Metrology laboratories such as LNE-SYRTE at Paris Observatory - PSL or INRIM in Turin hold extremely powerful atomic clocks used to maintain time scales that serve as French and international references, such as the International Atomic Time.
This precise calibration of time, as well as its inverse, frequency, is essential for precision measurements for fundamental physics or, for example, the physical chemistry of the atmosphere and for the synchronization of astrophysical observations or in high energy physics.
In order to be able to compare the signals delivered by these clocks, it is necessary that they can communicate between them. Since the technique using satellite networks has become too imprecise compared to the frequency stability of the best atomic clocks, another option has been developed, which consists in using optical fiber networks.
A Franco-Italian collaboration between the Systèmes de Référence Temps-Espace de l’Observatoire de Paris - PSL (SYRTE, Observatoire de Paris - PSL / CNRS / Sorbonne University), the Laboratoire de Physique des Lasers (LPL, CNRS / Sorbonne Paris Nord University), the Laboratoire national de métrologie et d’essais (LNE) and the Istituto nazionale di ricerca metrologica (INRIM) in Turin have just demonstrated the virtually uninterrupted operation for four months of a 1,023-km fiber-optic link between the Italian and French metrology institutes.
This is the first time that a link of this length has operated 24 hours a day for such a long time, which shows its potential to offer a continuous scientific service.
The French part relies on the research infrastructure REFIMEVE [[REMIFEVE was labeled in October 2021 by the Ministry of Higher Education and Research. REFIMEVE is piloted by LPL and LNE-SYRTE of Paris Observatory - PSL, which allows to disseminate on more than 6,000 km of optical links throughout the country, via the RENATER fiber optic network, the stability and accuracy of national time-frequency references developed by LNE-SYRTE.
The beam of a laser slaved to the reference atomic clock is injected into the optical fiber on one of the reserved channels of the multiplex, the other channels being used to transmit the telecommunication data without disturbing the signal. All along the route, relay stations allow the signal to be repeated to inject it from one link to another. Noise induced by thermal fluctuations and acoustic vibrations, which are likely to degrade the transmitted signal, are compensated with an electronic system.
The Franco-Italian link has allowed a four-month comparison of the Cs, Rb and Yb atomic clocks of the two countries, underlining the potential of fiber optic links to evaluate the uncertainty budgets of these clocks.
This reliable and robust tool is essential for new developments in metrology such as the development of more accurate time scales based on optical frequencies.
The characterization of the uncertainty induced by the use of the optical link (less than 6.10-19) confirms the idea that it could be used in the development of time scales based on the optical. Indeed the current definition of the second, based on an atomic transition of 133 Cs in the microwave domain, has a precision of the order of 10-16. By using transitions in the optical domain of atoms or ions instead, this precision could reach 10-18.
Multiple applications would result from this, such as the even more precise measurement of the Earth’s gravitational field or the fundamental constants of physics and the testing of physical theories beyond the Standard Model.
Reference
"Coherent optical fiber link across Italy and France", C. Clivati et al, Physical Review Applied, published November 03, 2022.
DOI : 10.1103/PhysRevApplied.18.054009