Yb/Sr optical clock comparison at 2.2E-18 uncertainty over a 1350-km fiber link
Authors: Lei Hou, Xiang-Pei Liu, Zhuan-Xian Xiong, Fa-Xi Chen, Wen-Xuan Song, Jian-Yu Guan, Qiang Zhu, Yu-Chen Zhang, Xiang Zhang, Bing Wang, De-Zhi Xiong, De-Quan Kong, Jia-Hao Zhang, Long-Fei Liao, Shao-Mao Wang, Xing-Yang Cui, Ping Xu, Peng Liu, Jie Li, Xiao Jiang, Ling-Xiang He, Li-Bo Li, Qi Shen, Youchao Xie, Lei Wang, Sheng-Kai Liao, Juan Yin, Cheng-Zhi Peng, Wen-Bin Shen, Bao-Long Lu, Han-Ning Dai, Yu-Ao Chen, Qiang Zhang, Hai-Feng Jiang, Jian-Wei Pan
Abstract
Redefining the International system of Units (SI) second requires independent frequency-ratio measurements between clocks based on different laboratories, with an overall uncertainty below 5E-18, to validate individual clock evaluations through consistency checks. Reaching this uncertainty over long distances has remained challenging. So far, comparisons at this level have been achieved only a few times, between co-located clocks, and their results disagree markedly. Previous high-precision comparisons have therefore been limited to local or kilometre-scale links. Here we compare a ytterbium optical lattice clock in Wuhan with a strontium optical lattice clock in Shanghai over a phase-stabilized fiber link spanning 1350 km, and determine the Yb/Sr frequency ratio to be 1.207 507 039 343 337 716 0(26), with a Bayesian statistical uncertainty of 1.14E-18 and a total uncertainty of 2.18E-18. To our knowledge, this is the first intercity long distance frequency-ratio measurement, whose total uncertainty reaches the level required by the redefinition roadmap. The measured ratio exhibits a significant correlation with Earth tides and its response agree with the model within the statistical uncertainty. At intercity scale, this comparison demonstrates the capability of an optical-clock network for frequency comparison and Earths gravitational-potential sensing, a key step toward a wide-area optical-clock network.
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International atomic clock comparison achieving $5\times 10^{-18}$ uncertaintyA. Tofful, M. Filzinger, T. Schmidt et al. · 2026 · Atomic Physics · AI summary available
The study compares optical clocks in separate countries over a long fiber link, measuring their frequency ratio at uncertainty below 5×10⁻¹⁸.
In short
Two optical clocks compared internationally over a 2220 km fiber link agree within 4.8 × 10⁻¹⁸ fractional uncertainty and tighten constraints on Lorentz-invariance violations.
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International optical clock comparison using the european optical fiber networkM. Pizzocaro, C. Zyskind, A. Amy-Klein et al. · 2026 · Physical Review Research · cited 8 times · Not summarized yet
The study compares seven optical clocks across European institutes over a fiber network to assess international frequency-ratio consistency.
Abstract
Optical clocks have achieved remarkable estimated fractional frequency uncertainties reaching the $10^{-18}$ level and below, enabling applications in fundamental physics, general relativity, and geodesy. However, the challenge of verifying the international consistency of optical clocks remains critical as efforts intensify toward redefining the SI second based on an optical transition or transitions. We report on a two-month international clock comparison campaign involving seven optical clocks in four national metrology institutes (INRIM, LNE-OP, NPL, and PTB) connected via the optical fiber network established in Europe. The campaign resulted in optical frequency ratios with uncertaintie…
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Coordinated international comparisons between optical clocks connected via fiber and satellite linksThomas Lindvall, M. Pizzocaro, R. Godun et al. · 2025 · Optica · cited 36 times · Not summarized yet
The study measures frequency ratios among clocks in several countries using fiber and satellite links, addressing coordinated international comparison.
Abstract
Optical clocks provide ultra-precise frequency references that are vital for international metrology as well as for tests of fundamental physics. To investigate the level of agreement between different clocks, we simultaneously measured the frequency ratios between ten optical clocks in six different countries, using fiber and satellite links. This is the largest coordinated comparison to date, from which we present a subset of 38 optical frequency ratios and an evaluation of the correlations between them. Four ratios were measured directly for the first time, while others had significantly lower uncertainties than previously achieved, supporting the advance towards a redefinition of the sec…
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Atomic Clock Frequency Ratios with Fractional Uncertainty ≤3.2×10^{-18}.A. Aeppli, Willa J. Arthur-Dworschack, K. Beloy et al. · 2026 · Physical Review Letters · cited 13 times · Not summarized yet
The study measures optical-clock frequency ratios at similarly low uncertainty using a shared ultrastable reference over a 3.6-km link.
Abstract
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-stabili…
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Frequency ratio of Yb and Sr clocks with 5 × 10−17 uncertainty at 150 seconds averaging timeN. Nemitz, T. Ohkubo, M. Takamoto et al. · 2016 · Nature Photonics · cited 223 times · Not summarized yet
The study measures the frequency ratio of ytterbium and strontium clocks, but uses co-located cryogenic clocks and common-mode laser-noise rejection.
Abstract
The most accurate ratio of the clock transition frequencies between Yb and Sr is measured by using a pair of cryogenic optical lattice clocks. Through common mode rejection of the clock laser noise, a uncertainty of 4.6 × 10−17 is achieved in 150 seconds. Transition frequencies of atoms and ions are among the most accurately accessible quantities in nature, playing important roles in pushing the frontiers of science by testing fundamental laws of physics, in addition to a wide range of applications such as satellite navigation systems. Atomic clocks based on optical transitions approach uncertainties of 10−18 (refs 1–3), where full frequency descriptions are far beyond the reach of the SI se…
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Improved systematic evaluation of a strontium optical clock with uncertainty below 1×10−18Zhi-Peng Jia, Jie Li, De-Quan Kong et al. · 2025 · Metrologia · cited 19 times · Not summarized yet
The study evaluates the Sr optical clock’s systematic uncertainty, establishing clock accuracy relevant to the Wuhan–Shanghai ratio measurement.
Abstract
We report a systematic uncertainty of 9.2×10−19 for the Sr1 optical lattice clock at the University of Science and Technology of China (USTC), achieving accuracy at the level required for the roadmap of the redefinition of the SI second. A finite-element model with in situ-validated, spatially-resolved chamber emissivity reduced blackbody radiation (BBR) shift uncertainty to 6.3×10−19. Concurrently, the externally mounted lattice cavity, by providing a larger beam waist, reduced the atomic density and thereby suppressed the density shift. Enhanced lattice depth modulation consolidated lattice light shift uncertainty to 6.3×10−19 by enabling simultaneous determination of key polarizabilities …
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Lu+ optical frequency references with accuracy verified at the 19th digitK. J. Arnold, M. Lee, Qi Zhao et al. · 2025 · Nature · cited 7 times · Not summarized yet
The study compares two lutetium-ion references to verify agreement at high accuracy, addressing clock consistency rather than intercity links.
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 relat…
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Geodesy and metrology with a transportable optical clockJ. Grotti, S. Koller, S. Vogt et al. · 2017 · Nature Physics · cited 468 times · Not summarized yet
The study uses a transportable strontium clock to measure a gravity-potential difference, applying optical clocks to geodesy.
Abstract
Optical atomic clocks, due to their unprecedented stability1–3 and uncertainty3–6, are already being used to test physical theories7,8 and herald a revision of the International System of Units9,10. However, to unlock their potential for cross-disciplinary applications such as relativistic geodesy11, a major challenge remains: their transformation from highly specialized instruments restricted to national metrology laboratories into flexible devices deployable in different locations12–14. Here, we report the first field measurement campaign with a transportable 87Sr optical lattice clock12. We use it to determine the gravity potential difference between the middle of a mountain and a locatio…
Prior work it builds on
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Optical atomic clocks: defining the future of time and frequency metrologyTara Fortier, A. Luiten, H. Margolis · 2025 · Optica · cited 30 times · Not summarized yet
The titled article surveys optical atomic clocks and their role in future time and frequency metrology.
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NTSC SrII optical lattice clock with uncertainty of 2×10−18Xiao-Tong Lu, Feng Guo, Yan-Yan Liu et al. · 2025 · Metrologia · cited 15 times · Not summarized yet
The study evaluates the NTSC SrII clock’s systematic uncertainty, characterizing a strontium standard relevant to clock comparisons.
Abstract
We report the systematic evaluation of the SrII optical lattice clock (OLC) at the National Time Service Center, achieving an uncertainty of 1.96×10−18. The newly established 87Sr OLC controls the black-body radiation shift by moving atoms into a temperature-stabilized chamber for clock transition detection. It mitigates the DC Stark shift by using conductive optical films on the inner windows, and reduces both the density shift and AC Stark shift by operating the clock with a relatively low lattice trap depth. Additionally, this clock accommodates changes in the angle between the lattice and gravity directions by rotating the entire setup, allowing for convenient adjustment of the tunneling…
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Atomic clock performance enabling geodesy below the centimetre levelW. McGrew, X. Zhang, R. Fasano et al. · 2018 · Nature · cited 600 times · Not summarized yet
The study demonstrates local optical-clock measurements for geodesy, where clock frequency responds to gravity potential.
Abstract
The passage of time is tracked by counting oscillations of a frequency reference, such as Earth’s revolutions or swings of a pendulum. By referencing atomic transitions, frequency (and thus time) can be measured more precisely than any other physical quantity, with the current generation of optical atomic clocks reporting fractional performance below the 10−17 level1–5. However, the theory of relativity prescribes that the passage of time is not absolute, but is affected by an observer’s reference frame. Consequently, clock measurements exhibit sensitivity to relative velocity, acceleration and gravity potential. Here we demonstrate local optical clock measurements that surpass the current a…
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Geodetic methods to determine the relativistic redshift at the level of 10$$^{-18}$$-18 in the context of international timescales: a review and practical resultsH. Denker, L. Timmen, C. Voigt et al. · 2018 · Journal of Geodesy · cited 111 times · Not summarized yet
The study reviews geodetic methods for determining relativistic redshift and reports practical results for international timescales.
Abstract
The frequency stability and uncertainty of the latest generation of optical atomic clocks is now approaching the one part in $$10^{18}$$1018 level. Comparisons between earthbound clocks at rest must account for the relativistic redshift of the clock frequencies, which is proportional to the corresponding gravity (gravitational plus centrifugal) potential difference. For contributions to international timescales, the relativistic redshift correction must be computed with respect to a conventional zero potential value in order to be consistent with the definition of Terrestrial Time. To benefit fully from the uncertainty of the optical clocks, the gravity potential must be determined with an a…
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Frequency ratio measurements at 18-digit accuracy using an optical clock networkKyle Martha I. Tobias Samuel M. Sarah L. Jwo-Sy Jean-Da Beloy Bodine Bothwell Brewer Bromley Chen Deschêne, K. Beloy, M. I. Bodine et al. · 2021 · Nature · cited 362 times · Not summarized yet
The study uses an optical-clock network to measure frequency ratios at 18-digit accuracy, addressing high-precision clock comparison.
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