Ultrastable Microwave Transfer of Cesium Frequency Standard over 20 km of Optical Fiber

Tests of fundamental physics, at low temperatures in tabletop experiments, often require precise absolute frequency measurements of transitions in atomic, ionic, or molecular systems. In many cases, the accuracy of an optical frequency measurement is limited by the accuracy of the local, commercial frequency reference used in the experiment. To address this limitation in measurement accuracy, we have recently partnered with the National Institute of Standards and Technology (NIST) to establish an optical fiber link between Colorado State University and the NIST radio station, WWV, for frequency transfer of the microwave signal generated by their cesium atomic clock ensemble. This frequency transfer scheme allows the NIST (WWV) timescale to act as the absolute frequency reference for measurements taking place at CSU. We have implemented an active stabilization scheme to eliminate drifts in the optical fiber path length throughout the course of a given day. The transferred cesium signal is compared to a local Rubidium reference as a preliminary test of the stability of the link.