Optical fiber interferometer compares clocks in Paris and Braunschweig
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.