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

The highest performance atomic clocks are based on the 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 10 mHz wide 1 S 0 → 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 2×10 −14 / τ for integration times up to 100 s, reaching a best performance of 1.9×10 −15 at 200 s. The clock demonstrated the same performance after transport and installation aboard a ship for field trial and operated uninterrupted for multiple days whilst at sea. These results show a pathway toward truly portable optical frequency references based on the interrogation of ultra-narrow transitions.