Scientists in Singapore have built an atomic clock that has reached the highest reported accuracy among optical atomic clocks.
The lutetium-based system measured time to 19 decimal places, with an uncertainty of just 1 x 10⁻¹⁹.
The results, published in Nature on September 23, mark a new measurement record for precision timekeeping.
The clock was developed by researchers at the Centre for Quantum Technologies (CQT) at the National University of Singapore.
The team has worked with lutetium for more than a decade, studying whether its atomic properties could support extremely precise timekeeping. Team leader Murray Barrett, a CQT principal investigator and associate professor of physics at NUS, said the measurements give the group confidence that its system is now the most accurate clock of its kind.
Atomic clocks measure time using changes in the energy state of atoms. A laser is tuned to a specific atomic transition, and the regular oscillations of the laser light provide a highly stable reference for measuring time. This method has been used for decades, with cesium serving as the basis of the international definition of the second since the 1960s.
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Cesium clocks remain central to modern timing systems. They support technologies such as satellite navigation and help synchronize communication and transport networks. Scientists, however, have been developing optical clocks based on other elements because they operate at much higher frequencies and can measure time with greater precision.
Elements such as ytterbium, strontium and aluminium have already produced leading optical clock systems. Data from these clocks are being considered by international standards bodies as part of work toward a possible new definition of the second in or after 2030. The Singapore research adds lutetium to this effort with a new level of measured accuracy.
Why Lutetium Matters
The CQT team chose lutetium because its atomic transition is less sensitive to changes in temperature and magnetic fields. Such environmental changes can shift the frequency used by an atomic clock and introduce small measurement errors. Lutetium’s relative stability allows researchers to maintain high accuracy across a wider range of conditions.
The researchers measured the frequency of their lutetium clock to 19 decimal places. They reported an uncertainty of 1 x 10⁻¹⁹, which they identify as the lowest uncertainty reported for an optical atomic clock. The team also developed a technique called hyperfine averaging to define the clock transition and improve the reliability of its measurements.
Each clock uses a single charged ¹⁷⁶Lu⁺ ion. The clock transition is linked to a laser operating at a wavelength of 848 nanometres. Researchers have spent more than 10 years refining the equipment and studying the properties that make lutetium suitable for precision timekeeping.
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Two Clocks Test Accuracy
The researchers did not rely on a single clock to establish their result. They built two lutetium clocks and compared their measurements over 200 hours using a method known as correlation spectroscopy. The two systems agreed to within an uncertainty of 5.7 x 10⁻¹⁹, which the researchers describe as the most precise clock comparison reported to date.
Comparing two clocks is an important test of reproducibility. Kyle Arnold, a senior research scientist at CQT and joint first author of the study, said that comparing independent clocks provides a way to test whether a time standard produces the same result repeatedly. The agreement between the two systems strengthened the team’s measurement of the clock’s performance.
At this level of precision, gravity itself becomes a factor. Optical clocks operating around the 10⁻¹⁹ level can detect differences in the rate at which time passes over height differences of only a few millimetres.
The Singapore team found that its comparison was sensitive enough to resolve a height difference of about 5 millimetres between the two clocks on the same table.
Researchers, therefore, measured the positions of the two lutetium ions independently and determined their height difference to within a millimetre. Differences in Earth’s gravitational field are not yet mapped accurately enough to make simple comparisons between clocks operating at this level in different locations.
From Laboratory To Field
The researchers now want to move the technology beyond a laboratory setting. Michael Lee, a joint first author and Ph.D. student at NUS, said the next stage is to reduce the size of the laboratory-scale system and develop a transportable version. The team expects that miniaturization can be achieved without giving up the clock’s measured accuracy.
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More accurate clocks have uses beyond setting international time standards. They can help scientists study fundamental physics, detect small changes in gravity and improve measurements of Earth’s gravitational field. Their results are also relevant to the international effort to determine whether the definition of the second should eventually move from cesium-based standards to optical atomic clocks.
The Singapore system, therefore, represents another step in the development of next-generation time standards. Further testing and comparisons with other leading optical clocks will help determine how lutetium performs against systems based on different elements. The team’s planned transportable version may also allow the technology to be tested in new environments and used for measurements outside the laboratory.
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