Singapore has moved to the front of one of modern physics’ quietest but most consequential races: the effort to measure time so precisely that the definition of a second, the mapping of Earth’s gravity and tests of fundamental physics can all be pushed into a new regime. Researchers at the Centre for Quantum Technologies and the National University of Singapore reported on September 23 that two optical atomic clocks based on singly charged lutetium-176 ions reached an evaluated fractional frequency uncertainty of about 1 × 10−19. In practical terms, that is a level of accuracy at which the clock’s rate is sensitive to gravitational differences associated with height changes measured in millimetres. The work, published in Nature, does not mean that tomorrow’s phones, satellites or financial networks will suddenly become orders of magnitude more precise. It does mean that a laboratory instrument has crossed a threshold that matters to the international system of measurement. The result gives metrologists another candidate technology as they prepare for a possible redefinition of the SI second around 2030 and gives physicists a more exact reference against which tiny effects in gravity, atomic structure and the laws of nature can be tested.
A record measured at the 19th digit
The central result is unusually easy to state and unusually difficult to achieve. The Singapore team measured the frequency of its lutetium clock with a systematic uncertainty at the level of one part in 1019, which the researchers describe as the lowest reported for an optical atomic clock. Nature’s accompanying news report characterized the device as the world’s most accurate timekeeper and said it is roughly four times more accurate than the previous best clock based on calcium ions. The comparison is important because modern clocks are no longer judged simply by how long they can run before appearing to drift. They are judged by an uncertainty budget: a painstaking accounting of every known physical effect that can push an atom’s transition frequency away from its ideal value.
That is why the headline number is not a marketing claim about a wristwatch that would “lose a second” only after an almost unimaginable period. It is a statement about fractional frequency uncertainty. A clock with an uncertainty near 10−19 can, in principle, discriminate one part in ten quintillion. Popular descriptions translate that scale into hundreds of billions of years per second of error because the analogy is intuitive, but the scientific achievement is more specific: the clock transition has been characterized and controlled so well that known systematic shifts add up to an uncertainty at the nineteenth decimal place.
The study was led by researchers including Kyle J. Arnold, Michael D. K. Lee and Murray D. Barrett at the Centre for Quantum Technologies, a Research Centre of Excellence hosted by NUS. The group has pursued lutetium as a clock species for more than a decade. That long development matters because the new result is not a sudden proof that lutetium is inherently “better” than every other atomic clock element. Optical clock performance depends on the atom or ion, the transition selected, laser stability, environmental control, interrogation methods and the ability to understand residual shifts. The Singapore team’s contribution is to show that one carefully chosen lutetium-ion transition can be controlled, reproduced and compared at a level that places it among the strongest contenders in precision timekeeping.
How an optical atomic clock keeps time
Atomic clocks work because atoms offer remarkably repeatable quantum transitions. Electrons in an atom or ion can occupy only certain energy states. When light of exactly the right frequency interacts with the atom, an electron can be driven from one state to another. That transition frequency acts as a natural reference. A clock locks an oscillator—in modern optical clocks, a laser—to the transition and counts the resulting oscillations. The faster and narrower the transition, and the more completely external disturbances can be understood and controlled, the better the potential time reference.
The international second is still defined through caesium-133. Since 1967, the second has been tied to 9,192,631,770 periods of radiation corresponding to a specific hyperfine transition in the caesium atom; the 2018 revision of the International System of Units preserved that numerical definition in terms of the caesium frequency. Caesium clocks operate in the microwave domain. Optical clocks use much higher-frequency transitions, meaning they effectively provide many more oscillations—or “ticks”—within a second. That higher carrier frequency is one reason optical systems can resolve smaller fractional differences.
The NUS clocks use a single ion of the isotope lutetium-176, written 176Lu+. The relevant clock transition is interrogated with laser light at a wavelength near 848 nanometres. In a simplified picture, the researchers trap the ion, cool and control it, tune the laser to the transition, and then measure how environmental conditions and experimental choices alter the observed frequency. The simplicity of the “atom as reference” concept hides an extraordinary amount of engineering. Electric fields, magnetic fields, temperature, motion, collisions, laser noise and relativistic effects can all matter once an experiment is trying to claim accuracy at the nineteenth decimal place.
At that level, the laboratory itself becomes part of the instrument. A slight thermal change can alter blackbody radiation around the ion. A magnetic-field shift that is negligible in ordinary spectroscopy can become significant. Motion changes frequencies through relativistic effects. Even gravity cannot be treated as a distant theoretical correction: because clocks run at different rates at different gravitational potentials, the physical height of one clock relative to another must be known with exceptional care. Precision timekeeping therefore sits at the intersection of quantum physics, laser engineering, thermometry, electromagnetism, geodesy and relativity.
Why lutetium is attracting attention
Lutetium is a rare-earth element and not the obvious household name in atomic timekeeping. Other optical clock programs have built world-class systems around strontium, ytterbium, aluminium, mercury, calcium and related species. The attraction of lutetium is that its chosen clock transition is relatively insensitive to some of the environmental disturbances that dominate uncertainty budgets elsewhere. The NUS team highlights low sensitivity to temperature and magnetic-field variations as important advantages.
Temperature matters because all objects at ordinary temperatures emit blackbody radiation. That radiation can shift atomic energy levels and therefore alter the transition frequency used as a clock reference. In 2018, members of the same Singapore research program reported that one promising lutetium transition had an exceptionally small blackbody radiation shift at room temperature. A clock that is naturally less sensitive to thermal radiation does not eliminate the need to understand temperature, but it can reduce the size of one important correction and make high accuracy less dependent on elaborate cryogenic operation.
The new clocks also work at room temperature and use commercially available laser technology, according to the researchers. Those details are strategically important. A metrology demonstration can be scientifically spectacular and still be too fragile or complex for broad use. Technologies that depend on highly specialized lasers, extreme cryogenic environments or delicate custom equipment are harder to reproduce in other laboratories and harder to transport. Lutetium’s appeal is therefore not only its present uncertainty number but the possibility that comparable performance can eventually be packaged into a more robust system.
That promise still needs to be demonstrated outside the controlled NUS environment. The Singapore group says it intends to miniaturize the laboratory clock into a transportable instrument. Transportability is not a cosmetic engineering goal. The most accurate clocks have become so sensitive to gravitational potential that comparing them across distant laboratories is itself difficult. A clock that can travel while retaining its performance could be brought to another metrology institute, a geophysical site or a calibration facility, creating direct comparisons that are otherwise limited by the precision of long-distance time and frequency transfer.
Two clocks matter more than one
One of the strongest parts of the Nature paper is that the researchers did not rely on a single clock. They built two independent lutetium-ion systems and compared them. The measured difference between the two was consistent with zero within an uncertainty of roughly 5.7 × 10−19 from statistical effects, with a further systematic component near 1.0 × 10−19. NUS describes this as the most precise comparison of atomic clocks yet performed.
That comparison addresses a central problem in precision measurement: a beautifully analyzed instrument can still hide an unknown bias. If two independent devices based on the same atomic reference agree after their corrections are applied, confidence rises that the uncertainty budget is capturing the dominant effects. Reproducibility is not identical to absolute accuracy, and two clocks in one laboratory do not settle every question, but the experiment is much stronger than a result inferred from one device alone.
The distinction is especially important as the global time-and-frequency community prepares for a future optical definition of the second. Any new standard must be reproducible in multiple laboratories and transferable across national metrology systems. The Bureau International des Poids et Mesures, or BIPM, has emphasized that a new definition cannot rest only on the best clock in the world. It must support continuity with existing time scales, international comparisons, contributions to International Atomic Time and practical dissemination to users.
The Singapore result therefore solves one problem and exposes the next. Same-species agreement inside one laboratory is a major validation. Inter-laboratory agreement at comparable levels is harder because the clocks must either be connected through extremely precise optical links, compared through satellite or fibre transfer methods, or physically brought together. At 10−19-class performance, differences in gravitational potential between laboratories become a leading practical issue. The challenge is no longer simply building a better clock; it is building a measurement network good enough to compare the best clocks without the network becoming the limiting instrument.
Accuracy, stability and the language of record clocks
Atomic-clock records can be confusing because “accuracy,” “stability” and “precision” are often used interchangeably in general reporting even though metrologists distinguish them. Stability describes how consistently a clock’s frequency behaves over a given averaging time. Accuracy, more formally expressed through evaluated uncertainty, describes how closely the corrected frequency is believed to represent the unperturbed atomic transition. A clock can be highly stable yet still carry a systematic offset; conversely, a clock with an excellent uncertainty budget may need long averaging times before statistical noise falls low enough to reveal that accuracy in a comparison.
The Singapore result is notable because the team addressed both sides of that problem. It evaluated systematic shifts in each lutetium reference and then compared two independent clocks to test whether their corrected frequencies agreed. The 1 × 10−19 figure refers to the evaluated fractional uncertainty of an individual clock, while the 5.7 × 10−19 figure describes the statistical uncertainty achieved in the direct comparison. Those numbers answer related but different questions, and neither should be reduced to a single slogan about a clock “never losing time.”
This distinction will matter increasingly as optical clocks approach the limits imposed not by one dominant perturbation but by a collection of ever smaller effects. At that frontier, progress can come from better atomic species, improved interrogation sequences, quieter lasers, more exact magnetic-field control, superior thermal characterization or longer comparisons. A new record may therefore reflect advances across an entire measurement system rather than one dramatic component.
It also explains why independent replication is so valuable. If another laboratory builds a lutetium clock with different hardware and reaches the same corrected frequency, confidence grows that neither group shares an unnoticed systematic error. In metrology, agreement is not a ceremonial confirmation after discovery; it is part of the discovery itself. The future SI second will have to be realized repeatedly, in different places, by different teams. A clock that cannot survive that test, however impressive its first uncertainty budget, cannot serve as a universal standard.
A clock precise enough to feel gravity
Einstein’s general theory of relativity predicts that clocks at different gravitational potentials tick at different rates. A clock lower in a gravitational field runs slightly slower than one higher up. For ordinary clocks the effect is invisible. For the best optical clocks, it is measurable over surprisingly small height differences. The NUS researchers say the precision of their comparison corresponds to sensitivity to a height difference of about five millimetres between the two clocks on the same table.
That does not mean the experiment mapped a five-millimetre change in terrain or that optical clocks are ready to replace conventional surveying instruments. The comparison was performed under carefully controlled laboratory conditions, and the researchers independently measured the relative height of their trapped ions to below the millimetre level so that gravity would not contaminate the clock comparison. The result instead demonstrates the scale at which relativity must now be treated as an engineering parameter.
The same sensitivity points toward “chronometric geodesy,” the use of clocks to measure differences in gravitational potential. Conventional geodesy combines satellite measurements, gravimeters, spirit levelling and models of Earth’s gravity field. Optical clocks offer a complementary technique: compare two clock frequencies, infer the difference in gravitational potential, and relate that difference to height or mass distribution. In principle, networks of sufficiently portable and stable optical clocks could help monitor changes associated with groundwater, ice, volcanic systems or tectonic processes.
The word “principle” is crucial. Turning laboratory clocks into geophysical instruments requires portability, uptime, reliable fibre or free-space links, environmental robustness and an accurate treatment of local gravity. It also requires cost and complexity to fall. The Singapore team’s plan to develop a transportable lutetium clock is therefore not merely an attempt to commercialize a laboratory trophy. It addresses one of the central conditions for moving optical timekeeping from metrology institutes into the field.
The race to redefine the second
The NUS paper lands at a consequential moment for international metrology. The BIPM’s Consultative Committee for Time and Frequency has been working for years on a roadmap to update the definition of the second. The current plan allows for a possible redefinition around 2030, though the process is deliberately cautious and the timetable depends on technical criteria being met. The General Conference on Weights and Measures must ultimately approve any change.
The motivation is straightforward: several optical frequency standards have already surpassed the best practical realizations of the caesium definition by large margins. BIPM guidance says a future optical definition should offer an immediate improvement of roughly ten to one hundred times while preserving continuity with the existing second. The new system must also be reproducible across national metrology institutes, support International Atomic Time and Coordinated Universal Time, and be disseminated without creating a standard that only a few elite laboratories can realize.
What has not been decided is equally important. A redefined second could be based on one selected optical transition, or the community could adopt an approach involving an ensemble of reference transitions. The international process is not a contest in which the latest record holder automatically becomes the new SI standard. Longevity, reproducibility, reliability, transfer methods, independent confirmation and broad institutional support matter as much as a single uncertainty figure.
That is why the Singapore result should be read as a strong new data point rather than the end of the race. Lutetium has demonstrated exceptional performance in one research program. Other clock species benefit from larger international communities, long comparison histories and existing roles as secondary representations of the second. The next phase will show whether lutetium’s apparent environmental advantages translate into repeatable performance in other laboratories and whether transportable implementations can retain the accuracy achieved at NUS.
Precision time is already critical infrastructure
The phrase “atomic clock” can sound esoteric, but timing already functions as hidden infrastructure across modern economies. Satellite navigation systems use atomic clocks to determine position from the travel time of radio signals. Telecommunications networks use precision timing to coordinate data transmission. Electrical grids depend on synchronized measurement and control. Financial markets timestamp transactions. Scientific facilities distribute stable frequencies across experiments. Transport, defence and space systems all depend on reliable time references.
None of those systems requires a 10−19-class clock at every node. In fact, the Singapore device is far beyond what routine networks need. The significance lies in the hierarchy of standards. National laboratories maintain the best references, compare them internationally and use them to calibrate lower-level clocks and time scales. Improving the top of that hierarchy can gradually improve the confidence, resilience and traceability of the systems beneath it.
There is also a technological spillover effect. Building an optical clock forces advances in ultra-stable lasers, vacuum systems, photonics, frequency combs, control electronics, quantum state preparation and environmental sensing. Those capabilities can migrate into other areas of quantum technology. The value of precision metrology has historically extended beyond the immediate measurement record because the tools needed to reach the record become technologies in their own right.
But the path from record-setting experiment to infrastructure is long. The NUS clock remains a laboratory system. It has not been deployed as a national time standard, has not replaced caesium and has not demonstrated continuous operation under the conditions demanded by telecommunications or navigation. The researchers’ next objective—miniaturization into a transportable clock—is therefore an engineering test of whether extraordinary accuracy can coexist with robustness and usability.
A new instrument for testing fundamental physics
Extreme clocks are also precision probes of the laws of nature. If two different atomic transitions are compared over time, physicists can search for tiny variations that might indicate changes in fundamental constants. Clock networks can test aspects of general relativity by measuring gravitational redshift. Some theoretical models of dark matter predict oscillations or transient effects that could slightly alter atomic transition frequencies. A sufficiently stable network of dissimilar clocks could search for correlated anomalies that conventional detectors would not see.
The strength of this approach is that timekeeping converts subtle physics into a frequency measurement, one of the most precisely measurable quantities in science. The weakness is that extraordinary claims require extraordinary control of systematic effects. A shift that looks like new physics may instead come from temperature, magnetic fields, electric-field gradients, laser noise, motion or an error in comparing distant clocks. Every improvement in clock accuracy therefore raises both the opportunity for discovery and the standard of evidence required to claim it.
The lutetium result is valuable precisely because its scientific importance does not depend on finding exotic physics. Even if no dark matter signal or violation of relativity ever appears, better clocks improve tests of theories by shrinking the space in which deviations could hide. They also improve geodesy and metrology. In precision science, a null result can be powerful when the instrument is trusted.
Researchers will now want independent comparisons with other leading optical clocks based on different elements. Those cross-species measurements are scientifically rich because each atomic transition responds differently to environmental effects and to some hypothetical variations in fundamental constants. A network that includes lutetium alongside strontium, ytterbium, aluminium and other references could be more informative than any one clock species alone.
Why the result is a Singapore technology story
The achievement is also notable for where it happened. Singapore has spent years building a research base in quantum science through the Centre for Quantum Technologies, the National University of Singapore and national funding programs. Much of the international conversation about quantum technology concentrates on quantum computing, encryption and investment valuations. Precision measurement is less visible, yet it is one of the areas in which quantum control already produces measurable, reproducible scientific performance.
The NUS clock shows a different model of technological leadership from the race to build the largest processor or the most commercially valuable software platform. The team selected a comparatively underused atomic species, spent more than a decade characterizing it, built the control systems required to extract its advantages, and then validated the result with a second independent clock. The payoff is not a mass-market product but a new frontier in measurement capability.
That distinction matters because scientific infrastructure increasingly has geopolitical as well as academic value. Countries that operate leading clocks, quantum networks and measurement institutes participate directly in the standards that define global technology. They train specialists in photonics, atomic physics and control systems and can anchor collaborations that link laboratories across continents. If the second is redefined around 2030, the laboratories that have demonstrated optical-clock performance and international comparability will help shape how the new standard is realized in practice.
Singapore’s result does not by itself determine that outcome. But it gives the city-state a more visible place in a field traditionally associated with large national metrology institutes in the United States, Europe and East Asia. The next test is whether the lutetium program can move from a singular record to a reproducible platform that other institutions can compare, adopt or challenge.
What the record does not prove
Record-breaking science often attracts language that runs ahead of the evidence. Several limits should remain clear. First, the new clock does not show that lutetium will become the basis of the future SI second. The international process remains open and weighs far more than headline accuracy. Second, the two clocks were built and compared by the same research group. That is a powerful reproducibility check, but independent replication elsewhere would provide a stronger test of hidden common-mode biases.
Third, the laboratory’s 10−19-level uncertainty does not automatically transfer to long-distance comparisons. At these scales, time transfer, gravitational-potential knowledge and the stability of links between laboratories can become limiting factors. The global system needs not only clocks this good but ways to compare them without throwing away most of their advantage. Fibre-optic frequency links can achieve remarkable performance on continental networks, while transportable clocks offer another route, but international coverage remains uneven.
Fourth, practical applications such as improved navigation or geological monitoring require systems that are smaller, easier to operate and more robust than a leading metrology experiment. The NUS team has explicitly identified transportability as a next step, which is an acknowledgment that the present device is not yet a field instrument.
Finally, the familiar statement that the clock would take hundreds of billions of years to lose a second should not be interpreted literally as a durability test. No one has run it for geological time. The figure is an extrapolation of measured fractional uncertainty. It communicates scale, but the underlying scientific result is the evaluated and experimentally checked stability of the clock frequency over real measurement intervals.
The comparison problem may now be harder than the clock problem
For much of the history of atomic timekeeping, the central technical challenge was building a more accurate reference. That remains difficult, but the frontier is shifting. Once multiple laboratories can operate clocks with uncertainties in the low 10−18 or 10−19 range, the ability to compare them becomes equally important. A standard that cannot be compared internationally is of limited value as the basis of a global unit.
The BIPM has highlighted transportable optical frequency standards as one path toward solving this problem. In May, it announced a collaboration with RIKEN, Japan’s National Metrology Institute and Shimadzu to study the role of transportable optical standards in future comparisons. The stated goal is to support independent realizations of the second at the 10−18 level and reliable contributions to Coordinated Universal Time. That effort illustrates the broader transition now underway: metrology is moving from isolated record clocks toward an ecosystem of clocks, links and operational procedures.
A transportable lutetium system could fit naturally into that ecosystem if the Singapore group succeeds in preserving its accuracy outside the current apparatus. It could be moved to compare directly with another institute’s strontium or ytterbium clock, used to validate a fibre link, or deployed at a geophysical site. Each of those tasks would provide a different test of whether lutetium’s laboratory advantages survive real-world constraints.
The practical stakes are high because the redefinition of the second is meant to improve the global time system, not merely the prestige of precision laboratories. The new definition must be realizable, comparable and useful. The best clock in the world is therefore only one piece of the answer. The future standard will depend on a network.
From measuring time to measuring the world
The history of measurement repeatedly shows that better standards create applications that were difficult to imagine when the standards were invented. Mechanical clocks transformed navigation. Quartz timing reshaped electronics. Atomic clocks made satellite navigation practical. Optical clocks may eventually have their largest impact in fields that are still developing today.
Chronometric geodesy is one example. If portable optical clocks can compare gravitational potential directly, national height systems could be linked in new ways. Regions separated by mountains, oceans or unstable ground could be connected through frequency measurements rather than only by traditional levelling. Repeated measurements might help track slow redistribution of water or ice. Combined with satellites and gravimeters, clocks could add a new type of sensor to Earth science.
Another opportunity is resilient timing. Modern societies rely heavily on satellite navigation signals for synchronization, yet those signals can be jammed, spoofed or disrupted. More capable terrestrial and transportable atomic clocks could support holdover timing and local references when satellite signals are unavailable. A 10−19 laboratory clock is far beyond what most resilience applications require, but progress at the frontier can drive components and techniques toward more deployable systems.
The most speculative opportunities lie in fundamental physics. Clocks could be distributed across Earth or placed in space to test relativity over larger gravitational differences, search for transient signals or compare different atomic species with unprecedented sensitivity. Space deployment would introduce its own engineering challenges, but precision timing is already central to missions that measure gravity and test relativity. Better optical references would expand what such missions could ask.
A milestone, not an endpoint
The Singapore team’s achievement is best understood as a milestone in a transition already under way. The world is moving from microwave atomic time toward optical atomic time. The old definition based on caesium remains authoritative, but the best optical clocks can already resolve frequency differences far smaller than today’s primary standards. The open questions are no longer whether optical clocks can outperform caesium in the laboratory, but how the international community should define the new second, how multiple realizations should be compared and how the improvement should be delivered to the global time system.
Lutetium now has a stronger claim to be part of that discussion. The NUS clocks combine a low evaluated uncertainty with a same-species comparison at the nineteenth digit, and the element’s relatively low sensitivity to temperature and magnetic fields offers a credible engineering advantage. Those are demonstrated results. A portable lutetium clock, routine operation outside the laboratory, independent reproduction by other groups and a role in international timekeeping remain future goals.
For the broader public, the immediate effect will be invisible. No phone will display more meaningful decimal places of a second, and no navigation receiver will suddenly become millimetre accurate because of one Nature paper. The importance is deeper. Measurement standards are the foundations on which technology builds, and improving the foundation changes what later becomes possible.
At the nineteenth digit, time is no longer just a coordinate used to order events. It becomes a sensor for gravity, a test bench for quantum theory, a reference for global infrastructure and a candidate tool for measuring changes in the planet itself. Singapore’s lutetium clocks have not finished the race to redefine the second. They have made the race more interesting—and made the boundary between a clock and a scientific instrument harder to see.
