Physicists at the University of Science and Technology of China have built a strontium optical lattice clock with a systematic uncertainty of 9.2 times 10 to the negative 19th power, a level of precision that places it among the most accurate timekeeping instruments ever constructed. The result, detailed in a preprint accepted by Metrologia, adds a significant Chinese entry to the short list of clocks capable of replacing the aging cesium-based definition of the second. With multiple laboratories worldwide now operating at similar performance thresholds, the question is no longer whether optical clocks can outperform cesium, but how fast the international metrology community can agree on a successor.
Inside the USTC Sr1 Clock’s Performance
The USTC Sr1 clock achieves its record-setting precision through careful control of the systematic effects that limit all optical lattice clocks. The team refined its modeling of blackbody radiation shifts and optimized the lattice cavity beam waist, two technical improvements that together drove the systematic uncertainty to 9.2 times 10 to the negative 19th. The clock also demonstrated frequency stability better than 1 times 10 to the negative 18th at 30,000 seconds of averaging time, meaning that over roughly eight hours of continuous operation, its tick rate holds steady to an extraordinary degree.
To put those numbers in plain terms: this clock would neither gain nor lose a full second over a span far exceeding the current age of the universe. The practical significance, though, is not cosmic timekeeping but the ability to detect tiny differences in gravitational potential, frequency offsets between distant clocks, and other physical quantities that matter for navigation, geodesy, and fundamental physics research.
How USTC Stacks Up Against Global Rivals
China is not the only country pushing optical clock performance into the sub-10-to-the-negative-18 range. A NIST aluminum-ion clock reported a systematic uncertainty of 5.5 times 10 to the negative 19th, making it the current record holder for the most accurate clock in the world. The USTC result, at 9.2 times 10 to the negative 19th, trails the NIST ion clock but operates on a fundamentally different platform. Optical lattice clocks trap thousands of neutral atoms simultaneously, while ion clocks work with a single trapped ion. Each architecture carries different strengths: lattice clocks tend to reach better short-term stability because they average over many atoms at once, while ion clocks can achieve extremely low systematic uncertainty in a simpler trapping environment.
Within China itself, the USTC clock is not an isolated effort. A separate strontium lattice clock at the National Time Service Center of the Chinese Academy of Sciences reported a total systematic uncertainty of approximately 1.96 times 10 to the negative 18th. That same NTSC group has also performed absolute frequency measurements of the strontium transition, reporting a value of 429,228,004,229,872.91(18) Hz by linking their clock to International Atomic Time. These cross-validated measurements from multiple Chinese institutions create an internal consistency check that strengthens any future claim for inclusion in international frequency standards.
What the Redefinition Roadmap Demands
Building a clock that outperforms cesium is necessary but not sufficient to change the definition of the second. A consensus roadmap published in Metrologia volume 61 lays out mandatory criteria that the metrology community must satisfy before any redefinition can proceed. These include demonstrating the maturity of optical frequency standards, establishing reliable contributions to Coordinated Universal Time and International Atomic Time, developing robust comparison methods between distant clocks, and resolving geopotential and gravitational redshift constraints so that clocks at different elevations can be meaningfully compared.
The gravitational requirement is particularly demanding. General relativity dictates that a clock at a lower elevation ticks slightly slower than one at a higher elevation. For clocks operating at the 10-to-the-negative-18 level, even a one-centimeter difference in height produces a measurable frequency shift. Before the second can be redefined, the international community needs agreement on the gravitational potential at each clock’s location to a precision that matches the clocks themselves. That challenge is as much a problem of geodesy as it is of atomic physics.
The CIPM Frequency List and the Path to Consensus
The mechanism for building international agreement runs through the Consultative Committee for Time and Frequency, which maintains a list of recommended values of standard frequencies under the authority of the International Committee for Weights and Measures. A 2021 update to that list, analyzed in a detailed preprint on secondary representations, describes how recommended frequency values are computed through data selection, uncertainty treatment, and consistency checks across laboratories worldwide. Strontium and ytterbium optical clocks are identified as central candidates for a future definition of the second.
The consistency-check process is where the proliferation of high-performance clocks in multiple countries becomes strategically important. A redefinition built on measurements from only one or two laboratories would lack the cross-validation that the metrology community requires. China’s growing roster of sub-10-to-the-negative-18 clocks, at both USTC and NTSC, could help mitigate single-lab biases in the global frequency data. The more independent measurements that agree, the stronger the case for adopting a new value.
Why Faster Oscillations Mean Better Clocks
The fundamental advantage of optical clocks over the current cesium standard comes down to frequency. Cesium clocks define the second by counting 9,192,631,770 oscillations of microwave radiation. Optical clocks, by contrast, use electronic transitions in atoms such as strontium and ytterbium that oscillate hundreds of thousands of times faster, in the visible or near-infrared part of the spectrum. Because each “tick” is shorter, a given measurement interval contains many more cycles, and statistical fluctuations average out more effectively. That allows optical clocks to reach both lower statistical uncertainty and finer resolution when comparing time and frequency.
In the USTC Sr1 system, thousands of strontium atoms are trapped in an optical lattice formed by interfering laser beams. The lattice holds the atoms nearly motionless, suppressing Doppler shifts that would otherwise blur the resonance. A highly stable laser probes a narrow optical transition, and feedback electronics lock the laser frequency to the atomic line. The result is a local oscillator whose frequency is defined by the atoms themselves rather than by any human-made resonator. By counting these optical cycles and dividing down to lower frequencies, the clock can produce time signals compatible with existing microwave-based systems while surpassing them in precision.
As more laboratories achieve similar performance, the role of international data repositories becomes more prominent. Many of the technical details of the USTC and NTSC clocks, as well as the analyses underlying the CIPM frequency recommendations, are disseminated through preprint servers supported by research institutions. These platforms allow rapid sharing of experimental methods and uncertainty budgets, enabling other groups to replicate and challenge reported results. Community-backed initiatives that invite scientists and the public to help sustain open-access dissemination indirectly accelerate progress toward a redefined second by keeping cutting-edge metrology work broadly accessible.
For now, the cesium definition remains legally binding, and optical clocks like USTC’s Sr1 function as “secondary representations” whose measured frequencies are compared against the official standard. But the performance numbers now being reported, systematic uncertainties in the low-10-to-the-negative-19 range and stabilities that reveal relativistic effects over laboratory length scales, indicate that the field has crossed a qualitative threshold. The next steps will depend less on any single record-setting device and more on coordinated international efforts to reconcile data sets, refine geodetic models, and ensure that the benefits of optical timekeeping can be integrated into global navigation, communications, and scientific infrastructure. When the second is finally redefined, clocks like Sr1 will have helped lay the groundwork, not just by keeping time, but by helping the world agree on what time is.
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*This article was researched with the help of AI, with human editors creating the final content.
