscopy experiments with ions of the Th-229 isomer have recently provided data on relevant properties of this unusual nucleus.