Optical atomic clocks, due to their unprecedented stability1,2,3 and uncertainty3,4,5,6, are already being used to test physical theories7,8 and herald a revision of the International System of Units9,10. However, to unlock their potential for cross-disciplinary applications such as relativistic geodesy11, a major challenge remains: their transformation from highly specialized instruments restricted to national metrology laboratories into flexible devices deployable in different locations12,13,14. Here, we report the first field measurement campaign with a transportable 87Sr optical lattice clock12. We use it to determine the gravity potential difference between the middle of a mountain and a location 90 km away, exploiting both local and remote clock comparisons to eliminate potential clock errors. A local comparison with a 171Yb lattice clock15 also serves as an important check on the international consistency of independently developed optical clocks. This campaign demonstrates the exciting prospects for transportable optical clocks.

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We would like to thank T. Zampieri for his technical support at LSM and A. Mura and Consorzio TOP-IX for technical help in the access to the optical fibre. The authors acknowledge funding from European Metrology Research Program (EMRP) Project SIB55 ITOC, the EU Innovative Training Network (ITN) Future Atomic Clock Technology (FACT), the DFG funded CRC 1128 geo-Q (Projects A03 and C04) and RTG 1728 and the UK National Measurement System Quantum, Electromagnetics and Time Programme. The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union.

Author information


  1. Physikalisch-Technische Bundesanstalt, Braunschweig, Germany

    • Jacopo Grotti
    • , Silvio Koller
    • , Stefan Vogt
    • , Sebastian Häfner
    • , Uwe Sterr
    •  & Christian Lisdat
  2. Institut für Erdmessung, Leibniz Universität Hannover, Hannover, Germany

    • Heiner Denker
    • , Christian Voigt
    •  & Ludger Timmen
  3. GFZ German Research Centre for Geosciences, Potsdam, Germany

    • Christian Voigt
  4. National Physical Laboratory, Teddington, Middlesex, UK

    • Antoine Rolland
    • , Fred N. Baynes
    •  & Helen S. Margolis
  5. Laboratoire Souterrain de Modane, Carré Sciences, Modane, France

    • Michel Zampaolo
  6. Istituto Nazionale di Ricerca Metrologica (INRIM), Physical Metrology Division, Torino, Italy

    • Pierre Thoumany
    • , Marco Pizzocaro
    • , Benjamin Rauf
    • , Filippo Bregolin
    • , Anna Tampellini
    • , Piero Barbieri
    • , Massimo Zucco
    • , Giovanni A. Costanzo
    • , Cecilia Clivati
    • , Filippo Levi
    •  & Davide Calonico
  7. Dipartimento di Elettronica e Telecomunicazioni, Politecnico di Torino, Torino, Italy

    • Benjamin Rauf
    • , Filippo Bregolin
    • , Anna Tampellini
    • , Piero Barbieri
    •  & Giovanni A. Costanzo


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D.C. coordinated the measurement campaign with contributions from C.L., H.S.M. and Mi.Z.; J.G., S.K., S.V., S.H., U.S. and C.L. designed, built and operated the transportable Sr lattice clock; H.D., C.V. and L.T. made the geodetic measurements and calculated the local gravity potential values; A.R., F.N.B. and H.S.M. prepared, characterized and operated the transportable frequency comb; M.P., P.T., B.R., F.B., and D.C. designed, built and operated the Yb lattice clock; G.A.C. and F.L. designed, built and operated the INRIM Cs fountain, C.C. and A.T. designed, characterized and operated the optical fibre link between INRIM and LSM; C.C., P.B. and Ma.Z. operated the frequency comb at INRIM. J.G., C.C., M.P., F.L., A.R., F.N.B., H.S.M., S.K. and C.L. contributed to the data analysis for the ratio and absolute frequency measurement. C.L. wrote the paper with support from H.S.M. and D.C. All authors discussed the results and commented on the paper.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Christian Lisdat.