Sébastien Bize received the European Frequency and Time Award from the European Frequency and Time Forum (EFTF) [1] on Monday 25 April, 2022, in recognition of his outstanding contribution to the field of Time and Frequency metrology. As deputy director of the laboratory in charge of its national metrology mission and head of the Optical Frequency Metrology research team, Sébastien Bize has been exceptionally involved in the laboratory and in the international community, with one common thread: developing state-of-the-art instrumentation that is as effective as it is available over time.

Sébastien Bize first distinguished himself by his important contribution to the construction of the "double fountain", a laser-cooled atomic clock that is unique in the world because it works simultaneously with cesium 133 and rubidium 87 atoms. This clock exploits the hyperfine transition of caesium, which defines the duration of the unit of time in the International System of Units, the second, but also the corresponding transition of rubidium, which has advantageous physical properties at very low temperatures. This exceptional clock, which has been operating almost continuously for nearly 20 years now, has enabled SYRTE to have the most accurate primary frequency standard in the world, and to have the 6.8 GHz hyperfine transition of rubidium 87 recognised as the first secondary representation of the time unit. Sébastien Bize has also worked to increase the impact of the laboratory and of France in time metrology, and has greatly contributed to the fact that the double fountain, together with the caesium atomic fountains already developed and operated in the laboratory, have contributed to more than 40% of the International Atomic Time calibrations over the last 15 years.

Such a high accuracy of time measurement, maintained over many years, allows to search for very small variations in space-time and thus to test the theory of gravitation and the standard model of particle physics, competing with, and often surpassing, cosmological and high energy tests. By comparing the frequencies of rubidium and caesium atomic transitions, the double fountain has served to set bounds on potential time variations of the fine structure constant and to contribute to the search for dark matter. By comparing the frequencies of the double fountain with those of hydrogen masers, Sébastien Bize and colleagues from France and Australia were able to perform one of the most stringent tests of local Lorentz invariance.

These are just a few facets of Sébastien Bize’s work. A visionary and pioneer, Sébastien has been involved since 2002 in the development of new generation clocks, no longer using transitions ’authorised’ by quantum physics, but ’forbidden’ transitions, where the transition frequencies are no longer in the 10 GHz range but in the 400-800 THz range, and which are then excited by lasers. The much higher frequency of the transition makes it possible to cut out and thus measure time with an increased precision of several orders of magnitude. To limit the disturbance of the transition frequency by the Doppler effect and by the effect of interactions between atoms (among others), optical clocks require the control of the movement and position of atoms in ultra-sophisticated traps. New quantum systems, with atoms or ions, are therefore being studied to produce these so-called "optical" clocks.

Sébastien Bize undertook the first optical clock with cold mercury atoms, showing that one of its transitions had particularly attractive quantum properties for tests of invariance of fundamental constants. The cold mercury atom clock is a challenge made particularly bold by the need to master ultraviolet optics and nonlinear optics, in addition to mastering quantum technologies.

Today, a whole set of clocks probing the transitions of different atoms (mercury, strontium, caesium and rubidium) are operated together at SYRTE and compared by optical frequency combs.

Sébastien Bize was thus able to play a central role in the precise determination of the frequency relationships between these new atomic transitions, and in the recognition of new secondary representations of the secondnew secondary representations of the second, some of which will play a role in the future redefinition of the second.

The coming years promise to be very exciting. Optical clocks, instruments of extreme precision, which can be compared without degradation by new optical means on the ground and in space, promise, beyond the redefinition of the second, the advent of chronometric geodesy and ever more stringent tests of elementary interaction theories.

For more information:
The European Frequency and Time Forum website
https://www.eftf.org/awards

The joint working group of the CCL and the CCTF:
https://www.bipm.org/fr/committees/cc/cctf/wg/ccl-cctf-wgfs

The Optical Frequency team page:
https://syrte.obspm.fr/spip/science/fop/