Photonic Generation and Free-Space Distribution of Millimeter Waves for Portable Optical Clocks

TL;DR

This paper presents a novel method for generating and distributing millimeter-wave signals using photonics, enabling portable optical clocks to achieve high-precision time transfer over free-space links, suitable for various advanced applications.

Contribution

It introduces an architecture for synthesizing 90 GHz millimeter waves with ultra-low residual instability and demonstrates a phase-stabilized free-space frequency comparison link over 110 meters.

Findings

Achieved 2x10^-15 residual instability at 1 second for millimeter-wave synthesis.

Demonstrated a 110 m free-space frequency comparison link with 10^-14 instability.

Provided a systematic analysis of uncertainties for future clock synchronization.

Abstract

Robust and portable optical clocks promise to bring sub-picosecond timing instability to smaller form factors, offering possible performance improvements and new scenarios for positioning and navigation, radar technologies, and experiments probing fundamental physics. However, there are currently limited methods suitable for broadly disseminating the sub-picosecond timing signals or performing frequency comparison of these clocks--particularly over open-air paths. Established microwave time transfer techniques only offer nanosecond level time synchronization, whereas optical techniques have challenging pointing requirements and lack the capability of all-weather operation. In this paper, we explore optically derived millimeter-wave carriers as a time-frequency link for full utilization of the next generation of portable optical clocks. We introduce an architecture that synthesizes 90 GHz millimeter waves with a one second residual instability of 2x10^-15, averaging into the 10^-17 range. In addition, we demonstrate a first-of-its-kind 110 m phase-stabilized free-space frequency comparison link over a millimeter-wave band with a one second instability in the 10^-14 region. Technical and systematic uncertainties are investigated and characterized, providing a foundation for future time and frequency transfer experiments among distributed portable optical clocks.