Transportable optical atomic clock for geodesy at the centimeter height level

Transportable high-performance optical atomic clocks are currently being developed worldwide. Apart from applications in metrology and fundamental physics studies, due to the relativistic gravitational redshift of clock frequencies they serve as quantum sensors of the local gravity potential. If they are connected to distant atomic clocks via high-performance frequency comparison links, the frequency difference between the clocks measured remotely enables a direct determination of gravity potential differences with high spatial and temporal resolution. This method is called chronometric leveling [1].

We have been operating a transportable strontium lattice clock for ten years and have used it for chronometric leveling. Here we describe our second-generation transportable atomic clock [2], which has been operational since 2023 and has demonstrated centimeter height resolution in the field. Taking all frequency shifts into account, its total systematic uncertainty is evaluated to be 2.1×10^-18, which via the gravitational redshift corresponds to a height uncertainty of 1.9 cm. This is close to the currently lowest achieved physical height uncertainties in geodesy, determined by the mostly satellite-based GNSS/geoid approach [3]. The atomic clock is installed in an air-conditioned car trailer and has been successfully operated after transportation several times.

We briefly review recent off-campus measurement campaigns in England [4], southern Germany and Italy, and we lay out plans for future measurements aiming to demonstrate how a next-generation height system with cm accuracy can be established and controlled in practice. These measurements are expected to benefit from the extended European core time-frequency network (C-TFN), which forms a tripod in Germany with PTB at the hub, thus enabling, for example, the detection of 2D tilt in the current height reference system.

References:

[1] T. E. Mehlstäubler et al., Rep. Prog. Phys. 81(6), 064401, 2018.

[2] I. Nosske et al., Quant. Sci. Technol. 10(4), 045076, 2025.

[3] H. Denker et al., J. Geod. 92(5), 487-516, 2018.

[4] International Clock and Oscillator Networking Collaboration, arXiv:2410.22973, 2024.