Interrogation of caesium atoms in a fountain clock by a femtosecond laser microwave oscillator

TL;DR

This paper demonstrates a caesium fountain clock that uses a laser-based microwave oscillator derived from a femtosecond frequency comb, significantly improving stability and approaching quantum projection noise limits.

Contribution

It introduces a laser-derived microwave oscillator for fountain clocks, replacing traditional quartz oscillators, and shows improved stability close to quantum noise limits.

Findings

Frequency instability reached 0.74E-14 at 100 s

Quantum projection noise limits the instability over a range of atom numbers

The new microwave source does not limit the clock's frequency stability

Abstract

A caesium fountain clock is operated utilizing a microwave oscillator that derives its frequency stability from a stable laser by means of a fiber-laser femtosecond frequency comb. This oscillator is based on the technology developed for optical clocks and replaces the quartz based microwave oscillator commonly used in fountain clocks. As a result, a significant decrease of the frequency instability of the fountain clock is obtained, reaching 0.74E-14 at 100 s averaging time. We could demonstrate that for a significant range of detected atom numbers the instability is limited by quantum projection noise only, and that for the current status of this fountain clock the new microwave source poses no limit on the achievable frequency instability.