Al+ optical clocks for fundamental physics, geodesy, and quantum metrology Till Rosenband, Physicist, National Institute of Standards and Technology

Laser-cooled trapped atoms have long been recognized as potentially very accurate frequency standards for clocks. Ultimate accuracies of 10-18 or 10-19 were projected from the time-dilation of trapped ions that move at laser-cooled velocities. Al+ quantum-logic clocks, which have a low sensitivity to background field perturbations, are approaching this accuracy, but further control of atomic motion is needed to reach the goal of 10-18. The current accuracy of 8.6x10-18 has allowed a geo-potential-difference measurement that detected a height change of 37+/-17 cm due to the gravitational red-shift. We have also observed quantum coherence of Al+ clock ions with a record Q-factor of 3x1016, and compared the Al+ resonance frequency to that of a single Hg+ ion to place limits on the temporal variation of the fine-structure constant.