Geodesy and metrology with a transportable optical clock

Optical atomic clocks, due to their unprecedented stability(1-3) and uncertainty(3-6), are already being used to test physical theories(7,8) and herald a revision of the International System of Units(9,10). 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 locations(12-14). Here, we report the first field measurement campaign with a transportable Sr-87 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 Yb-171 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.