Take a second and divide it into a trillion pieces. Then divide each of those again. A clock good enough to count what is left is no longer an instrument. It is a window onto physics itself.
Researchers at Singapore’s Centre for Quantum Technologies (CQT) have now built the most accurate atomic clock ever reported, verified to the 19th decimal place of its frequency. The results were published in Nature on 23 September 2026, and they mark the first time a clock based on lutetium has entered the race to redefine the second.
“I am confident that what we have now is the most accurate clock in the world,” says Murray Barrett, Associate Professor at the National University of Singapore and Principal Investigator at CQT, who led the team. “In the future, I just don’t see how this clock can be beat.”
What the Team Actually Achieved
The headline figure is a systematic uncertainty of 1 x 10-19, the lowest ever reported for an optical atomic clock. Systematic uncertainty is the hard floor on accuracy. It is the accumulated error from every known source of imprecision in the hardware itself, and unlike random noise, it cannot simply be averaged away. Beating it requires physically redesigning the instrument.
Just as importantly, the team did not simply build one very good clock and trust it. They built two, independently, and then measured the two against each other. Running for 200 hours using a technique called correlation spectroscopy, the pair agreed to an uncertainty of 5.7 x 10-19, the most precise comparison between two atomic clocks ever performed.
Each clock consists of a single trapped ion of the isotope lutetium-176, stripped of one electron. Its clock transition is matched to a laser tuned to 848 nanometres. The laser is locked to the ion, and the stability of that lock is the clock.
Why Two Clocks Matter More Than One
“There is a humorous saying that ‘A man with a watch knows what time it is. A man with two watches is never sure,'” says Dr Kyle Arnold, Senior Research Scientist at CQT and joint first author of the paper. “It basically tells you that the only way to test the accuracy of a standard is to compare clocks and demonstrate reproducibility.”
This is the difference between precision and accuracy. A single clock can be measured beautifully and still be wrong, because the error lives in the instrument. Two independently built clocks that agree have shown the error is not in the hardware.
Why Lutetium, and Why It Took a Decade
Lutetium is an unusual choice, and that is precisely the point. It is the heaviest and least reactive member of the lanthanide series of rare earth elements, with a relative atomic mass of 174.97. That bulk buys it two practical advantages for timekeeping.
- Insensitivity to magnetic fields. Magnetic shifts nudge the frequency of a clock transition. Lutetium’s electron states barely move when the surrounding field changes.
- Insensitivity to temperature. Blackbody radiation from warm surfaces shifts many optical clocks. Lutetium’s transition is unusually resistant to it.
“The good properties mean that high accuracy can be achieved even in a wide range of environments,” Barrett says. “The lutetium clock would be stable even if you went from the hottest place recorded on Earth in Death Valley to the coldest place in the Antarctic plateau.”
The downside is scarcity. Lutetium is the rarest of the naturally occurring rare earth elements, and until this result, CQT was, to the team’s knowledge, the only group in the world working on it for timekeeping. Ten years of precision engineering went into the apparatus, including inventing a method called hyperfine averaging to define which internal transition would serve as the clock signal in the first place.
How Atomic Clocks Work
The underlying idea is elegant. Every atom has a fixed, universal oscillation built into its structure, a kind of natural pendulum at the quantum level. An atomic clock measures that oscillation and counts swings to keep time.
Since 1967, the second has been defined by caesium. Specifically, it is 9,192,631,770 transitions of the hyperfine state of the caesium-133 atom, probed by microwaves. That definition is what keeps GPS satellites, mobile networks, power grids, and financial trading systems synchronised. It is also, for all its elegance, a 1960s technology. The National Institute of Standards and Technology explains the process in detail and why researchers keep pushing past it.
Optical clocks replace microwaves with visible light. Light oscillates tens of thousands of times faster than microwaves, so each second contains far more cycles to count. Ytterbium, strontium, and aluminium-ion clocks have all recently taken the lead, each improving on the last in different ways.
The Records This Clock Broke
- Previous best optical clock: a cryogenic calcium-ion design reported at 4.4 x 10-19 by the Chinese Academy of Sciences in February 2026, which had to be cooled to liquid nitrogen to reach that figure.
- Previous optical clock accuracy record: a NIST aluminium-ion clock at 5.5 x 10-19, published in Physical Review Letters in July 2025. Ytterbium and strontium designs have also recently taken the lead.
- Previous best clock comparison: superseded by the 5.7 x 10-19 two-clock agreement reported here.
Lutetium beats the previous best by roughly a factor of four, which is a large margin in a field where progress is normally measured in incremental improvements. The researchers at CQT note that more measurement time could reduce the uncertainty further. It also reaches that accuracy at room temperature, where the calcium record required cooling the apparatus to liquid nitrogen.
The Millimetre Problem
Here is where the story stops being abstract. Clocks accurate at the 10-19 level are so sensitive that they detect the slowing of time caused by gravity across height differences of just a few millimetres. The CQT team’s own comparison could resolve a 5 mm height difference between two clocks sitting on the same table.
That sensitivity is a gift for a field called relativistic geodesy, which uses clocks to map gravitational variations across the Earth’s surface. It is also a headache for metrology. To be sure the 5.7 x 10-19 figure measured the clocks rather than the table they sat on, the team separately measured the height difference between the two trapped ions to sub-millimetre precision.
One limitation remains. The team would like to compare their lutetium clock against the best optical clocks in other laboratories, but the difference in gravitational field strength between distant locations is not known well enough to support a comparison at this level. Until someone builds a system that can measure the Earth’s gravity field to the necessary precision, the comparison stays within one laboratory.
What Redefining the Second Would Mean
Caesium has defined the second for nearly six decades, and it will not be replaced lightly. The formal process for revising the second is slow by design, and any change requires multiple independent laboratories to demonstrate results that agree, followed by years of scrutiny.
A redrawn definition would likely tie the second to a fixed count of optical transitions, in the same way the present one ties it to caesium. The gain would be stability: an optical standard is theoretically far more stable than its microwave predecessor. The risk is that the new definition would be accurate to a level nobody can verify on a routine basis, which is exactly why the community insists on repeated clock-to-clock comparisons before the change is made.
Michael Lee, joint first author and a PhD student on the team, says the next step is to shrink the laboratory apparatus into something transportable. “The next step is to take the lab-scale clock and miniaturise it into a transportable system,” he says. The team expects it can be made smaller without losing accuracy. A portable clock at this level of precision would let it be compared directly against the best clocks in the world and eventually taken out of the laboratory entirely.
Why This Record Actually Matters
Optical clocks are not merely improving the measurement of time. They are instruments for fundamental physics. Only at this level of precision can experiments seriously attempt to test general relativity against quantum mechanics, search for the minute changes in constants predicted by some models of new physics, and measure whether the Earth’s gravitational field varies in the way current theory says it should.
The broader shift here is worth noting. Quantum sensing is quietly becoming a practical field, and a clock that holds its accuracy across a temperature range from Death Valley to the Antarctic plateau is a fundamentally different kind of instrument from one that must be thermally stabilised to the millikelvin. Quantum sensing’s wider promise extends well beyond timekeeping, into navigation without GPS, mineral detection and medical imaging. Related advances in single-photon detection and cancer treatment monitoring show how the same underlying principles translate into entirely different applications.
The complete paper, Lu+ optical frequency references with accuracy verified at the 19th digit, is published in Nature with the full error budget and characterisation data.
Whether or not the second is ultimately redefined around an optical transition, a clock that measures time to 19 decimal places is a reminder of how much precision is left to extract. The universe is 13.8 billion years old. At this clock’s accuracy, it would not have gained or lost a single second since the Big Bang.
