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Optical fiber interferometer compares clocks in Paris and Braunschweig


The worldwide network of optical glass fiber connections for the transmission of data allows access to remote instruments from several places at the same time. By modifying such glass fiber links to glassfiber- based optical interferometers, optical frequencies can be compared over very long distances. This has now been implemented on a 1400 km long glass fiber link from Braunschweig to Strasbourg and back with a resolution of only a few μHz, which corresponds to a relative resolution of the frequency of 2 · 10–20.

The frequencies of strontium lattice clocks in the French metrology institute SYRTE (Paris) and PTB were compared by means of fiber connections to Strasbourg (705 km from Paris, 710 km from Braunschweig). The individual setups comprising an interrogation laser, an optical lattice, an fs frequency comb, a transfer laser and a stabilized link are shown. The difference frequency of the two transfer lasers is measured in Strasbourg. (Figure from an original publication: C. Lisdat, G. Grosche, N. Quintin, C. Shi, S.M.F. Raupach et al.: A clock network for geodesy and fundamental science. Nature Communications, 7:12443 (2016), DOI 10.1038/NCOMMS12443)

A cloud of laser-cooled atoms in the strontium atomic clock at PTB is visible to the naked eye thanks to the fluorescence light emitted by the atoms. Laser-cooling is the first step in preparing ultracold atoms, in which the clock transition near 429 THz is interrogated.

Parallel to this, clocks have developed at a spectacular pace. Optical clocks, which stabilize the frequency of laser light to atomic transitions, supply the frequency with an uncertainty of a few 10–18; this is more accurate than the current Cs fountain clocks by a factor of 100. The high precision of optical clocks has, to date, been available only locally since satellite transmission causes a frequency uncertainty > 10–16.

The most interesting experiments carried out with optical clocks are, however, based on comparing them with each other, e.g. in order to detect the time-dependent change in fundamental constants. In addition, comparing the frequency of two clocks yields the height difference between them – via the gravitational redshift which the light experiences on its way from one clock to the other. The comparison thus provides data points for the geodetic reference surface, the so-called “geoid”. This research approach is being pursued jointly by physicists and geodesists in the Collaborative Research Centre 1128 (“geo-Q”).

A glass-fiber-based connection between PTB and its partner institute LNE-SYRTE in Paris has now been established which allows fast and precise clock comparisons with an uncertainty < 10–18. Frequency fluctuations in the glass fiber are actively reduced by up to 5 orders of magnitude, and power losses of 200 dB (1020) are compensated for by special amplifiers. PTB and SYRTE have compared their most stable optical clocks, which are based on ultra-cold neutral strontium atoms, via the new connection: after only 1000 seconds, the instability between the clocks was close to 2 · 10–17.

The 22.7 m difference in height between the two institutes was confirmed by means of the gravitational redshift measured within the combined uncertainty of the two clocks of 5 · 10–17. The excellent consistency of the measurements is also an important step towards the redefinition of the second.