Japan's Optical Lattice Clocks Advance Toward a New Definition of the Second

A Japanese invention moves closer to setting world time

A clock that measures roughly 429 trillion vibrations of light every second is helping drive an international effort to replace the world's current time standard. The technology, proposed in 2001 by University of Tokyo physicist Hidetoshi Katori, is now a commercial product and the subject of a collaboration with the organisation responsible for coordinating international measurement standards.

RIKEN, Shimadzu Corporation and the International Bureau of Weights and Measures (BIPM) signed a memorandum of understanding in Kawasaki, Japan, on May 12, 2026, to work on practical uses of optical lattice clocks. The university's collaboration announcement identifies RIKEN President Makoto Gonokami, advanced photonics director Koichiro Tanaka and Katori among those attending.

The agreement follows Shimadzu's April 2025 release of the Aetherclock OC020, which the company describes as the world's first commercial optical lattice clock. The broader technology has reached precision commonly expressed as an error equivalent to about one second over 10 billion years.

The international target is a possible decision on a new definition of the second in 2030, with implementation afterward. Scientists must still agree on the definition and demonstrate that laboratories can reproduce and share it reliably.

For ordinary users, a second would retain its familiar length. The proposed change concerns the physical reference used to measure it. For laboratories, national time services and systems that depend on precise synchronisation, it would allow a much finer measurement of that same interval.

Why the current second is becoming a limit

Today's second is defined by fixing a particular microwave frequency of caesium 133 atoms at exactly 9,192,631,770 vibrations per second. Counting that many cycles gives an interval of one second.

Atomic clocks work by tuning electromagnetic radiation to a frequency that atoms absorb. An atom can move between particular energy states, a process called a transition. The radiation frequency associated with that transition supplies a repeatable reference, allowing clocks built in different places to measure the same unit.

Optical clocks use light rather than microwaves. In a strontium optical lattice clock, the relevant light vibrates about 429 trillion times per second, roughly 47,000 times the caesium reference frequency. This provides much finer divisions of time, although controlling disturbances remains essential to making an accurate clock.

The BIPM's redefinition guidance says some optical standards already have systematic uncertainties about 100 times smaller than the best caesium fountain clocks. Systematic uncertainty describes how much a measurement might be shifted by effects such as the clock's environment or imperfections in its operation.

The practical problem is that scientists can now build clocks that outperform the reference defining their unit. A new definition aims to deliver an immediate improvement in accuracy of 10 to 100 times while preserving continuity with the existing second.

How a lattice of light holds atoms still

Katori's approach addresses a basic difficulty: moving atoms are harder to measure precisely. Their motion changes how they encounter the peaks and troughs of a light wave, introducing shifts into the measured frequency.

Researchers first cool the atoms close to absolute zero, minus 273.15 degrees Celsius, to reduce their movement. They then trap them using laser light separate from the laser that measures the clock transition.

Laser beams travelling in opposite directions overlap to create a standing wave, with alternating regions of stronger and weaker light. Under suitable conditions, atoms become trapped in the resulting array of tiny spaces. This array is the optical lattice.

Many trapped atoms can be measured together. Averaging their signals reduces random fluctuations more quickly than measuring a single atom repeatedly. Another leading optical clock design instead confines one electrically charged atom, or ion, using electric fields. Both approaches are part of the debate over the future second.

Japan's National Institute of Information and Communications Technology (NICT) says optical lattice clocks using strontium and ytterbium are being developed across national standards laboratories. Their role already extends beyond experiments: they help calibrate the international atomic timescale.

One atomic reference or a combination?

The central decision is whether the new second should rely on one optical transition or a combination of several. Strontium's strong performance makes it a candidate, alongside other atomic and ionic transitions.

Option 1 would follow the current model: choose one transition and give its frequency an exact numerical value. Its main attraction is simplicity. Laboratories would have a single reference, while clocks using other transitions could serve as secondary standards.

Choosing that transition would require international agreement among institutions developing different clocks. A much better reference emerging later could also create pressure for another redefinition.

Option 2 would define the second using a weighted geometric average of several optical frequencies. In simple terms, several atomic references would contribute to one defining value, with some given more influence than others. The mathematical average combines frequencies by multiplication and powers rather than ordinary addition.

The BIPM describes versions with fixed weights and versions in which the weights and participating transitions could change as technology improves. Such flexibility could accommodate better clocks without starting the entire redefinition process again. It would also require more complicated rules and make it harder for an isolated laboratory to reproduce the definition independently.

Japanese and French researchers offer a way to compare them

NICT and the Paris Observatory have developed a common mathematical and graphical framework for examining both options. Their December 2025 announcement says the work also proposes an objective way to calculate weights using measurement data collected worldwide.

The study, by Jérôme Lodewyck and Tetsuya Ido, appeared in Metrologia on November 14, 2025, under the title Properties of a definition of the SI second based on several optical transitions.

Their explanation begins with a useful distinction: nature determines the ratio between two atomic frequencies, while people choose the unit used to express them. A graph can show the measured frequency ratio as one line and the chosen definition as another. Their intersection determines the numerical frequencies.

Within this framework, a definition based on one transition becomes a special case of the combined approach: one transition receives all the weight and the others receive none. Researchers can therefore compare the proposals within the same picture.

The proposed weighting method uses the collected frequency comparisons and their uncertainties rather than selecting only some results. NICT says this reduces subjective choices about which measurements should determine each transition's contribution.

The Consultative Committee for Time and Frequency (CCTF), which handles technical discussions on international time standards, had chosen neither option at its September 18 and 19, 2025, meeting. NICT said participants agreed that understanding of the combined approach had advanced enough to proceed with discussions on selecting a method.

Japan's optical clocks already help keep time

NICT's optical clock programme shows how the technology has moved into operational timekeeping. The strontium 87 transition at a wavelength of 698 nanometres became a secondary representation of the second in October 2006. NICT measurements began contributing to the internationally adopted strontium frequency value in September 2012.

In 2016, NICT generated a continuous time signal based on an optical clock. A hydrogen maser supplied a reliable ongoing signal, while optical clock measurements adjusted its frequency. Across a test lasting five months, the resulting second intervals were closer to the SI definition than those of Coordinated Universal Time (UTC).

NICT restarted this optical timescale in 2021, and it now contributes to the stability of Japan Standard Time. Combining a dependable continuous signal with precise optical measurements helps maintain accurate time without depending on uninterrupted connections to other time services.

The institute's NICT-Sr1 clock was recognised as a secondary frequency standard in November 2018. The following month, it began contributing data to calibrate International Atomic Time (TAI), which forms the basis of UTC and national timescales.

International comparisons have also expanded. In 2020, NICT compared its strontium clock with an ytterbium optical lattice clock at Italy's national metrology institute, INRIM, using very long baseline interferometry, a technique associated with radio astronomy. The experiment confirmed agreement between optical clocks on different continents.

From a laboratory room to a commercial clock

Shimadzu's commercial development account describes the engineering required to take optical lattice clocks outside specialist laboratories. When Katori approached the company in 2015, the equipment filled a large laboratory room.

An early goal was to compare clocks at ground level and on the observation deck of Tokyo Skytree, 450 metres higher. Engineers reduced the clock's volume to 920 litres, about one twentieth of its original size, for the experiment that began in 2018.

The clocks measured time passing 4.26 nanoseconds per day faster at the observation deck. A nanosecond is one billionth of a second. The difference reflected general relativity: clocks at different gravitational potentials run at slightly different rates.

Commercial development brought further demands. The system used six types of lasers, each requiring precise control. Shimadzu revised its size target from 400 litres to 250 litres in 2020 and developed automatic recovery functions to handle disruptions.

Yuya Sakai, an assistant manager in Shimadzu's Laser Applications Group who worked on the software, explained why automatic adjustment was necessary:

If the wavelength of any one of the six lasers shifts out of adjustment, it could prevent the system from functioning as a clock.

Shimadzu says its software can reset laser conditions and restore operation after environmental disturbances. The company reports inquiries from research institutions worldwide, while identifying longer laser life and greater resistance to vibration as areas needing further improvement.

Better clocks also need better connections

National time laboratories must compare their clocks to build a shared international timescale. A clock's precision is useful to the wider network only if the connection can carry that precision between institutions.

The BIPM says current time transfer through global navigation satellite systems, including GPS, is adequate for exploiting the best microwave standards over practical averaging periods of about a day. Optical clocks are 10 to 100 times more accurate, demanding better links.

Optical fibre connections have demonstrated the required performance over distances of around 1,000 kilometres. Connecting continents remains more difficult. Researchers are investigating advanced exchanges of signals in both directions, optical links through space and transportable optical clocks.

Existing caesium equipment would continue to have a role after redefinition. Caesium fountains would become secondary standards, their measurements would remain traceable through the BIPM's Circular T publication, and they could continue contributing to UTC.

The international clock pool already combines about 450 clocks, including commercial caesium clocks and hydrogen masers. Optical standards would improve the reference used to guide this network, while existing equipment continued operating.

The checkpoints before a possible 2030 decision

The updated international roadmap makes readiness, rather than the calendar alone, the deciding factor. Eight mandatory criteria and six supporting conditions cover matters including reliable optical standards, international comparisons, contributions to atomic time and distribution of the new unit to users.

The key stages are:

• 2022: The General Conference on Weights and Measures adopted a policy of working toward redefinition in 2030.
• 2026: The conference is scheduled to review progress and continued work toward a proposal.
• 2028 and 2029: The CCTF is to evaluate readiness, narrow the options and finalise a proposal.
• 2030: The conference could decide on the definition and its implementation date if mandatory criteria are met and consensus is reached.

The final definition and implementation date have not been settled. National authorities may also need to revise laws where legal time is tied to the existing definition.

Japan's contribution now spans the invention of the optical lattice approach, operational contributions to international time, commercial equipment and research on how to choose the definition itself. The next task is to turn those advances into a standard that laboratories worldwide can reproduce and share.

Key Points

• RIKEN, Shimadzu and the BIPM signed a collaboration agreement on May 12, 2026, to support practical use of optical lattice clocks.
• Shimadzu released the Aetherclock OC020 in April 2025, describing it as the first commercial optical lattice clock.
• The proposed new second would use optical frequencies, targeting an immediate accuracy improvement of 10 to 100 times.
• Scientists are considering one defining optical transition or a weighted combination of several transitions.
• Japan's optical clocks already contribute to international atomic time and the stability of Japan Standard Time.
• A decision is possible in 2030, subject to agreement and technical readiness, with implementation afterward.
• Existing caesium clocks would remain usable and continue contributing to international timekeeping.