# Optical-Clock-Based Time Scale

## TL;DR

This paper introduces a hybrid microwave-optical time scale that leverages intermittent optical clocks and an ensemble of microwave clocks to achieve unprecedented stability and accuracy over months, surpassing traditional methods.

## Contribution

The paper presents the first implementation of a hybrid optical-microwave time scale that maintains high accuracy with intermittent optical clock operation and ensemble-based flywheel, improving upon existing timekeeping systems.

## Key findings

- Achieved sub-nanosecond accuracy over several months.
- Attained fractional frequency uncertainty of 1.45×10⁻¹⁶ at 30 days.
- Reached the 10⁻¹⁷ fractional frequency level at 50 days.

## Abstract

A time scale is a procedure for accurately and continuously marking the passage of time. It is exemplified by Coordinated Universal Time (UTC), and provides the backbone for critical navigation tools such as the Global Positioning System (GPS). Present time scales employ microwave atomic clocks, whose attributes can be combined and averaged in a manner such that the composite is more stable, accurate, and reliable than the output of any individual clock. Over the past decade, clocks operating at optical frequencies have been introduced which are orders of magnitude more stable than any microwave clock. However, in spite of their great potential, these optical clocks cannot be operated continuously, which makes their use in a time scale problematic. In this paper, we report the development of a hybrid microwave-optical time scale, which only requires the optical clock to run intermittently while relying upon the ensemble of microwave clocks to serve as the flywheel oscillator. The benefit of using clock ensemble as the flywheel oscillator, instead of a single clock, can be understood by the Dick-effect limit. This time scale demonstrates for the first time sub-nanosecond accuracy for a few months, attaining a fractional frequency uncertainty of 1.45*10-16 at 30 days and reaching the 10-17 decade at 50 days, with respect to UTC. This time scale significantly improves the accuracy in timekeeping and could change the existing time-scale architectures.

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Source: https://tomesphere.com/paper/1902.06858