An Yb transportable clock connected to the REFIMEVE fiber network for chronometric geodesy

As a direct consequence of the gravitational time dilation predicted by Einstein, atomic clock frequencies depend on the local gravitational potential. Several groups in the world have developed the capacity to control the frequency of a new generation of optical clocks at the 18 digits level, which corresponds to the ability to detect 1 cm height changes. Therefore, it raises the perspective of contributing to a refined definition of the geoid by rivaling traditional geodesy techniques, based either on satellites or on spirit leveling.

 

Transportable optical clocks have drawn considerable interest in the last years [1-3], as they are the only ground-based instruments able to perform a mapping of geopotential variations [4]. In this context, SYRTE has started the development of a new optical lattice clock based on neutral Ytterbium, on top of 3 stationary optical clocks (neutral strontium or mercury) already operational in the lab. It is designed to be transportable and aims at an uncertainty in the 10^-18 range. This new optical frequency standard will exploit the research infrastructure REFIMEVE, a metrological fiber network disseminating throughout the French territory a 1542 nm ultrastable frequency reference [5]. The ~60 outputs spread along the link will allow us to remotely compare it to the ~12 stationary European optical clocks that are already connected to the network.

 

We will present a description of the clock design, stressing the technological and conceptual choices that we did in prevision for the field conditions outside of a well-controlled lab. We will notably discuss the strategy we follow to reduce the deadtime in order to adapt to the reduced stability of the clock. It allows us to adapt to the spectral degradation of the narrow laser probing the metrological transition due to field conditions (vibrations, temperature gradients …). Thanks to the operational capacity of the REFIMEVE infrastructure to deliver not only a stable but also accurate optical signal, we will have the possibility to generate locally, with a transportable optical frequency comb, an accurate RF signal able to reference all the instruments attached to the transportable clock. We will therefore conclude by showing how the device under development can be operated exclusively by optical referencing to the REFIMEVE signal.

This work has received support from: Agence Nationale de la Recherche (ANR) with project ROYMAGE (ANR-20-CE47-0006), DIM SIRTEQ, and Labex First-TF with project PATHYNAGE.

[1] M. Takamoto et al., "Test of general relativity by a pair of transportable optical lattice clocks", Nature Photonics 14.7, 411-415 (2020)

[2] J. Grotti et al., "Geodesy and metrology with a transportable optical clock", Nature Physics 14.5, 437-441 (2018)

[3] J. Cao et al., "A compact, transportable single-ion optical clock with 7.8×10⁻¹⁷ systematic uncertainty", Applied Physics B 123.4, 1-9 (2017)

[4] G.Lion, I.Panet, P.Wolf, C.Guerlin, S.Bize and P.Delva, "Determination of a high spatial resolution geopotential model using atomic clock comparisons", Journal of Geodesy, 91(6), 597-611 (2017)

[5] E. Cantin, M. Tønnes, R. Le Targat, A. Amy-Klein, O. Lopez and P.-E. Pottie, “An accurate and robust metrological network for coherent optical frequency dissemination”, New Journal of Physics, vol. 23, p. 053027 (2021)