Fiber-optic connection between French and German optical clocks compares their precision
The link supports fast clock comparisons with an uncertainty below 10-18.
|A metrological optical fiber link has been made between LNE-SYRTE in Paris, France and PTB in Braunschweig, Germany, connected in Strasbourg. The overall link length is 1400 km. (Graphic: PTB)|
In the past few years, optical atomic clocks have become 100 times more precise than the best cesium clocks. But so far their precision has been available only locally, as frequency transfer via satellite does not provide sufficient resolution. Now, a high-precision 1400-km direct fiber-optic connection between France and Germany has been established through the joint work of Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig, Systèmes de Référence Temps-Espace (LNE-SYRTE) in Paris, and Laboratoire de Physique des Lasers (LPL) in Villetaneuse.1
The connection is based on standard telecom optical fibers; optical power losses of 200 dB are compensated by means of custom-developed amplifiers. In addition, frequency fluctuations that arise during the propagation along the fiber are actively suppressed by up to six orders of magnitude.
The first comparison between the French and German optical strontium clocks confirms the high expectations placed in the connection; the fully independent clocks agree with an unrivalled fractional uncertainty of 5 x 10-17. Their collaboration is a first step towards a European network of optical clocks providing ultrastable high-precision optical reference signals to diverse users, benefiting various research areas with applications in fundamental physics, astrophysics, and geoscience.
Difference in gravitational redshift
Comparisons of clocks at the highest resolution allow a wide range of very sensitive physical experiments, for instance, the search for time-dependent changes of fundamental constants. Also, the apparent rate of a clock depends on the local gravitational potential: comparing two clocks measures the gravitational redshift between them, and thus yields their height difference. Such measurements provide data points for the geodetic reference surface, the so-called "geoid". This research approach is pursued jointly by physicists and geodesists in the Collaborative Research Centre 1128 ("geo-Q") of the German Science Foundation (DFG).
Frequency fluctuations between the two strontium optical lattice clocks of less than 2 x 10-17 were observed after 2000 s of averaging time, and the link itself supports fast clock comparisons with an uncertainty below 10-18. As both clocks are based on the same atomic transition they should theoretically supply exactly the same frequency – except for the gravitational redshift due to the 25 m difference in height between the two institutes. This was indeed confirmed within the clocks’ combined uncertainty of 5 x 10-17, corresponding to a height uncertainty of 0.5 m.
Among other benefits, this work clears the path towards a redefinition of the unit of time, the SI second, through regular international comparisons of optical clocks.
1. C. Lisdat et al., Nature Communications 7:12443 (2016); doi: 10.1038/NCOMMS12443