Optical clocks at sea

Deployed optical clocks will improve positioning for navigational autonomy¹, provide remote time standards for geophysical monitoring² and distributed coherent sensing³, allow time synchronization of remote quantum networks^4,5 and provide operational redundancy for national time standards. Although laboratory optical clocks now reach fractional inaccuracies below 10^-18 (refs. ^6,7), transportable versions of these high-performing clocks^8,9 have limited utility because of their size, environmental sensitivity and cost¹⁰. Here we report the development of optical clocks with the requisite combination of size, performance and environmental insensitivity for operation on mobile platforms. The 35 l clock combines a molecular iodine spectrometer, fibre frequency comb and control electronics. Three of these clocks operated continuously aboard a naval ship in the Pacific Ocean for 20 days while accruing timing errors below 300 ps per day. The clocks have comparable performance to active hydrogen masers in one-tenth the volume. Operating high-performance clocks at sea has been historically challenging and continues to be critical for navigation. This demonstration marks a significant technological advancement that heralds the arrival of future optical timekeeping networks.