Synchronization of Distant Optical Clocks at the Femtosecond Level

TL;DR

This paper demonstrates the unambiguous synchronization of two optical clocks over a free-space link with femtosecond precision, overcoming atmospheric turbulence and path variations, enabling advanced optical network applications.

Contribution

It introduces a method for femtosecond-level synchronization of distant optical clocks using two-way reciprocity over free-space links, a significant advancement in precision timing.

Findings

Time deviation below 1 fs over 0.1 to 6500 seconds

Time wander of 40 fs peak-to-peak over several days

Reciprocal optical link valid to below 225 attoseconds

Abstract

The use of optical clocks/oscillators in future ultra-precise navigation, gravitational sensing, coherent arrays, and relativity experiments will require time comparison and synchronization over terrestrial or satellite free-space links. Here we demonstrate full unambiguous synchronization of two optical timescales across a free-space link. The time deviation between synchronized timescales is below 1 fs over durations from 0.1 s to 6500 s, despite atmospheric turbulence and kilometer-scale path length variations. Over several days, the time wander is 40 fs peak-to-peak. Our approach relies on the two-way reciprocity of a single-spatial-mode optical link, valid to below 225 attoseconds across a turbulent 4-km path. This femtosecond level of time-frequency transfer should enable optical networks using state-of-the-art optical clocks/oscillators.