Portable laser-cooled ytterbium beam clock based on an ultra-narrow optical transition

The highest performance atomic clocks are based on interrogation of ultra-narrow optical transitions. There is now significant interest in developing these systems as a source of GNSS-independent time in deployed, dynamic environments. We report on the development and field trial of a portable optical atomic clock interrogating the 10mHz wide $^1$S$_0\rightarrow ^3$P$_0$ transition in ytterbium-171. To enable measurement of this ultra-narrow transition in a deployed setting we combine an atom-vapor based pre-stabilization reference with all-digital control and continuous clock spectroscopy of a transversely-cooled thermal atomic beam. Characterization of the short-term frequency stability within the lab demonstrates a modified Allan deviation of $2\times 10^{-14}/\sqrt{\tau}$ for integration times up to 100s, reaching a best performance of $1.9\times 10^{-15}$ at 200s. The clock demonstrated the same performance after transport and install aboard a ship for field trial, and operated uninterrupted for multiple days whilst at sea. These results show a pathway towards truly portable optical frequency references based on the interrogation of ultra-narrow transitions.