Finnish optical clock sets new accuracy record and brings us closer to a new definition of the second

Press release 2 December, 2025

A research team at VTT MIKES has set a new record in optical-clock absolute frequency measurements using a strontium single-ion clock with exceptionally low uncertainty and high uptime.

The official definition of the second is set to be updated for the first time in decades. The change will be based on new optical clocks, which are far more precise than today’s standards.

Now, researchers at VTT MIKES have demonstrated a strontium single-ion optical clock with an exceptionally low systematic uncertainty of 7.9×10⁻¹⁹, among the lowest ever reported. Over 10 months, the clock’s frequency was measured against International Atomic Time (TAI) with an impressive 84% uptime. The record-setting total uncertainty of this measurement was just 9.8×10⁻¹⁷, limited by the cesium clocks that realise the current definition of the second and calibrate TAI.

As society becomes more digital, the need for more precise clocks grows

Accurate timekeeping is essential for modern life, since it enables everything from reliable internet connections to scientific research and secure banking. VTT’s breakthrough puts Finland at the forefront of a global effort to ensure our timekeeping keeps pace with technological progress.

“Optical clocks with low systematic uncertainty and high reliability as well as continuity with the current cesium-atom-based definition are among the criteria that need to be fulfilled before a redefinition can take place. Our work made a significant contribution towards these,” says Thomas Lindvall, Senior Scientist at VTT MIKES.

The new definition of the second is expected to be adopted internationally within the next decade.

Link to publication: https://doi.org/10.1103/czlf-bfvp

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Optical clock at the laboratory of VTT MIKES. (Photos source: VTT)

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