Optical clocks at sea

Deployed optical clocks will improve positioning for navigational autonomy1, provide remote time standards for geophysical monitoring2and distributed coherent sensing3, allow time synchronization of remote quantum networks4,5and 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 clocks8,9have 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.