Development of NPL Rb fountain frequency standard

TL;DR

This paper characterizes the distributed cavity phase frequency shift in the NPL Rb fountain standard, demonstrating how to optimize the vertical angle to reduce uncertainty and improve frequency stability.

Contribution

It provides the first detailed analysis of the distributed cavity phase shift in the NPL Rb fountain and shows how to minimize its impact on frequency uncertainty.

Findings

Distributed cavity phase shift estimated at 1.25×10^-16

Uncertainty reduction lowers total type B uncertainty to 2.4×10^-16

Vertical angle adjustment effectively minimizes phase shift

Abstract

The first characterization of the distributed cavity phase frequency shift of the NPL Rb fountain frequency standard is reported. It is shown how the symmetric part of the distributed cavity phase shift can be used to set up the vertical angle of the fountain. The estimated uncertainty of the distributed cavity phase frequency shift of 1.25\times10^-16 reduces the total type B uncertainty of the Rb fountain down to 2.4\times10^-16.