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Intercontinental comparison of optical atomic clocks through very long baseline interferometry

Nature Physics volume 17, pages 223–227 (2021)Cite this article

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Abstract

The comparison of distant atomic clocks is foundational to international timekeeping, global positioning and tests of fundamental physics. Optical-fibre links allow the most precise optical clocks to be compared, without degradation, over intracontinental distances up to thousands of kilometres, but intercontinental comparisons remain limited by the performance of satellite transfer techniques. Here we show that very long baseline interferometry (VLBI), although originally developed for radio astronomy and geodesy, can overcome this limit and compare remote clocks through the observation of extragalactic radio sources. We developed dedicated transportable VLBI stations that use broadband detection and demonstrate the comparison of two optical clocks located in Italy and Japan separated by 9,000 km. This system demonstrates performance beyond satellite techniques and can pave the way for future long-term stable international clock comparisons.

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Fig. 1: Schematic of the frequency links between the optical clocks.
Fig. 2: Comparison of Yb/Sr frequency ratio measurements.

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Data availability

Data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request. Source data are provided with this paper.

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Acknowledgements

Development of the broadband feed system was supported by K. Fujisawa of Yamauchi University. We acknowledge S. Hasegawa and Y. Miyauchi of NICT/Kashima Space Technology Center, F. Nakagawa and H. Ishijima of NICT headquarters and T. Ikeda of KDDI Research Laboratory for their contribution to this work. We thank A. Tabellini of IRA/INAF, A. Dondi and M. Alberghini of the Bologna Customs Agency for the management of the temporary importation of the transportable VLBI antenna from Japan to Italy. The broadband feeds were realized through a grant of Joint Development Research on ‘Development of Broadband Receiver System of Kashima 34m’ (2013–2015) supported by the Research Coordination Committee, the National Astronomical Observatory of Japan (NAOJ), the National Institutes of Natural Sciences (NINS). The development of the ytterbium optical clock and the optical link was supported by the European Metrology Program for Innovation and Research (EMPIR) projects 15SIB03 OC18, 15SIB05 OFTEN, 17IND14 WRITE and 18SIB05 ROCIT, by the Italian Ministry of Education, University and Research (LIFT and Metgesp projects) and by the Horizon 2020 Marie Skłodowska-Curie Research and Innovation Staff Exchange (MSCA-RISE) project Q-SENSE (grant agreement no. 691156). The EMPIR initiative is co-funded by the European Union’s Horizon 2020 research and innovation programme and the EMPIR Participating States. J.L. was supported by the French National Research Agency under the LABEX Cluster of Excellence FIRST-TF (ANR-10-LABX-48-01). Fast data transfer from Italy to Japan for quick turnaround of the experiments was realized with the support of the high-speed research networks JGN, Internet2, TransPAC, APAN, GEANT and GARR. IPPP results make use of the GINS software developed by the French Space Agency, CNES.

Author information

Author notes

  1. Kazuhiro Takefuji

    Present address: Japan Aerospace Exploration Agency (JAXA), Nagano, Japan

  2. Filippo Bregolin

    Present address: Toptica Photonics AG, Gräfelfing, Germany

Authors and Affiliations

  1. Istituto Nazionale di Ricerca Metrologica (INRIM), Torino, Italy

    Marco Pizzocaro, Cecilia Clivati, Filippo Bregolin, Piero Barbieri, Alberto Mura, Elena Cantoni, Giancarlo Cerretto, Filippo Levi, Roberto Ricci & Davide Calonico

  2. National Institute of Information and Communications Technology (NICT), Kashima, Japan

    Mamoru Sekido, Kazuhiro Takefuji, Hideki Ujihara, Tetsuro Kondo & Eiji Kawai

  3. National Institute of Information and Communications Technology (NICT), Koganei, Tokyo, Japan

    Hidekazu Hachisu, Nils Nemitz, Masanori Tsutsumi, Ryuichi Ichikawa, Kunitaka Namba, Yoshihiro Okamoto, Rumi Takahashi, Junichi Komuro & Tetsuya Ido

  4. Chinese Academy of Sciences, Shanghai Astronomical Observatory, Shanghai, China

    Tetsuro Kondo

  5. Istituto Nazionale di Astrofisica (INAF), Istituto di Radioastronomia (IRA), Bologna, Italy

    Giuseppe Maccaferri, Mauro Roma, Claudio Bortolotti, Monia Negusini, Roberto Ricci, Giampaolo Zacchiroli, Juri Roda & Federico Perini

  6. Bureau International des Poids et Mesures (BIPM), Sèvres, France

    Julia Leute & Gérard Petit

  7. LNE-SYRTE, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Paris, France

    Julia Leute

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  1. Marco Pizzocaro
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Contributions

D.C., T.I. and M.S. designed, prepared and coordinated the experiment with contributions from F.P. and F.L. M.S., K.T., H.U., M.T., J.K. and R.I. designed and built the broadband VLBI system. M.S., K.T., H.U., M.T., E.K., K.N., Y.O. and R.T. set up and operated the VLBI stations in Koganei and Kashima. M.S., K.T., M.T., H.U., K.N., Y.O. and R.T. set up and operated the VLBI station in Medicina with support from G.M., M.R., C.B., G.Z., J.R. and F.P. M.P., F.B. and P.B. evaluated and operated the ytterbium clock and the comb in Torino. C.C. and A.M. evaluated and operated the optical-fibre link and the combs in Torino and Medicina with support from M.R. and C.B. H.H., N.N. and T.I. evaluated and operated the strontium clock and the comb in Koganei. M.S., K.T. and T.K. carried out the correlation analysis of the VLBI delay data. M.S., M.N. and R.R. carried out the geodetic VLBI analysis. E.C., G.C., R.I., J.L. and G.P. carried out the data analysis of the satellite links. M.P. and M.S. performed the data analysis for frequency comparisons, with contributions from N.N., C.C., J.L. and G.P. M.P. wrote the manuscript with support from M.S., N.N. and C.C. All authors discussed the results and commented on the manuscript.

Corresponding author

Correspondence to Mamoru Sekido.

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Competing interests

The authors declare no competing interests.

Additional information

Peer review information Nature Physics thanks Jacob Covey, Rüdiger Haas and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data

Extended Data Fig. 1 Delay resolution function of GALA-V and standard geodetic VLBI.

a, Delay resolution functions for the frequency array of GALA-V described in the text and for standard geodetic VLBI (IVS-T2 session, experiment code T2126). GALA-V delay resolution function shows a single peak and fine delay resolution that allows the derivation of the absolute group delay without ambiguity. b, The delay resolution function formed from the frequency array of standard geodetic VLBI shows a delay ambiguity with a period equal to the reciprocal of the greatest common denominator of the frequency interval.

Source data

Extended Data Fig. 2 Node-hub style observations with transportable broadband VLBI stations.

a, Transportable small VLBI stations A and B can be used for intercontinental baseline by using a Node-hub style VLBI scheme with the station R. Delay data of AB baseline is computed by linear combination of delay data on RA and RB baselines. b, Block diagram of the Node-hub measurement with the broadband VLBI system.

Extended Data Fig. 3 Extrapolations over dead times using the masers as a flywheel.

a, Uptimes of IT-Yb1 (yellow), optical link (green), VLBI link (blue) and NICT-Sr1 (red) marked as colored regions as a function of MJD. Gray horizontal bars represent evaluation intervals corresponding to each VLBI session, as marked by a vertical notch. For each evaluation interval the extrapolation was calculated for the masers at INRIM (between IT-Yb1 and the optical link), INAF (between the optical link and VLBI), and NICT (between VLBI and NICT-Sr1). b, Noise models used for extrapolations for the masers at INRIM, INAF, and NICT, describing the Hadamard variance σH2 = h −2(τ/s)−2 + h−1(τ/s)−1 + h0 + h2(τ/s)2.

Extended Data Fig. 4 Yb/Sr frequency ratio measurements using IPPP or PPP.

a, Plot of the frequency ratio measurements. Green points represent the IPPP comparison with its total uncertainty. Green shaded region is the weighted mean of the IPPP comparison with its total uncertainty. Purple points represent the PPP comparison with its total uncertainty. Purple shaded region is the weighted mean of the PPP comparison with its total uncertainty. Horizontal offset between the two dataset is just for clarity. b, Uncertainty budget for the satellite comparisons. All uncertainties correspond to one standard deviation.

Source data

Source data

Source Data Fig. 2

Source data for the frequency comparison of Yb and Sr clocks using VLBI and IPPP (csv file).

Source Data Extended Data Fig. 1

Source data for delay resolution function of GALA-V and standard geodetic VLBI (csv file).

Source Data Extended Data Fig. 4

Source data for the frequency comparison of Yb and Sr clocks using IPPP and PPP (csv file).

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Pizzocaro, M., Sekido, M., Takefuji, K. et al. Intercontinental comparison of optical atomic clocks through very long baseline interferometry. Nat. Phys. 17, 223–227 (2021). https://doi.org/10.1038/s41567-020-01038-6

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