Search

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.

Optical atomic clocks, due to their unprecedented stability1–3and uncertainty3–6, are already being used to test physical theories7,8and 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 locations12–14. 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. 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.

We implemented a compact, robust, and stable device for simultaneous frequency stabilization of lasers with different wavelengths used for the cooling and trapping of Yb atoms in an optical lattice clock. The lasers at 399, 556, and 759 nm are locked to a single ultra-stable cavity using the offset sideband locking technique, a modified version of the Pound–Drever–Hall method. For the most demanding stabilization here, the 556 nm laser, this system exhibits a 300 Hz linewidth for an integration time of 80 ms. We observed a long-term drift of less than 20 kHz per day at 759 nm that is suitable for operating the lattice laser with a light shift uncertainty below 1×10^−18. We successfully tested the system for operating the clock during a typical working day by simultaneously locking the three lasers to the cavity.