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International collaboration sets new record for comparison of distant ultrastable lasers through an optical fibre network

Led by NPL, an international consortium has set a new record for comparing distant ultrastable lasers through a metrological optical fibre network.

Using cutting-edge optical frequency standards developed at PTB and NPL, the team has been able to demonstrate that their measurement precision is preserved even when transmitted across thousands of kilometers of optical fibre.

This network has been specifically set up for comparing optical atomic clocks, the next generation of atomic clocks which is expected to improve our ability to measure time and frequency by several orders of magnitude. The extreme precision of optical atomic clocks will enable new applications, such as using the change of rate of a clock to measure gravity - an effect described by Einstein’s theory of general relativity. Other applications include improvement in communication, synchronisation and localisation.

The metrological optical fibre network utilised in the measurements is operated by multiple research institutes, as a result this activity has been highly collaborative and international, involving 32 authors from five institutions, namely: NPL in the UK, RENATER, LPL and LNE-SYRTE in France and PTB in Germany.

Just like quartz oscillators are key components in conventional microwave clocks, ultrastable lasers play a critical role in optical clocks. State-of-the art ultrastable lasers like the ones used in this study achieve a stability better than 6 parts in 100,000,000,000,000,000. For instance, fractionally this is equivalent to a wobble of the distance between the Earth and the Sun of less than the width of a hair. Such precision is realised using optical reference cavities providing an optical standing wave with a frequency stability set by the distance of the mirrors. The stability of the optical references demonstrated in this work corresponds to an average variation of the position of the mirrors 50 times smaller than the diameter of a proton, over a cavity length of about half a metre.

Being able to transfer such precision to distant locations not only proves once again that optical fibre networks can be used to compare optical atomic clocks, it also enables more widespread, efficient utilisation of the cutting-edge research infrastructure otherwise only available at a few national laboratories. In this work we discuss how this measurement precision can be made available through optical fibre to a much larger community of users, ranging from other national laboratories, academia and industry.

This work is available at: M. Schioppo et al. “Comparing ultrastable lasers at 7 × 10−17 fractional frequency instability through a 2220 km optical fibre network”, Nature Communications (2022).

Marco Schioppo, Senior Research Scientist, NPL states: “This work is important as it demonstrates that it is possible to compare two geographically distant ultrastable lasers and measure their fundamental noise using an optical fibre network. We show that ultrastable lasers can be used to effectively measure the total fibre link noise. We also assess the possibility to disseminate ultrastable light to distant users through optical fibre. Our work is an important step towards making available the time and frequency high-precision measurement capability of a national metrological institute to a wide range of users.”


Jochen Kronjäger, Senior Research Scientist, NPL states: “I find it absolutely fascinating to see how two world-leading ultrastable lasers, developed independently and both pushing the boundaries of optical technology, can be compared to reveal their fundamental noise, despite being separated geographically by almost 800 km. It’s a phantastic demonstration of the power of light, made possible by many years of international collaboration.”



Corresponding authors:

Marco Schioppo    

Jochen Kronjäger  

Image: Schematic layout of the experiment for comparing two ultrastable lasers through a 2220 km long metrological optical fibre link network.

11 Jan 2022