Reaching the quantum limit in a fountain clock using a microwave oscillator phase locked to an ultrastable laser

A cesium 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 in the frequency instability of the fountain clock is obtained, reaching $7.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}{[\ensuremath{\tau}\text{ }(\text{s})]}^{\ensuremath{-}1/2}$. 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 microwave source poses no limit on the achievable frequency instability.