Optical fiber : a revolution for the comparison of atomic clocks

Optical fiber : a revolution for the comparison of atomic clocks
Extrait du Observatoire de Paris centre de recherche et enseignement en astronomie et
astrophysique relevant du Ministère de l'Enseignement supérieur et de la Recherche.
https://www.obspm.fr/optical-fiber-a-revolution.html
Optical fiber : a revolution for
the comparison of atomic
clocks
Date de mise en ligne : vendredi 19 août 2016
Observatoire de Paris centre de recherche et enseignement en astronomie et
astrophysique relevant du Ministère de l'Enseignement supérieur et de la
Recherche.
Copyright © Observatoire de Paris centre de recherche et enseignement en astronomie et astrophysique relevant du Ministère de l'Enseignement
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Optical fiber : a revolution for the comparison of atomic clocks
The first direct comparison between optical strontium clocks through a 1400 km optical fiber
link has been made by a team of French and German physicists. This work is published in the
current edition of Nature Communications. Comparing optical clocks at the highest
resolution opens the way to a wide range of very sensitive physical experiments.
In the past few years, optical atomic clocks have made spectacular progress. They have become 100 times more
precise than the best cesium clocks, and their frequency can be controlled up to the 18th digit. Unfortunately, this
precision has, to date, been available only locally since conventional satellite transmission cannot provide such a
high resolution.
This has recently changed thanks to a new direct connection between France and Germany : high-precision
frequencies can now "travel" through a 1400 km optical fiber link between the Physikalisch-Technische
Bundesanstalt (PTB) in Braunschweig, Germany, and LNE-SYRTE (Systèmes de Référence Temps-Espace) in
Paris, France. The first comparison realized between the French and German optical strontium clocks proved that the
clocks tick at the same frequency with a relative uncertainty of 5 × 10-17, although they were developed in two
independent institutes with different experimental set-up and environment.
Recherches sur la future génération d'horloge atomique L'horloge à réseau optique à atomes de strontium. Pour
qu'on puisse interroger avec précision les atomes, ceux-ci doivent être refroidis, c'est-à-dire ralentis, et piégés dans
un réseau optique, l'ensemble étant sous ultra-vide. Ici le nuage d'atomes de strontium piégé par des faisceaux
laser. Laboratoire Foucault, LNE-SYRTE, (Systèmes de Référence Temps-Espace, Observatoire de
Paris/CNRS/UPMC/LNE), à l'Observatoire de Paris. © X. Francolon - Sipa press
This result is far from being reachable with other comparison means than optical links, which provided here a
resolution of 3x10-19. This work is published in the current edition of Nature Communications. In the future, such
optical fiber links could build a whole network of optical clocks which could be compared with each other within
minutes, with benefits in fundamental physics, astronomy and geoscience.
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Optical fiber : a revolution for the comparison of atomic clocks
The experimental set-up for the clocks comparison is shown on Figure below. The strontium clocks are built with cold
atoms in an optical lattice. They rely on light in the visible spectral range which can be measured very precisely,
thanks to its high frequency and optical frequency combs - a technology award by the Nobel prize of Physics in
2005-. The 1400 km long link is based on standard telecom optical fibers for which we compensate power losses of
20 orders of magnitude by means of special amplifiers. We also actively suppress frequency fluctuations added by
the propagation by up to 6 orders of magnitude, enabling the transmission of optical signals with very high stability.
Chemin de la liaison optique de 1415 km entre le Syrte en France et le PTB en Allemagne Les points rouges,
bleus et verts marquent les lieux où les équipements scientifiques conçus par les chercheurs franco-allemands ont
été installés. Meynadier / Le Targat / Pottie / LNE-Syrte
Approximately midway, the signals from LNE-SYRTE and PTB meet at the University of Strasbourg, so that the
clocks of the two institutes can be compared there. The German part of the link uses commercially rented optical
fiber and facilities of the German National Research and Education Network (DFN). The French part of the link is
operated by the French Ministry of Education and Research within the framework of the RENATER network and was
developed jointly by Laboratoire de Physique des Lasers (LPL) and LNE-SYRTE.
Comparing optical clocks at the highest resolution opens the way to a wide range of very sensitive physical
experiments. For instance, the rate of a clock slightly depends on the local gravitational potential : comparing two
clocks yields the height difference between them - by measuring the gravitational redshift - thus providing data points
for the geodetic reference surface, the so-called "geoid". This work also clears the path towards a redefinition of the
unit of time, the SI second, through regular and practical international comparisons of optical clocks.
This successful collaboration is a first step towards a European network of optical clocks which could provide
ultrastable high-precision optical reference signals to the most diverse users. Various research areas will benefit from
this - fundamental research (to test the fundamental laws of physics), astrophysics (to improve synchronization
between telescopes), geoscience and, last but not least, metrology.
Besides PTB, LNE-SYRTE (CNRS/Observatoire de Paris/Laboratoire National d'Essais/University Paris 6/PSL
Research University and Sorbonne) and LPL (CNRS/University Paris 13 Sorbonne Paris Cité), the following partners
are involved : RENATER, LP2N (Institut d'Optique Graduate School, CNRS, and Université de Bordeaux) and Institut
für Erdmessung (Leibniz Universität Hannover).
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Optical fiber : a revolution for the comparison of atomic clocks
We acknowledge funding support from the Agence Nationale de la Recherche (ANR blanc, Labex First-TF, Equipex
REFIMEVE+), the European Metrology Research Programme (contract NEAT-FT and ITOC), Centre National
d'Études Spatiales (CNES), Conseil Régional Île-de-France (DIM Nano'K), CNRS with Action Spécifique Gravitation,
Références, Astronomie, Métrologie (GRAM) and the German Research Foundation DFG within RTG 1729 and CRC
1128 geo-Q. Research at PTB was supported by the Centre of Quantum Engineering and Space-Time Research
(QUEST).
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