10 Atomic Clocks Connected Across 6 Countries In Most Ambitious Timekeeping Experiment Ever

Study Finds ^ | June 13, 2025 | Rachel Godun and Helen Margolis (National Physical Laboratory, UK)

[Red Badger]

In a nutshell

Scientists connected 10 ultra-precise atomic clocks across six countries in the largest coordinated timekeeping experiment ever conducted

While many clocks agreed to extraordinary precision, the study revealed concerning discrepancies that highlight challenges in global clock synchronization

This research is a crucial step toward redefining the international second using optical clocks by 2030, potentially revolutionizing global timekeeping

******************************************************************

EUROPE — Every clock in your house probably tells a slightly different time. Now picture those clocks as the most precise instruments humanity has ever built, so accurate that they wouldn’t lose or gain a second for billions of years. That’s the challenge a team of international scientists recently tackled when they connected 10 of the world’s most advanced atomic clocks across six countries in the largest coordinated timekeeping experiment ever conducted.

The results, published in the journal Optica, reveal both incredible precision achievements and surprising discrepancies that could reshape how we define time itself. While some clocks agreed to within their expected precision limits, others showed unexpected differences that have major implications for the future of global timekeeping.

These optical clocks are being considered as replacements for the current international time standard, with scientists hoping to redefine the second — the fundamental unit of time — by 2030. But first, they need to prove these clocks can work together reliably on a global scale.

Why Ultra-Precise Timekeeping Matters

Accurate time underpins everything from GPS navigation to financial markets to internet communications. “The accurate time and frequency signals provided by atomic clocks are essential for many everyday technologies — like GPS, managing power grids and keeping financial transactions in sync,” said Helen Margolis, head of time and frequency at the National Physical Laboratory (NPL) in the United Kingdom, in a statement.

The current international time standard relies on cesium atomic clocks. Optical clocks are now about 100 times more accurate than the best cesium clocks and can measure time so accurately that they would lose or gain less than one second over billions of years.

Building incredibly precise clocks is one challenge; getting them to agree with each other across vast distances is entirely another. “Optical clocks provide ultraprecise frequency references that are vital for international metrology as well as for tests of fundamental physics,” the researchers wrote in their paper.

The Global Experiment: 45 Days of Synchronized Science

For 45 days starting February 20, 2022, research teams in Finland, France, Germany, Italy, the United Kingdom, and Japan ran their optical clocks simultaneously while connected through fiber optic cables and satellite links as part of a European collaborative project called ROCIT.

Each clock traps and cools individual atoms to near absolute zero, then uses laser light to measure their natural vibrations with extraordinary precision. The clocks use different types of atoms: some use strontium, others use ytterbium, and one uses indium ions. Each atomic species vibrates at its own unique frequency, like different musical notes. By comparing these frequencies, scientists can test whether the fundamental constants of physics remain truly constant across time and space.

European teams relied on a network of fiber optic cables stretching thousands of kilometers, including a 1,023-kilometer link between France and Italy. For distant locations like Japan, scientists used satellite-based GPS technology through a technique called Integer Precise Point Positioning (IPPP). However, satellite linking has limited precision due to measurement uncertainties caused by factors like signal noise or instrument limits.

Mixed Results Reveal Both Promise and Problems For Atomic Clocks

The experiment produced 38 different frequency ratio measurements between pairs of clocks. Four of these comparisons were conducted directly for the first time, and many others were measured with much greater accuracy than before. Many clocks agreed to within their expected precision limits, sometimes to extraordinarily high precision.

However, several concerning discrepancies emerged. Most notably, there appeared to be systematic problems with the Italian team’s equipment during the measurement period.

“Not all the results confirmed what we expected, and we observed some inconsistencies in the measurements,” said Rachel Godun, principal scientist at NPL. “However, comparing so many clocks at once and using more than one technique for linking the clocks made it easier to identify the source of the problem.”

These findings demonstrate a key point about precision timekeeping: “This serves to illustrate the importance of carrying out large, coordinated measurement campaigns with multiple clocks and links running simultaneously in order to identify and eliminate such inconsistencies,” the authors write in their paper.

Beyond timekeeping applications, this experiment marks several scientific firsts. These measurements help scientists search for signs that the fundamental constants of nature might be changing over time, a possibility that could revolutionize our understanding of physics. Marco Pizzocaro, a senior researcher at the Instituto Nazionale Di Ricerca Metrologica (INRiM) in Italy, believes their work “could also be used for carrying out tests of fundamental physics, such as searching for dark matter or testing the basic rules of physics.”

The Path Forward for Global Timekeeping

The research builds a key step toward the international physics community’s goal of redefining the International System of Units second to use optical clocks instead of the current cesium-based standards. The transition, targeted for 2030, requires demonstrating that different optical clocks worldwide can maintain consistent, reliable operation.

“Comparing multiple clocks at the same time and using more than one type of link technology provides far more information than the mostly pairwise clock comparisons that have been carried out to date,” said Thomas Lindvall, senior scientist at VTT MIKES in Finland. “With a coordinated set of measurements, it becomes possible to check consistency while also providing more trusted results. These results can help determine which optical clock(s) should be used in the new definition of the second.”

However, major challenges remain. “These measurements provide critical information about what work is still needed for optical clocks to achieve the precision and reliability required for use in international timekeeping,”

The experiment identified areas where more work is needed. To confirm that all clocks are performing as expected, measurement uncertainties must be reduced to match the precision of the clocks themselves. Repeated measurements will then be needed to confirm reliable operation necessary to build confidence in both the clocks and the links.

Scientists plan more extensive comparison campaigns as optical clock technology continues improving. Future experiments will likely include more clocks, longer measurement periods, and better fiber optic connections to reduce uncertainties further.

“Our findings could help to improve the performance of next-generation optical clocks, unleashing entirely new applications and advancing scientific endeavors that rely on time and frequency,” said Margolis.

While this experiment demonstrated that a global network of optical clocks is feasible, it also showed that achieving perfect synchronization across continents remains a formidable technical challenge. In an age where milliseconds can mean millions of dollars in financial markets and nanoseconds matter for GPS accuracy, this research brings us closer to a future where humanity’s most fundamental measurement — time itself — will be based on the quantum mechanical behavior of individual atoms, synchronized across the globe with previously unimaginable precision.

Paper Summary

Methodology

Scientists from six countries operated 10 optical atomic clocks simultaneously for 45 days starting February 20, 2022, as part of the European collaborative project ROCIT. These clocks use different atomic species (strontium, ytterbium, and indium) trapped and cooled to near absolute zero, with laser light measuring their natural oscillations. The clocks were connected via fiber optic cables spanning thousands of kilometers across Europe and satellite-based IPPP (Integer Precise Point Positioning) technology for more distant locations. Data was recorded every second and synchronized to Coordinated Universal Time.

Results

Researchers measured 38 frequency ratios between different clock pairs. Four of these comparisons were conducted directly for the first time, and many others were measured with much greater accuracy than before. The customized optical fiber links allowed measurements with 100 times greater precision than satellite techniques. However, the study revealed systematic discrepancies, including problems with signal distribution at INRIM that caused a 4 × 10^-16 offset in satellite measurements compared to fiber links, and evidence of frequency instabilities in some German and French clocks.

Limitations

Satellite-based measurements had higher uncertainties than fiber optic connections due to measurement uncertainties caused by factors like signal noise or instrument limits. Some clock discrepancies couldn’t be definitively attributed to equipment problems versus fundamental issues. The measurement campaign was limited to 45 days, and coordinating the simultaneous operation of ten high-performance clocks in various countries required extensive advance planning.

Funding and Disclosures

Research was conducted under the European collaborative project ROCIT as part of the European Metrology Programme for Innovation and Research. Additional support came from national research councils and agencies in participating countries. The authors declared no conflicts of interest.

Publication Information

“Coordinated international comparisons between optical clocks connected via fiber and satellite links” by Thomas Lindvall, Marco Pizzocaro, Rachel M. Godun, and colleagues, published in Optica, Vol. 12, No. 6, June 2025, pages 843-852.

---

TOPICS: Computers/Internet; History; Military/Veterans; Science
KEYWORDS: astronomy; clocks; physics; science; stringtheory; ticktock

[HartleyMBaldwin]

Does anybody really know what time it is?

[abb]

Time has come today.

[abb]

If I Could Turn Back the Hands of Time.

[Red Badger]

Time is on my side. Yes, it is................

[HartleyMBaldwin]

Time is too slow for those who wait
And time is too swift for those who fear
Time is too long for those who grieve
And time is too short for those that laugh

[HartleyMBaldwin]

If I could put time in a bottle...

[Red Badger]

Time - Pink Floyd.......................

[abb]

If I could save time in a bottle
The first thing that I’d like to do
Is to save every day
‘Til eternity passes away

[Red Badger]

Time of the Season..................

[Red Badger]

Haven’t Got Time for the Pain....................

[Telepathic Intruder]

Does anybody really care?

[HartleyMBaldwin]

If so, I can’t imagine why.

[waredbird]

My wife will still be late.

[HartleyMBaldwin]

My work on this thread is done.

[Vaduz]

That reminds me I have to reset my sun dial for summer time.

[Seruzawa]

Baby baby baby you’re out of time.

[omni-scientist]

“The significance of the passage of time, right? The significance of the passage of time. So when you think about it, there is great significance to the passage of time.”

Maybe Kamala Harris was on to something, or just on something.

[abb]

Time waits for no one
And nobody gets away (nobody gets away)
There’s no exception
We all got our dues to pay (all got our dues to pay)

[Leaning Right]

To help advance science, I volunteered my clock to be part of the experiment.
But the humorless snobs turned me down.