Mobile Ytterbium Optical Clock Applied to Geodesic Exploration – ROYMAGE

In this project, we propose to build a transportable optical clock based on ytterbium atoms and to explore applications of this device to geodesy and geodynamics. The frequency of an atomic clock being sensitive to the geopotential caused by mass distribution , measuring the frequency shift between two clocks can be interpreted in terms of potential difference, or height difference. The perspective of controlling the clock frequency at the level of 18 significant digits is now a reality, which opens the possibility of measuring height differences at the cm level, or equivalent geopotential variations at 0.1 m2/s2 . In parallel, the deployment of optical fiber networks in charge of disseminating an ultrastable reference at 1542 nm is ongoing across Europe, and particularly in France where it takes the form of the Equipex REFIMEVE+. In the future, the transportable clock will enable the resolution of height changes at the 1 cm level between a reference point and any access point to this European networks, even for distances of several thousands of kilometers. Such a measurement is presently unreachable for any instrument, ground-based or in orbit around the Earth.

These unprecedented measurements will lead to disruptive applications in operational geodesy and in Earth Sciences. In operational geodesy, accurate and high-resolution measurements of height differences over long distances will enable the correction of biases specific to traditional leveling methods. It will also build homogeneous height references at continental scales. Moreover, a better knowledge of geopotential differences will considerably improve the mapping of the equipotential surfaces of the gravity potential , particularly the reference corresponding to the mean sea level, called the geoid. This will have an impact on the determination of marine references, and on studies of coastal currents. Additionally, considering the original spectrum (range of several 100 km) of clock-based measurements compared to usual gravimetric surveys, the monitoring of geopotential variations in a given location will give access to underground deep mass transfers due to many phenomena (tectonic deformation, volcanism, seismic cycle, or change of the mean sea level …). These aspects can possibly increase public awareness of natural hazards and draw the attention of top decision makers.

Countless technological and conceptual challenges must be tackled to transfer a device as precise as an atomic clock from a well-controlled lab environment to outdoor uncontrolled conditions. Several approaches are presented in the ROYMAGE project, notably to preserve the stability and the low uncertainty, to reduce electrical consumption, and to reference all the instruments attached to the clock only to the ultrastable 1542 nm carrier provided by the European fiber network. To this end, we propose innovative techniques of seismometers-assisted vibration compensation, of dual Ytterbium clouds to minimize deadtimes, or of “bootstrapping” of an optical frequency comb permanently attached to the device. The clock will be assembled at SYRTE (Observatoire de Paris), and prior to transport at nodes of the European fiber link, metrological performances will be assessed by comparison to the 6 atomic clocks (strontium, mercury, cesium) already operational in the laboratory.

To conclude, the project aims at building the core of the clock, demonstrate the feasibility of the instrument, and evaluate its possible impact for targeted applications. The consortium submitting the proposal gathers specialists of quantum technologies, of geodesy and geophysics, and operational experts in terrestrial and marine geodetic references..