Researchers at the National University of Singapore compared two optical clocks based on separately trapped lutetium ions. Across 11 measurements totaling 200 hours, their weighted-mean fractional-frequency difference was [−0.1 ± 5.7 (statistical) ± 1.0 (systematic)] × 10−19.
A fractional-frequency difference expresses an offset relative to the clocks’ frequency. Here the central value is smaller than the statistical uncertainty, so the comparison did not resolve a difference between the two references.
An optical clock uses a laser locked to a specific change in an atom’s energy to provide a stable frequency reference. The two systems each interrogated a singly ionized lutetium-176 atom in a separate trap and used the same 848-nanometer transition.
The team compared the clocks using correlation spectroscopy. Both ions were interrogated with a shared laser, and correlations between the measurements helped reject noise common to both. This reduced the laser’s fluctuations as a limit on how closely the references could be compared.
The comparison is consistent with the uncertainty estimates the NUS team assigned to each system. Both clocks were tested within one research program, so the result is not an independent cross-laboratory validation.
Sources
- Nature: “Lu+ optical frequency references with accuracy verified at the 19th digit” (published September 23, 2026; DOI: 10.1038/s41586-026-11072-8).
- Illustrative image: NIST mercury-ion trap, photo by J. Bergquist and D. Wineland/NIST. The photograph shows a mercury-ion apparatus, not the lutetium clocks in the study. NIST reuse and credit information.