Since 1975, the Simone and Cino Del Duca Foundation has been awarding grants to promote science, arts, and literature in France and abroad. Since 2005, it has been hosted by the Institut de France.
This year, the jury, made up of members of the Academy of Sciences, awarded Rodolphe Le Targat (Laboratoire temps espace, Optical Frequencies team) the Del Duca 2025 scientific grant in the category “Sciences of the Universe and their applications.” This award recognizes his work on transportable optical clocks (ROYMAGE project), which are capable of mapping space-time to contribute to chronometric geodesy.
Ytterbium team from the Time-Space Laboratory, in front of the ROYMAGE portable clock
Since the 1970s, atomic clocks based on the frequency of the cesium microwave transition have been the guardians of international atomic time accuracy. This reference frequency, which is virtually unchanging due to the very nature of the energy structure of atoms, is controlled to a level of 16 significant digits by the world’s best cesium clocks (microwave fountains).
At the turn of the 21st century, a new field of research emerged : a few pioneering groups around the world, including LTE, began developing clocks based on ultra-deep light traps (optical lattices) to cool previously unexploited gases (Sr, Hg, Yb, etc.) and thus explore transitions in the optical domain (frequency 100,000 times higher). Progress has been so rapid that it is now possible to control this reference frequency to 18 significant digits, with unrivalled statistical resolution since thousands of these neutral atoms can be interrogated at the same time. The excitement in the time-frequency community, driven by parallel progress made by a dozen laboratories around the world, is such that a new definition of the second will be introduced in 2030 or 2034. The LTE is playing a leading role in preparing for this upcoming “leap” : it has been tasked by the LNE(Laboratoire National de Métrologie et d’Essais) with operating France’s high-precision atomic clocks – currently numbering eight – and producing and disseminating the UTC(OP) time scale.
Far beyond simple metrological interest, these advances open the door to numerous applications that were previously unthinkable : tests to detect minute deviations from the laws of fundamental physics, research into dark matter, and even determining the shape of the Earth’s gravitational potential.
Ytterbium magneto-optical cell, in the center of the vacuum chamber of the ROYMAGE instrument
In 2021, the Laboratoire temps espace began construction of transportable optical network clocks based on ytterbium (project ROYMAGE), with the aim of contributing to chronometric geodesy, the science that measures the shape of the Earth using clock data. It is through an effect of general relativity known as gravitational time dilation that the frequency of atoms depends on the local geopotential. Thus, by comparing two clocks at a distance, it is possible to measure the difference in potential, and therefore altitude, between these two instruments : a fractional frequency difference of 1×10−18 corresponds to a change in height of 1 cm. This project relies on the fiber optic network REFIMEVE, which distributes an ultra-low noise and accurate 1542 nm infrared carrier to numerous academic, industrial, civil, and space agency stakeholders. This signal is available across much of mainland France, with connections to the German, British, and Italian networks. Transportable instruments positioned along this network can thus be compared to the dozen or so stationary optical clocks currently in operation in Europe.
The use of these comparisons by the geodetic community will contribute to our understanding of the Earth’s geoid, a reference equipotential surface whose realization is an essential prerequisite for a new definition of the second. It will also be a new tool for unifying vertical reference systems between countries, as well as for detecting natural events affecting the geopotential, whether sudden (tsunamis, earthquakes) or only perceptible over the long term (rising sea levels).
It is therefore the ability of clocks to map space-time that has been recognized this year by the jury of the Academy of Sciences. This direct application of quantum mechanics to Earth sciences highlights the use of new-generation quantum sensors for fundamental research as well as for technological and societal issues.
Le LTE, UMR 8255 (Observatoire de Paris PSL - CNRS - Sorbonne Université - LNE - Univ Lille) was created in January 2025 from the merger of SYRTE (Systèmes de références temps-espace) and IMCCE (Institut de mécanique céleste et de calcul des éphémérides).
The ROYMAGE project is a collaboration between LTE, IPGP, IGN, and SHOM. It is funded by the French National Research Agency (ANR-20-CE47-0006), EURAMET’s EMP program, CNES, the First-TF labex, the Scientific Council of the Paris Observatory, the GRAM national program, DIM SIRTEQ, DIM QUANTIP, and the Del Duca Foundation.
Contact
Rodolphe Le Targat, LNE researcher at the Laboratoire temps espace
Rodolphe.Letargat@observatoiredeparis.psl.eu
