Geodesy and metrology with a transportable optical clock

Optical atomic clocks, due to their unprecedented stability1–3 and uncertainty3–6, 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 geodesy 11 , a major challenge remains: their transformation from highly specialized instruments restricted to national metrology laboratories into flexible devices deployable in different locations12–14. Here, we report the first field measurement campaign with a transportable 87Sr optical lattice clock 12 . 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 clock 15 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. An atomic clock has been deployed on a field measurement campaign to determine the height of a mountain location 1,000 m above sea level, returning a value that is in good agreement with state-of-the-art geodesy.