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A “light highway” for highprecision frequencies

Comparison of optical atomic clocks via new 1,400 km optical fiber link between Braunschweig and Paris – closer to goal of European network of the best clocks

PTB-News 3.2016
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fundamental research in physics

In the past few years, optical atomic clocks have made spectacular progress. They have become 100 times more precise than the best cesium clocks. So far, their precision has been available only locally, since frequency transfer via satellite cannot provide sufficient resolution. This has now changed thanks to a novel 1,400 km optical fiber link between Braunschweig and Paris. This link allows frequencies to “travel” and optical atomic clocks to be compared across national borders. In the first comparison between PTB’s optical strontium clocks and those of the French LNE-SYRTE, an unrivaled fractional uncertainty of 5 · 10−17 was achieved.

The German part of the link uses commercially rented optical fibres and facilities of the German National Research and Education Network (DFN). The French part of the link uses the Network for Education and Research, RENATER, which is operated by the GIP RENATER. Approximately midway, signals from LNE-SYRTE and PTB meet at the IT Centre of the University of Strasbourg, so that the clocks of the two institutes can be compared there. (Fig.: PTB)

Comparisons of clocks at the highest resolution allow a wide range of very sensitive physical experiments to take place, such as the search for t ime-dependent changes of fundamental constants. In addition, 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. This research approach is pursued jointly by physicists and geodesists in Collaborative Research Centre 1128 (“geo-Q”) of the German Science Foundation (DFG).

Today’s most precise atomic clocks are based on optical transitions. Such optical clocks can provide a stable frequency with a fractional uncertainty of only a few 10–18. This is approximately 100 times more precise than the best cesium fountain clocks, which realize the unit of time, the SI second. However, clock comparisons using frequency transfer via satellites are limited to a frequency resolution near 10–16.

For this reason, scientists from PTB and from two French institutes in Paris (Systèmes de Référence Temps-Espace, LNE-SYRTE, and Laboratoire de Physique des Lasers, LPL) have been working for several years on an optical fiber connection between the German and French national metrology institutes, PTB and LNE-SYRTE. The 1,400 km link is now complete. Frequency shifts which are generated across the long distance are actively suppressed by up to 6 orders of magnitude, while power losses of 200 dB (1020) are compensated by means of special amplifiers. This allows optical signals to be transmitted with very high stability.

The link enables fast clock comparisons with an uncertainty of less than 10–18. When comparing the two strontium optical lattice clocks, frequency fluctuations of less than 2 · 10–17 were observed after only 2,000 seconds of averaging time, thereby demonstrating the high stability of the clocks. The 22.7 m difference in height between the two institutes was confirmed by means of the gravitational redshift measured within the clocks’ uncertainty of 5 · 10-17.

This development represents progress toward the goal of a European network of optical clocks connected by optical fiber links which additional European metrology institutes could successively join. This should place them in a leading role for the dissemination of optical reference – a role which, to date, has only been available in a few metrology institutes. In addition, this work clears the path towards a redefinition of the unit of time, the SI second, now that regular international comparisons of optical clocks are possible.

Contact (optical fiber link)

Gesine Grosche
Department 4.3 Quantum Optics and Unit of Length
Phone: +49 (0)531 592-4340

Contact (strontium clock)

Christian Lisdat
Department 4.3 Quantum Optics and Unit of Length
Phone: +49 (0)531 592-4320

Scientific publication

C. Lisdat, G. Grosche et al.: A clock network for geodesy and fundamental science. Nature Communications 7:12443 (2016)