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arXiv:2512.07346 · Nature

Lu+ optical frequency references with accuracy verified at the 19th digit

K. J. Arnold, Michael Dao Kang Lee, Qi Zhao, Qichen Qin, Zhao Zhang, N. Jayjong, M. D. Barrett

Published Sep 23, 2026• 1 citation
arXivHTMLPDF

Abstract

Optical atomic frequency references1 have far surpassed their microwave frequency predecessors, leading to an anticipated redefinition of the SI (International System of Units) second2. However, as the scientific community seeks consensus on a new standard, and several state-of-the-art optical standards now report evaluated fractional uncertainties below 10−18, verification by same-species comparisons to comparable levels remains an outstanding challenge. Here we report two 176Lu+ single-ion optical frequency references, each with evaluated fractional frequency uncertainty near 1 × 10−19, which are directly compared by correlation spectroscopy and demonstrate agreement, with a measured relative frequency difference of [−0.1 ± (5.7)stat ± (1.0)sys] × 10−19, where ‘stat’ and ‘sys’ indicate the statistical and systematic uncertainty, respectively. Our optical references have been comprehensively assessed with evaluated uncertainties below 10−18, supported by a same-species comparison of independent systems to the 5.7 × 10−19 level. This accuracy, achieved in practical room-temperature systems, will contribute to improving international timekeeping and towards chronometric levelling3,4 at the millimetre scale, as well as tests of fundamental physics5 such as Lorentz invariance6, general relativity7, searches for dark matter8,9 and variation of fundamental constants10,11. Two independent 176Lu+ single-ion optical clocks achieve fractional frequency uncertainties near 10−19 and agree at 5.7 × 10−19, advancing precision timekeeping.

DOI
10.1038/s41586-026-11072-8

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Optical atomic frequency references1 have far surpassed their microwave frequency predecessors, leading to an anticipated redefinition of the SI (International System of Units) second2. However, as the scientific community seeks consensus on a new standard, and several state-of-the-art optical standards now report evaluated fractional uncertainties below 10−18, verification by same-species comparisons to comparable levels remains an outstanding challenge. Here we report two 176Lu+ single-ion optical frequency references, each with evaluated fractional frequency uncertainty near 1 × 10−19, which are directly compared by correlation spectroscopy and demonstrate agreement, with a measured relative frequency difference of [−0.1 ± (5.7)stat ± (1.0)sys] × 10−19, where ‘stat’ and ‘sys’ indicate the statistical and systematic uncertainty, respectively. Our optical references have been comprehensively assessed with evaluated uncertainties below 10−18, supported by a same-species comparison of independent systems to the 5.7 × 10−19 level. This accuracy, achieved in practical room-temperature systems, will contribute to improving international timekeeping and towards chronometric levelling3,4 at the millimetre scale, as well as tests of fundamental physics5 such as Lorentz invariance6, general relativity7, searches for dark matter8,9 and variation of fundamental constants10,11. Two independent 176Lu+ single-ion optical clocks achieve fractional frequency uncertainties near 10−19 and agree at 5.7 × 10−19, advancing precision timekeeping.
The extreme precision of optical atomic clocks has led to an anticipated redefinition of the second by the International System of Units. Furthermore, accuracies pushing the boundary of 1 part in 10 18 and beyond will enable new applications, such as in geodesy and tests of fundamental physics. The 1 S 0 to 3 D 1 optical transition in 176 Lu + has exceptionally low sensitivity to external perturbations, making it suitable for practical clock implementations with inaccuracy at or below 10 −18 . Here, we perform high-accuracy comparisons between two 176 Lu + references using correlation spectroscopy. A comparison at different magnetic fields is used to obtain a quadratic Zeeman coefficient of −4.89264(88) Hz/mT for the reference frequency. With a subsequent comparison at low field, we demonstrate agreement at the low 10 −18 level, statistically limited by the averaging time of 42 hours. The evaluated uncertainty in the frequency difference is 9 × 10 −19 and the lowest reported in comparing independent optical references.
International atomic clock comparison achieving 5×10−185\times 10^{-18}5×10−18 uncertainty
Two single-ion optical clocks, located in the United Kingdom and in Germany, are compared remotely via a 2220\,km long optical fiber link. Both clocks are based on the 2S1/2(F=0)↔2F7/2(F=3){}^2S_{1/2} (F=0)\leftrightarrow {}^2F_{7/2} (F=3)2S1/2​(F=0)↔2F7/2​(F=3) electric octupole (E3) transition in 171Yb+^{171}\text{Yb}^+171Yb+. Using more than 10610^6106 seconds of measurement data, we determine a fractional frequency difference of 1.0(4.8)×10−181.0(4.8)\times 10^{-18}1.0(4.8)×10−18, demonstrating agreement of the independently developed systems. To our knowledge, this constitutes the first international clock comparison reaching a fractional uncertainty below 5×10−185\times 10^{-18}5×10−18, an important milestone towards an anticipated optical redefinition of the SI second. The measurement uncertainty is dominated by that of the relativistic redshift correction between the two sites, which includes time-varying gravity potential differences. We exploit the large baseline between the two clocks to search for violations of Lorentz invariance. In the absence of statistically significant modulations, we improve constraints on the Robertson-Mansouri-Sexl parameter ααα by approximately a factor of five in the uncertainty, and provide limits on corresponding parameters within the Standard-Model Extension framework.
We report high-precision frequency ratio measurements between optical atomic clocks based on ^{27}Al^{+}, ^{171}Yb, and ^{87}Sr. With total fractional uncertainties at or below 3.2×10^{-18}, these measurements meet an important milestone criterion for redefinition of the second in the International System of Units. Discrepancies in ^{87}Sr ratios at approximately 1×10^{-16} and the Al^{+}/Yb ratio at 1.6×10^{-17} in fractional units compared to our previous measurements underscore the importance of repeated, high-precision comparisons by different laboratories. A key upgrade from our previous work is the use of a common ultrastable reference delivered to all clocks via a 3.6-km phase-stabilized fiber link between two institutions, enabling better accuracy and stability in frequency transfer. Derived from a cryogenic single-crystal silicon cavity, this reference improves comparison stability by a factor of 2-3 over previous systems, with an optical lattice clock ratio achieving a fractional instability of 1.3×10^{-16} at 1 s. Identifying individual clock stabilities with a three-corner-hat measurement, we demonstrate the most stable optical lattice and trapped-ion clocks used in a multispecies comparison. By enabling faster comparisons, this stability will improve sensitivity to nonwhite noise processes and other underlying limits of state-of-the-art optical frequency standards.
Atomic clock frequency ratios with fractional uncertainty ≤3.2×10−18\leq 3.2 \times 10^{-18}≤3.2×10−18
We report high-precision frequency ratio measurements between optical atomic clocks based on 27^{27}27Al+^++, 171^{171}171Yb, and 87^{87}87Sr. With total fractional uncertainties at or below 3.2×10−183.2 \times 10^{-18}3.2×10−18, these measurements meet an important milestone criterion for redefinition of the second in the International System of Units. Discrepancies in 87^{87}87Sr ratios at approximately 1×10−161\times10^{-16}1×10−16 and the Al+^++/Yb ratio at 1.6×10−171.6\times10^{-17}1.6×10−17 in fractional units compared to our previous measurements underscore the importance of repeated, high-precision comparisons by different laboratories. A key innovation in this work is the use of a common ultrastable reference delivered to all clocks via a 3.6 km phase-stabilized fiber link between two institutions. Derived from a cryogenic single-crystal silicon cavity, this reference improves comparison stability by a factor of 2 to 3 over previous systems, with an optical lattice clock ratio achieving a fractional instability of 1.3×10−161.3 \times 10^{-16}1.3×10−16 at 1 second. By enabling faster comparisons, this stability will improve sensitivity to non-white noise processes and other underlying limits of state-of-the-art optical frequency standards.
A multi-ion optical clock with 5×10−19\mathbf{5 \times 10^{-19}}5×10−19 uncertainty
Today's most accurate clocks are based on laser spectroscopy of electronic transitions in single trapped ions and feature fractional frequency uncertainties below 1×10−181\times10^{-18}1×10−18. Scaling these systems to multiple, simultaneously interrogated ions reduces measurement times, driving recent advances in multi-ion clocks. However, maintaining state-of-the-art systematic uncertainties while increasing the number of ions remains a central challenge. Here, we report on a multi-ion optical atomic clock with a fractional frequency uncertainty of 5.3×10−195.3\times10^{-19}5.3×10−19 and up to 10 \Sr ions. Ion-resolved state detection enables minimization of position-dependent shifts, with residual effects suppressed below the 10−2010^{-20}10−20-level. Clock operation with eight to ten ions reduces the measurement time by a factor of 4.8 compared to single-ion operation. A comparison with an established \Yb single-ion clock yields an unperturbed frequency ratio of 0.6926711632159660405(20)0.6926711632159660405(20)0.6926711632159660405(20), with a statistical uncertainty of 0.9×10−180.9\times10^{-18}0.9×10−18 and a combined uncertainty of 2.9×10−182.9\times 10^{-18}2.9×10−18. These results demonstrate robust multi-ion clock operation with reduced averaging time and state-of-the-art accuracy.
Optical atomic clocks based on laser-cooled trapped ions and atoms have advanced rapidly over the past decade. With fractional frequency uncertainties now surpassing 10 −18 , they are some of the most precise measurement tools ever built. Yet researchers are still pursuing new avenues of research to explore the fundamental limits to their stability, accuracy, and reproducibility. In this mini-review, we provide a survey of the current state of the art by describing the fundamental principles and techniques that underpin this progress, the architectures used to realize optical clocks, and the supporting laser technologies that are essential to their operation. We also examine the progress that has been made toward a redefinition of the second in the International System of Units and the inclusion of optical clocks into the global time and frequency metrology infrastructure. Finally, we discuss emerging applications of optical clocks and look at the prospects for making their precision more readily accessible to end users.