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Optical two-tone time transfer

Jonathan D. Roslund*, Abijith S. Kowligy, Junichiro Fujita, Micah P. Ledbetter, Akash V. Rakholia, Martin M. Boyd, Jamil R. Abo-Shaeer, and Arman Cingöz†

  • Vector Atomic, Inc., Pleasanton, California, USA

  • *Contact author: jon@vectoratomic.com
  • †Contact author: arman@vectoratomic.com
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Phys. Rev. Applied 24, 014001 – Published 1 July, 2025

DOI: https://doi.org/10.1103/9w7h-zzyd

Abstract

Subpicosecond time synchronization can enable future optical timekeeping networks [F. Riehle, Nat. Photonics 11 (1), 25 (2017)], including coherent phased array radar imaging at gigahertz levels [Collaboration, Event Horizon Telescope, Astrophys. J. Lett. 875 (2019)], intercontinental clock comparisons for the redefinition of the second [Dimarcq et al., Metrologia 61 (1), 012001 (2024)], chronometric leveling [Takamoto et al., Nat. Photonics 14 (7), 411 (2020)], and synchronization of remote assets, including future satellite-based optical time standards. With optical clocks now operating on mobile platforms [Roslund et al., Nature 628 (8009), 736 (2024)], free-space synchronization networks with compatible performance and the ability to operate under platform motion are essential to expand the reach of precision timing. Recently, femtosecond-level optical time-transfer techniques have been developed that can operate over hundreds of kilometers despite atmospheric turbulence, signal fade, and dropouts [Giorgetta et al., Nat. Photonics 7, 434 (2013); Caldwell et al., Nature 618 (7966), 721 (2023); Shen et al., Nature 610 (7933), 661 (2022)]. However, practical challenges remain for these pulsed techniques on moving platforms, including dispersion management for variable link lengths and Doppler-induced pulse ambiguities. Continuous-wave techniques overcome these limitations but are prone to femtosecond-level cycle slips during signal fades and dropouts [Gozzard et al., Phys. Rev. Lett. 128, 020801 (2022)]. Here we report an optical time-transfer scheme with femtosecond-level performance that overcomes the practical limitations of dispersion and dropout-induced cycle slips and can also support moving platforms. Importantly, the setup reduces hardware requirements by employing an integrated photonics transceiver and telecom-band lasers that are compatible with full photonic integration. Using this scheme, subfemtosecond synchronization was demonstrated over a free-space link of about 100 m for several hours. Similar performance was demonstrated over a 1-km fiber non-polarization-maintaining link without dispersion compensation. Finally, the link was used to syntonize two iodine optical clocks and then compare them over 4 days. Importantly, the system maintains femtosecond-level performance for received optical power as low as 1 picowatt, compatible with ground-to-satellite link-loss budgets.

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Phys. Rev. Applied 24, 014001– Published 1 July, 2025
  • Received 27 February 2025
  • Accepted 21 May 2025
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

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