# 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.

## Key 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.

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Source: https://tomesphere.com/paper/1704.01506