Distributed cavity phase frequency shifts of the caesium fountain PTB-CSF2

TL;DR

This paper assesses the frequency errors and uncertainties in the PTB-CSF2 caesium fountain clock caused by distributed cavity phase and microwave lensing, combining measurements and calculations for improved accuracy.

Contribution

It provides a detailed evaluation of distributed cavity phase shifts and microwave lensing effects, with combined measurement and ab initio calculations to refine systematic uncertainty estimates.

Findings

Uncertainty from distributed cavity phase is 1.3E-16.

Frequency bias due to cavity phase is 0.4E-16.

Total systematic uncertainty of PTB-CSF2 is 4.1E-16.

Abstract

We evaluate the frequency error from distributed cavity phase in the caesium fountain clock PTB-CSF2 at the Physikalisch-Technische Bundesanstalt with a combination of frequency measurements and ab initio calculations. The associated uncertainty is 1.3E-16, with a frequency bias of 0.4E-16. The agreement between the measurements and calculations explains the previously observed frequency shifts at elevated microwave amplitude. We also evaluate the frequency bias and uncertainty due to the microwave lensing of the atomic wavepackets. We report a total PTB-CSF2 systematic uncertainty of 4.1E-16.