# SI-traceable measurement of an optical frequency at low $10^{-16}$ level without a local primary standard

## TL;DR

This paper demonstrates a method for SI-traceable optical frequency measurement at the low 10^{-16} uncertainty level without a local primary standard, using TAI and a combined hydrogen maser oscillator.

## Contribution

It introduces a novel approach combining TAI-based measurements and a dual hydrogen maser oscillator to achieve high-accuracy optical frequency measurement without a local primary standard.

## Key findings

- Achieved fractional uncertainty of 4.3×10^{-16} in optical frequency measurement.
- Reduced uncertainty in TAI-based measurements to low 10^{-16} level.
- Validated results with other state-of-the-art cesium fountain measurements.

## Abstract

SI-traceable measurements of optical frequencies using International Atomic Time (TAI) do not require a local primary frequency reference, but suffer from an uncertainty in tracing to the SI second. For the measurement of the $^{87}$Sr lattice clock transition, we have reduced this uncertainty to low $10^{-16}$ level by averaging three sets of ten-day intermittent measurements, in which we operated the lattice clock for $10^4$ s on each day. Moreover, a combined oscillator of two hydrogen masers was employed as a local flywheel oscillator (LFO) in order to mitigate the impact of sporadic excursion of LFO frequency. The resultant absolute frequency with fractional uncertainty of $4.3\times10^{-16}$ agrees with other measurements based on local state-of-the-art cesium fountains.

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Source: https://tomesphere.com/paper/1704.00893