Engineering news
In the past few years, optical atomic clocks have become almost 100 times more precise than the best caesium clocks.
So far, this precision has only been available locally, because frequency transfer using satellite connections cannot provide the resolution needed. However, a direct optical connection between laboratories in different countries - France and Germany is breaking new ground in precision measurement.
After its completion, highly precise signals can now "travel" through a 1400 km optical fibre link between the Systèmes de Référence Temps-Espace (SYRTE) in Paris and the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig, Germany, where it is claimed the most precise optical clocks in Europe are operated.
Engineers at the laboratories have been celebrating after the first successful connection the French and German optical strontium clocks. The connection represents the first frequency comparison of its kind across national borders. The fully independent clocks agree with an unrivalled fractional uncertainty of 5 x 10–17.
Research applications
According to the researchers, whose work on the connection is published in August’s Nature Communications, the direct link is the first step towards a European network of optical clocks. This network will provide ultra stable, high-precision optical reference signals, like those currently available from metrology institutes to a more diverse range of users.
Various research areas will benefit from this, including research to test the fundamental laws of physics, geoscience and, last but not least, metrology. This work also clears the path towards a redefinition of the unit of time, the SI second, through regular international comparisons of optical clocks.
Comparisons of the clocks at the the highest resolutions enables a wide range of very sensitive physical experiments, such as the search for time-dependent changes of fundamental constants.
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 25m difference in height between the two institutes. This was indeed confirmed within the clocks’ combined uncertainty of 5×10-17, corresponding to a height uncertainty of only 0.5 m.
Such measurements provide data points for the geodetic reference surface, the so-called "geoid".
Stable signals
The connection is the result of several years of work by French and German researchers. The 1400km link is based on standard telecom optical fibres. The high stability of the transmissions is enabled by two main measures.
Optical power losses of 200dB are compensated for by means of amplifiers developed bespoke for the project. Frequency fluctuations added during the propagation along the fibre are actively suppressed by up to 6 orders of magnitude.
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 network.
At around the midpoint midway, the signals meet at the IT Centre of the University of Strasbourg, so that the clocks of the two institutes can be compared there.