Two room-temperature lutetium clocks agree at the 19th decimal place

Independent clocks based on single lutetium ions showed a relative frequency difference consistent with zero at this level of precision.

Editorial illustration — not from the study.

Researchers compared two independent optical clocks, each using a single lutetium-176 ion. The clocks showed a measured relative frequency difference of [−0.1 ± 5.7 statistical ± 1.0 systematic] × 10⁻¹⁹, indicating agreement at roughly the 19th decimal place.

Each clock had an evaluated fractional frequency uncertainty near 1 × 10⁻¹⁹, while the same-species comparison supported agreement at the 5.7 × 10⁻¹⁹ level. The systems operate at room temperature and could contribute to international timekeeping and measurements of height differences at the millimetre scale using clocks.

The clocks agree

The study reports two independent ¹⁷⁶Lu⁺ single-ion optical clocks with evaluated fractional frequency uncertainties near 1 × 10⁻¹⁹. A direct comparison using correlation spectroscopy measured their relative frequency difference as [−0.1 ± (5.7)stat ± (1.0)sys] × 10⁻¹⁹. The result supports agreement between the clocks at the 5.7 × 10⁻¹⁹ level, with statistical and systematic uncertainty reported separately.

Why matching clocks matters

Optical clocks have surpassed older microwave-based standards in precision, and the international community is considering a future redefinition of the SI second. Agreement between independent clocks using the same atom is an important check for such standards. The authors also say these room-temperature systems could aid international timekeeping, millimetre-scale measurements of height differences, and tests of fundamental physics, including general relativity and searches for changes in fundamental constants or dark matter.

Evidence and limits

This is a direct comparison of two independent single-ion clocks based on the same lutetium isotope, with each system’s uncertainty evaluated and the comparison performed using correlation spectroscopy. The reported result applies to these two clock systems and their stated uncertainty budget; the abstract does not establish how the performance will transfer to other clock designs or operating conditions. The abstract also does not describe a larger network of clocks or a new definition of the second.

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