World’s Most Ambitious Clock Test Brings Us Closer to a New Definition of the Second

In an extraordinary international effort, scientists across six countries have completed the largest and most advanced comparison of atomic clocks ever attempted. The result is a major milestone in redefining something as fundamental as the second.

The experiment, which ran for 45 days between February and April 2022, involved ten of the most advanced optical atomic clocks on the planet. These were located in national metrology institutes in Germany, France, Italy, Finland, the United Kingdom, and Japan. The clocks were linked through a combination of ultra-stable fiber optic cables and a high-precision satellite technique known as integer precise point positioning. Altogether, 65 researchers took part in this unprecedented campaign.

At stake is nothing less than the global definition of the second, the very unit by which we measure time.

The second, redefined through time

The journey to today’s definition of a second is a long one. In the early 20th century, a second was defined as 1/86,400 of a mean solar day. That worked until scientists realised that Earth’s rotation wasn’t as consistent as once believed. In 1956, the second was redefined again, this time based on Earth’s orbit around the Sun. But that too was soon replaced.

In 1967, the international scientific community agreed on the current standard: a second is defined as the time it takes for a caesium-133 atom to emit 9,192,631,770 cycles of microwave radiation as it jumps between two energy levels. These caesium-based atomic clocks have served science and society reliably ever since. The National Physical Laboratory in New Delhi, for instance, maintains five such clocks to keep India’s official time, disseminated via INSAT satellites, telecommunications networks, and fiber links.

Yet, as technology and scientific needs have grown more sophisticated, caesium clocks are no longer enough.

Why optical clocks are the future

The push toward redefining the second comes from the increasing demand for timekeeping that is accurate to 17 or even 18 decimal places. Caesium clocks, while remarkably precise, can’t deliver that. That’s where optical atomic clocks come in.

Optical clocks function similarly to caesium clocks but operate at much higher frequencies, in the range of visible light rather than microwaves. That means more oscillations per second and therefore a finer measurement. Some optical clocks can measure time so precisely they would drift by just one second in 15 billion years.

These clocks are based on atoms such as strontium, ytterbium, and indium. For example, the frequency of radiation emitted by a strontium atom jumping between two specific energy states is about 429 trillion hertz, almost 50,000 times higher than that used in a caesium clock. The more waves there are to count in a given second, the more precisely time can be measured.

Optical clocks are already used in scientific applications that need ultra-precise timing, from GPS and space navigation to radio astronomy and climate science. But before they can replace caesium clocks as the world’s official timekeepers, scientists need to prove they all agree with one another, regardless of where in the world they are located.

The most ambitious test in history

This latest test, described in a paper published in Optica on June 12, brought together ten optical atomic clocks based on five different atoms: strontium-87, ytterbium-171, charged ytterbium ions in two configurations (E2 and E3), charged strontium-88 ions, and indium-115 ions.

Two clocks in Germany, located in the same building, were connected with short optical fibers. Clocks in France, Germany, and Italy were linked using telecommunication fibers already running between the countries. For the more distant connections, across the English Channel, the Baltic Sea, and all the way to Japan, scientists used satellite-based GPS techniques.

Because optical clocks occasionally require maintenance and brief shutdowns, each lab also maintained simpler backup clocks. These backups used GPS data to keep time and filled in during periods when the optical clocks were offline.

Between February 20 and April 6, 2022, scientists conducted 38 independent frequency ratio measurements, comparisons of how different clocks tick against each other. Four of these ratios had never been directly measured before: the ratios between In⁺ and Yb, Yb⁺(E3) and Yb, Sr⁺ and Sr, and Sr⁺ and Yb.

The most accurate result came from a comparison between the In⁺ and Yb⁺(E3) clocks in Germany, with an uncertainty of just 4.4 × 10⁻¹⁸ - one of the tightest margins ever recorded.

Clocks in harmony, mostly

The results show that most clocks agreed with each other to a level of precision between 10⁻¹⁶ and 10⁻¹⁸. In other words, they ticked in harmony even across vast distances. For example, the strontium clocks in Germany and France differed by less than 2 × 10⁻¹⁶, whether compared via fiber optic cable or satellite link.

Similarly, clocks in Germany and the UK matched within 3 × 10⁻¹⁶ despite being separated by the North Sea. These results confirm that current communication networks—including long-haul optical fibers and GPS, are up to the task of handling time signals with extraordinary accuracy.

Ratios between clocks using the same atoms, such as strontium-to-strontium or ytterbium-to-ytterbium, reinforced that many of the devices were functioning properly and consistently.

Finding the flaws before 2030

Not everything was flawless. The Italian ytterbium clock, for example, showed consistent offsets of around 4 × 10⁻¹⁶ when measured via satellite. Fiber-based comparisons, on the other hand, did not show these errors. This discrepancy pointed to a previously unknown glitch in the Italian lab’s signal distribution setup.

In another case, small but meaningful offsets of up to 2 × 10⁻¹⁶ were observed between the strontium clocks in France and Germany. These inconsistencies are significant enough to matter when finalising a new global definition of the second.

Rather than being setbacks, researchers said such discoveries are valuable. They highlight where technical adjustments are needed before the optical clocks can fully take over as the global time standard.

A future-proof definition of time

To prevent future errors or misinterpretation, the research teams created a 38-by-38 matrix of correlation coefficients—242 in total. These numbers show how measurements were interconnected, whether through shared clocks, communication links, or backups. Publishing this data will help future analysts combine results responsibly and avoid double-counting or bias.

In the final analysis, the experiment demonstrated that optical atomic clocks in different parts of the world can agree to within one part in a billion billion. That level of accuracy, maintained across multiple countries and systems, clears many of the final hurdles in the global effort to redefine the SI second.

The official transition to optical clocks is expected to happen around 2030. When it does, the second will still be the same length—but it will be measured with far greater clarity, consistency, and confidence than ever before.

Also Read: Sensex, Nifty Hit 2025 Highs as Ceasefire Boosts Market Sentiment