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Comparison of geographically separated ultrastable lasers, over the longest ever reported metrological optical fibre link network, measuring 2,220 km

The research from four projects within EURAMET’s European Metrology Programme for Innovation and Research about ultrastable lasers is described in a recent article in the journal Nature Communications.

The four EMPIR projects are:

• Optical clocks with 1E-18 uncertainty (15SIB03, OC18)
• Optical frequency transfer - a European network (15SIB05, OFTEN)
• Robust Optical Clocks for International Timescales (18SIB05, ROCIT)
• Advanced time/frequency comparison and dissemination through optical telecommunication networks (18SIB06, TiFOON)

Abstract from the article

The article entitled Comparing ultrastable lasers at 7 × 10−17 fractional frequency instability through a 2220 km optical fibre network explains what has been achieved in the projects:

Ultrastable lasers are essential tools in optical frequency metrology enabling unprecedented measurement precision that impacts on fields such as atomic timekeeping, tests of fundamental physics, and geodesy. To characterise an ultrastable laser it needs to be compared with a laser of similar performance, but a suitable system may not be available locally. The article reports a comparison of two geographically separated lasers, over the longest ever reported metrological optical fibre link network, measuring 2,220 km in length, at a state-of-the-art fractional-frequency instability of 7 × 10−17 for averaging times between 30 s and 200 s.

The measurements also allow the short-term instability of the complete optical fibre link network to be directly observed without using a loop-back fibre. Based on the characterisation of the noise in the lasers and optical fibre link network over different timescales, we investigate the potential for disseminating ultrastable light to improve the performance of remote optical clocks

These EMPIR projects are co-funded by the European Union's Horizon 2020 research and innovation programme and the EMPIR Participating States.

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