Comptes Rendus
The new International System of Units / Le nouveau Système international d'unités
The unit of time: Present and future directions
Comptes Rendus. Physique, Volume 20 (2019) no. 1-2, pp. 153-168.

Some 50 years ago, physicists, and after them the entire world, started to found their time reference on atomic properties instead of motions of the Earth that have been in use since the origin. Far from being an arrival point, this decision marked the beginning of an adventure characterized by an improvement by 6 orders of magnitude in the uncertainty of realization of atomic frequency and time references. Ever-progressing atomic frequency standards and time references derived from them are key resources for science and for society. We will describe how the unit of time is realized with a fractional accuracy approaching 1016 and how it is delivered to users via the elaboration of the international atomic time. We will describe the tremendous progress of optical frequency metrology over the last 20 years that led to a novel generation of optical frequency standards with fractional uncertainties of 1018. We will describe work toward a possible redefinition of the SI second based on such standards. We will describe existing and emerging applications of atomic frequency standards in science.

Il y a une cinquantaine d'années, les physiciens, et après eux le monde entier, ont commencé à fonder leur référence temporelle sur les propriétés atomiques au lieu des mouvements de la Terre, qui étaient utilisés depuis l'origine. Loin d'être un point d'arrivée, cette décision a marqué le début d'une aventure caractérisée par une amélioration par six ordres de grandeur de l'incertitude de la réalisation des références atomiques de fréquence et de temps. Les étalons de fréquence atomique en constante progression et les références de temps qui en découlent sont des ressources essentielles pour la science et pour la société. Nous décrirons comment l'unité de temps est réalisée avec une précision relative approchant 1016 et comment elle est mise à la disposition des utilisateurs via l'élaboration du temps atomique international. Nous montrerons les progrès considérables de la métrologie des fréquences optiques au cours des vingt dernières années, qui ont conduit à une nouvelle génération d'étalons de fréquence optique avec des incertitudes relatives de 1018. Nous décrirons les travaux en vue d'une éventuelle redéfinition de la seconde du SI basée sur ces étalons. Nous décrirons les applications scientifiques existantes et émergentes des étalons atomiques de fréquence.

Published online:
DOI: 10.1016/j.crhy.2019.02.002
Keywords: Atomic fountain, Timescale, Optical frequency standard, Fundamental physics test, Chronometric geodesy, Redefinition of the SI second
Mot clés : Fontaine atomique, Échelle de temps, Étalon de fréquence optique, Test de physique fondamentale, Géodésie chronométrique, Redéfinition de la seconde du SI

Sébastien Bize 1

1 SYRTE, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, LNE, 61, avenue de l'Observatoire, 75014 Paris, France
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Sébastien Bize. The unit of time: Present and future directions. Comptes Rendus. Physique, Volume 20 (2019) no. 1-2, pp. 153-168. doi : 10.1016/j.crhy.2019.02.002. https://comptes-rendus.academie-sciences.fr/physique/articles/10.1016/j.crhy.2019.02.002/

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