Precision atomic spectroscopy for improved limits on variation of the fine structure constant and local position invariance.

Measurements comparing the relative rates of two atomic clocks that are based on very narrow transitions between well-resolved and relatively unperturbed atomic energy levels currently report among the highest precision attained for any fundamental quantity. Such measurements, which can yield greater than 15 digits of frequency accuracy, permit tests of fundamental postulates of physics, including searches for variations of fundamental constants. In this Letter, we analyze measurements of the frequency