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Geopotential measurements with synchronously linked optical lattice clocks

Nature Photonics volume 10, pages 662–666 (2016)Cite this article

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Abstract

According to Einstein's theory of relativity, the passage of time changes in a gravitational field1,2. On Earth, raising a clock by 1 cm increases its apparent tick rate by 1.1 parts in 1018, allowing chronometric levelling3 through comparison of optical clocks1,4,5. Here, we demonstrate such geopotential measurements by determining the height difference of master and slave clocks separated by 15 km with an uncertainty of 5 cm. A subharmonic of the master clock laser is delivered through a telecom fibre6 to synchronously operate7 the distant clocks. Clocks operated under such phase coherence reject clock laser noise and facilitate proposals for linking clocks8,9 and interferometers10. Taken over half a year, 11 measurements determine the fractional frequency difference between the two clocks to be 1,652.9(5.9) × 10−18, consistent with an independent measurement by levelling and gravimetry11. Our system demonstrates a building block for an internet of clocks, which may constitute ‘quantum benchmarks’, serving as height references with dynamic responses.

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Figure 1: Schematic of the experimental set-up.
Figure 2: Excitation probability and instability of the clock comparison.
Figure 3: Frequency differences between clocks.
Figure 4: Calculated tidal perturbation on the geopotential for 15 days, from 1 September 2015.

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Acknowledgements

This work is partially supported by the Japan Society for the Promotion of Science through its Funding Program for World-Leading Innovative R&D on Science and Technology Program and by the Photon Frontier Network Program of the Ministry of Education, Culture, Sports, Science and Technology, Japan. The authors thank M. Das for the development of cryogenic clocks, N. Nemitz and K. Gibble for careful reading of the manuscript and for useful comments, and M. Musha for the loan of a phase-locked loop circuit for the clock laser.

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Author notes

  1. Yuki Kuroishi

    Present address: Present address: The Earthquake Research Institute, The University of Tokyo, Bunkyo-ku, Tokyo 113-0032, Japan,

Authors and Affiliations

  1. Department of Applied Physics, Graduate School of Engineering, The University of Tokyo, Bunkyo-ku, 113-8656, Tokyo, Japan

    Tetsushi Takano, Noriaki Ohmae & Hidetoshi Katori

  2. Innovative Space-Time Project, ERATO, Japan Science and Technology Agency, Bunkyo-ku, 113-8656, Tokyo, Japan

    Tetsushi Takano, Masao Takamoto, Ichiro Ushijima, Noriaki Ohmae, Tomoya Akatsuka, Atsushi Yamaguchi & Hidetoshi Katori

  3. Quantum Metrology Laboratory, RIKEN, Wako-shi, 351-0198, Saitama, Japan

    Masao Takamoto, Ichiro Ushijima, Noriaki Ohmae, Tomoya Akatsuka, Atsushi Yamaguchi & Hidetoshi Katori

  4. RIKEN Center for Advanced Photonics, Wako-shi, 351-0198, Saitama, Japan

    Masao Takamoto, Ichiro Ushijima, Tomoya Akatsuka, Atsushi Yamaguchi & Hidetoshi Katori

  5. Geospatial Information Authority of Japan, Tsukuba-shi, 305-0811, Ibaraki, Japan

    Yuki Kuroishi, Hiroshi Munekane & Basara Miyahara

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Contributions

T.T., M.T. and I.U. operated the clocks at UTokyo and RIKEN and analysed data. T.A. and A.Y. were responsible for the fibre link and M.T. and N.O. for the clock lasers. Y.K. evaluated the tidal perturbation. B.M. supervised the spirit-levelling measurements. Y.K., B.M., H.M. and H.K. discussed the geodetic applications of a quantum benchmark. T.T., Y.K. and H.K. wrote the manuscript. H.K. planned and supervised the experiments. All authors discussed the results and commented on the manuscript.

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Correspondence to Hidetoshi Katori.

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Takano, T., Takamoto, M., Ushijima, I. et al. Geopotential measurements with synchronously linked optical lattice clocks. Nature Photon 10, 662–666 (2016). https://doi.org/10.1038/nphoton.2016.159

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