The U.S. Naval Observatory provides the master clock for the DoD. To support this mission, we have built and fielded 4 rubidium atomic fountain clocks at our Washington D.C. site. This ensemble of clocks has been running continuously for slightly less than two years and is contributing to our larger ensemble of atomic clocks.
I will talk about the construction, operation, and underlying physics of these clocks. Each clock is a continuously running spectroscopy experiment that measures an atomic frequency to better than 10^-15 in one day. The performance of these clocks over the previous two years will be presented along with comparisons to international timescales.
Finally, I will present an experiment where we use this ensemble of clocks to set the most stringent limits on Local Position Invariance (LPI) violations through a “solar null test.” We make this comparison by looking for variations between atomic clocks based on different atomic references over the year. The presence or lack of variations driven by the annual variation of the solar gravitational potential sets improved limits on LPI violations and several fundamental constants’ coupling to the gravitational potential .