Abstract: The optical frequency comb has enabled a wide range of frequency, time, and distance metrology applications due to its precise, rigid, and referenced optical output. However, the very rigidity that makes the frequency comb an excellent time-frequency ruler limits its applicability to comb-based sensing, with many operating far from quantum-limited sensitivity. For instance, frequency-comb-based two-way time transfer relies on the detection of an incoming optical comb pulse train from a distant location. These incoming comb pulse trains may be weak, and amplification is costly — operation close to the quantum limit would dramatically increase the scope of what is possible in terms of range, SWaP (size, weight and power) and link loss. In turn, that increased scope would enable comparison of state-of-the-art optical clocks for the future redefinition of the second, a wide range of fundamental physics tests, and chronometric geodesy.

Here, I'll present our development of a quantum-limited approach to optical time transfer that relies on a time-programmable frequency comb to overcome the inherent trade-offs that arise from the rigid operation of a traditional comb. These programmable combs have pulse time and phase that can be digitally controlled with ±2-attosecond accuracy, enabling their use as an optical tracking oscillator. Using frequency combs as optical tracking oscillators to reach the quantum limit for optical time transfer, we have demonstrated sub-femtosecond time transfer across a 300-km terrestrial free-space link with more than 100 dB of loss, a factor of 10,000 lower received-power threshold than previous frequency-comb-based approaches. I'll show results from this 300-km demonstration as well as more recent work connecting optical atomic clocks across open-air paths.

About the Speaker: Laura Sinclair is the optical time transfer project lead in the Fiber Sources and Applications Group, which is part of the Communications Technology Laboratory at the National Institute of Standards and Technology (NIST) in Boulder, Colorado. She received a bachelor's degree in physics from the California Institute of Technology in 2004, a doctorate in physics from the University of Colorado Boulder in 2011, and was a post-doc at NIST Boulder, including a National Research Council post-doctoral fellowship, before joining the staff. She's was awarded a Presidential Early Career Award for Scientists and Engineers (2019), a Department of Commerce Gold Medal for Scientific/Engineering Achievement as part of the Boulder Atomic Clock Optical Network Collaboration (2019), a NIST Excellence in Technology Transfer Award (2024), the Arthur S. Flemming Award for Basic Science (2024) and an Optica Fellow Award (2026). Her research focuses on the development of optical frequency combs and their wide-ranging applications, particularly to optical time transfer and ranging. With the optical time transfer project team, she has recently demonstrated optical time transfer at the quantum limit, achieving sub-femtosecond time synchronization over 300 kilometers of air.