Microwave-clock timescale with instability on order of $10^{-17}$

TL;DR

This paper presents a highly stable microwave-clock timescale using rubidium fountains, achieving mid 10^{-17} instability and near-zero drift over 5.5 years, surpassing individual clocks and approaching fundamental limits.

Contribution

It demonstrates a long-term stable rubidium fountain timescale with record low instability and drift, highlighting fundamental performance limits of atomic fountain clocks.

Findings

Rubidium fountain timescale reaches mid 10^{-17} instability.

Zero drift of 1.3×10^{-19} per day achieved.

Fundamental limits due to fountain behavior discussed.

Abstract

The fundamental limits of atomic fountains as operational clocks are considered. Four rubidium fountains in operation at the U.S. Naval Observatory for over 5.5 years have demonstrated unprecedented long-term stability for continuously running clocks [1,2]. With only these rubidium fountains, a post-processed timescale can be created that demonstrates superior long-term performance to any individual clock by compensating for occasional frequency steps. By comparing to the world's primary standards we demonstrate instability of this rubidium fountain timescale reaching the mid $10^{-17}$'s and zero drift at the level of $1.3 \times 10^{-19}$/day. We discuss fundamental limits due to common mode behaviour or individual fountain performance that cannot be corrected.