The USNO rubidium fountains

TL;DR

This paper reports on the long-term stability and performance of four USNO rubidium fountains, highlighting their potential for precise timekeeping and fundamental physics tests over 4.5 years.

Contribution

It presents the operational stability, frequency behavior, and ensemble performance of USNO rubidium fountains, and discusses their application in testing fundamental constants and gravitational redshift.

Findings

Fountains maintained stability with deviations below 10^-16.

The ensemble achieved a white-frequency noise level of 10^-13.

Fountains exhibit occasional frequency changes of about 1.5 times per year.

Abstract

Four rubidium fountains at the U.S. Naval Observatory (USNO) have been in operation for 4.5 years. Each fountain has demonstrated periods of stability marked by Total or Theo deviation below 10^-16. Occasional frequency changes, on order of 1.5 times per year per fountain, introduce deviations from white-frequency noise behavior. Averaged together, the four fountains form an ensemble with a white-frequency noise level of 10^-13 and excellent long-term stability as compared to the primary frequency standards contributing to TAI. Progress on using the clocks at USNO for improving limits on coupling of fundamental constants to gravity by measuring the universality of the gravitational redshift for different types of clocks is discussed.