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Comparison of atomic clocks via the information superhighway

Especially interesting for
  • improving the time scale
  • users of optical frequencies, e.g. in geodesy, astronautics and radioastronomy

In future, it will be possible to exploit the outstanding stability and accuracy of an optical atomic clock also in places other than the  metrology institute where the clock is located. The prerequisite for this is for the clock to be connected to a standard optical fibre telecommunications link. This has been demonstrated on a 920 km optical fibre link between Braunschweig and Munich.

920 km optical fibre link used to transmit the frequency from a PTB laser located in Braunschweig to the Max Planck Institute of Quantum Optics in Garching, near Munich.

Today, optical atomic clocks are already ticking so precisely that two clocks of the latest generation differ only in the 17th place after the decimal point. Thus, they allow highaccuracy tests of fundamental theories ranging from cosmology to quantum physics. Previously, the problem was that measurements were possible only locally at very few institutes worldwide (such as, e. g., PTB).  Over the last few years, PTB, the Max Planck Institute of Quantum Optics in Garching, the Institute of Quantum Optics of Leibniz  University in Hanover and the Excellence Cluster QUEST, with the support of the European Space Agency (ESA), of the Deutsches  Forschungsnetz (the German National Research and Education Network – DFN) and of GasLINE, a telecommunications network  provider of German gas distribution companies, have been investigating how other users could benefit from the outstanding stability and accuracy of these optical clocks.

Since standard procedures for time and frequency comparisons based on satellites cannot achieve the accuracy and stability required for optical clocks, highly stable optical frequencies were transmitted via telecommunications optical fibres. In order to compensate for the attenuation of the signals due to the length of the link, optical amplifiers were installed along curthe way. To prevent accuracy and stability losses, it was necessary to detect and record variations of the laser frequency caused by mechanical, acoustic and thermal disturbances (e. g. due to changes in temperature, motor traffic or road works along the section). For this purpose, the receiver sends the laser light back to the emitter via the same link. After comparing the received signal with the signal sent, a correction can be stamped on to the sender, so that the signal received is undisturbed.

For the first time, it was possible to demonstrate that this is nearly perfectly possible in real time, even on a glass fibre connection of more than 900 km in length, so that the frequency of approx. 200 THz which is provided to the user deviates by less than 1/10 000 hertz from the frequency originally sent. This corresponds to a relative frequency uncertainty of less than 5 ∙ 10–19.

Optical frequencies of a quality as good as that normally provided only locally at metrology institutes can now be disseminated. Using an existing optical fibre infrastructure – as available, for example, in the national research networks – will, in future, allow the national network to be transformed into a European network that will eventually interconnect all optical clocks in Europe.

Fundamental research will be the first sector to benefit from this, for instance for determining fundamental constants with great accuracy, checking the General Theory of Relativity and for quantum electrodynamics forecasting, but also for future applications in geodesy, radioastronomy or in the aerospace industry.

Scientific publication

Predehl, K.; Grosche, G.; Raupach, S.M.F.; Droste, S.; Terra, O.; Alnis, J.; Legero, Th.; Hänsch, T.W.; Udem, Th.; Holzwarth, R.; Schnatz, H.: A 920 km Optical Fiber Link for Frequency Metrology at the 19th Decimal Place. Science 336 (2012) 441–444