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  • Featured in Physics
  • Open Access

Synchronization of Distant Optical Clocks at the Femtosecond Level

Jean-Daniel Deschênes*, Laura C. Sinclair, Fabrizio R. Giorgetta, William C. Swann, Esther Baumann, Hugo Bergeron, Michael Cermak, Ian Coddington, and Nathan R. Newbury†

  • National Institute of Standards and Technology, 325 Broadway, Boulder, Colorado 80305, USA

  • *jean-daniel.deschenes@gel.ulaval.ca
  • †nathan.newbury@nist.gov
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Phys. Rev. X 6, 021016 – Published 11 May, 2016

DOI: https://doi.org/10.1103/PhysRevX.6.021016

Abstract

The use of optical clocks or oscillators in future ultraprecise 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 time scales across a free-space link. The time deviation between synchronized time scales is below 1 fs over durations from 0.1 to 6500 s, despite atmospheric turbulence and kilometer-scale path length variations. Over 2 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 or oscillators.

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Physics Subject Headings (PhySH)

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Next Generation Clock Networks

Published 11 May, 2016

Free-space laser links have been used to synchronize optical clocks with an unprecedented uncertainty of femtoseconds.

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Popular Summary

Optical clocks have advanced dramatically over the last three decades. However, the scientific motivations for optical clocks, beyond the fascinating atomic physics, often rely on comparing times between distant clocks either on Earth or in space. In other words, the clocks are only one component in a much larger time network. Before researchers can embark on using “time networks” for sensing, relativistic tests, geodesy, navigation, etc., they must be able to connect distant optical time scales generated by optical clocks over long, intermittent, and varying optical free-space links. Here, we demonstrate that distant optical time scales can be both compared and synchronized with femtosecond-level precision and accuracy.

Our two clocks are connected only by a free-space link across 4 km of turbulent air yet maintain the same time to within ±20 fs over 50 h. The optical clock is based on a stabilized frequency comb laser and optical cavity, but future systems could use a full atomic optical clock to provide the overall absolute time base for a clock network. Our approach extends conventional radio-frequency two-way time transfer to the optical domain and achieves a 1000-times improved performance.

We expect that our findings will pave the way for high-precision applications of optical clock technology, such as the next generation of atomic time scales and navigation systems, clock-based geodesy, tests of general relativity, and coherent sensing across distributed platforms.

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Phys. Rev. X 6, 021016– Published 11 May, 2016
  • Received 11 December 2015
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Published by the American Physical Society under the terms of the Creative Commons Attribution 3.0 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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