This paper presents a high-precision international comparison of two optical atomic clocks, testing whether independently developed systems in different countries agree at a fractional frequency uncertainty below 5 × 10⁻¹⁸. Such comparisons help establish the consistency of advanced timekeeping systems and support preparations for a possible optical redefinition of the SI second. The researchers also use the comparison to test Lorentz invariance, a fundamental principle of relativity. Only the paper’s title, author list, and abstract are available, so this summary cannot independently assess the experimental methods, uncertainty calculations, or supporting analyses. The experiment compares two single-ion optical clocks, one in the United Kingdom and the other in Germany, connected by a 2,220 km optical fiber link. Both clocks operate on the electric octupole (E3) transition in singly ionized ytterbium-171, connecting the ²S₁/₂ state with total angular momentum F = 0 to the ²F₇/₂ state with F = 3. Using the same atomic transition enables a direct test of agreement between the independently developed systems, while the fiber link allows their frequencies to be compared remotely. From more than one million seconds of measurement data, the researchers determine a fractional frequency difference of 1.0 × 10⁻¹⁸ with an uncertainty of 4.8 × 10⁻¹⁸, expressed in the abstract as 1.0(4.8) × 10⁻¹⁸. The result is consistent with zero within the reported uncertainty. According to the authors, this is the first international clock comparison to achieve a fractional uncertainty below 5 × 10⁻¹⁸. The abstract does not specify the uncertainty’s coverage convention or provide a detailed uncertainty budget. The dominant source of measurement uncertainty is the relativistic redshift correction between the two sites. Clocks at different gravitational potentials run at different rates, and this effect must be accounted for when comparing their frequencies. The correction incorporates time-varying gravitational-potential differences rather than treating the difference between the sites as fixed. This finding underscores the importance of precise gravitational-potential measurements in remote comparisons of highly accurate clocks. However, the abstract does not explain how these potential differences were determined or how their variations were modeled. The researchers also use the clocks’ large geographic separation to search for violations of Lorentz invariance. They report no statistically significant modulations in the comparison data. This null result reduces the uncertainty in constraints on the Robertson–Mansouri–Sexl parameter alpha by approximately a factor of five and establishes limits on corresponding parameters in the Standard-Model Extension framework. The abstract does not give numerical bounds or identify the specific Standard-Model Extension parameters constrained. Overall, the study demonstrates agreement between two internationally separated ytterbium-ion optical clocks at a reported fractional uncertainty of 4.8 × 10⁻¹⁸ while strengthening tests of fundamental physics. It marks an important step toward an optical redefinition of the SI second. The dominant uncertainty from the redshift correction also highlights a key challenge: further improvements require not only accurate clocks and reliable frequency transfer, but also sufficiently precise knowledge of gravitational-potential differences between sites.
Optical Clocks Agree Across Borders at an Extraordinary Level of Precision
How can scientists know that two exceptionally precise clocks, built independently in different countries, tell the same time? They connect them, compare their ticking rates, and account for effects so small that gravity itself becomes a leading concern. A new international comparison reports agreement between optical atomic clocks in the United Kingdom and Germany with a fractional frequency uncertainty below 5 × 10⁻¹⁸.
The paper, “International atomic clock comparison achieving 5 × 10⁻¹⁸ uncertainty,” by A. Tofful, M. Filzinger, T. Schmidt, A. Amy-Klein, E. Benkler, E. Cantin et al., describes both a milestone in precision timekeeping and a test of fundamental physics. Its findings support preparations for a possible optical redefinition of the SI second, while revealing how strongly future progress depends on understanding gravity at each clock’s location.
Two Clocks, One Atomic Reference
The experiment compares two single-ion optical clocks separated by national borders and connected through a 2,220 km optical fiber link. Both use the same reference: an electric octupole transition, known as the E3 transition, in singly ionized ytterbium-171.
Specifically, the transition connects the ²S₁/₂ state with total angular momentum F = 0 to the ²F₇/₂ state with F = 3. These labels identify the atomic states that provide the clocks’ reference frequency. Using the same transition makes the comparison a direct test of whether independently developed systems reproduce the same frequency.
The fiber connection allows the researchers to compare the clocks remotely. This matters because excellent performance at a single laboratory is only part of the challenge. For a widely usable time standard, independent clocks must also demonstrate consistent results when compared across substantial distances.
What the Numbers Mean
Using more than one million seconds of measurement data, the researchers report a fractional frequency difference of 1.0 × 10⁻¹⁸, with an uncertainty of 4.8 × 10⁻¹⁸. The abstract expresses this as 1.0(4.8) × 10⁻¹⁸.
A fractional frequency difference describes the mismatch relative to the clocks’ operating frequency. Here, the measured difference is smaller than the reported uncertainty, making the result consistent with zero. In other words, the comparison finds no resolved disagreement between the clocks at this level of precision; it does not establish that their frequencies are exactly identical.
According to the authors, this is the first international clock comparison to achieve a fractional uncertainty below 5 × 10⁻¹⁸. That claim marks an important threshold for cross-border comparisons, where the clocks, the frequency-transfer connection, and corrections for their environments all contribute to the final result.
Why Gravity Becomes the Limiting Factor
The dominant source of measurement uncertainty was the relativistic redshift correction between the two sites. Clocks at different gravitational potentials run at different rates. To compare their underlying atomic frequencies meaningfully, researchers must correct for that difference.
At the precision reached here, this is not a minor adjustment. Uncertainty in the correction can outweigh other contributions to the comparison’s uncertainty. Better clocks alone therefore cannot guarantee a better international result: scientists also need sufficiently precise knowledge of the gravitational potentials where those clocks operate.
The reported correction incorporates time-varying gravitational-potential differences rather than treating the relationship between the sites as fixed. This highlights the importance of tracking changes as well as determining the overall difference. However, the abstract does not explain how those potentials were measured or how their variations were modeled.
Testing a Principle of Relativity
The researchers also use the clocks’ large geographic separation to test Lorentz invariance, a foundational principle of relativity. Such tests look for departures from the expected behavior of physical systems, which could point toward physics beyond established descriptions.
The team reports no statistically significant modulations in the comparison data. This is a null result, not evidence of a violation. Nevertheless, a sufficiently sensitive null result can tighten the limits on how large any possible departure could be.
According to the abstract, the comparison reduces the uncertainty in constraints on the Robertson–Mansouri–Sexl parameter alpha by approximately a factor of five. It also establishes limits on corresponding parameters within the Standard-Model Extension framework. The abstract supplies neither numerical bounds nor the specific Standard-Model Extension parameters involved.
A Step Toward a New Second—with Important Caveats
Only the paper’s title, author list, and abstract are available for this account. That limits what can be assessed independently: the detailed experimental methods, uncertainty calculations, and supporting analyses are not accessible here. The abstract also does not specify the uncertainty’s coverage convention or provide a full uncertainty budget.
Within those limits, the central result is clear: two internationally separated ytterbium-ion optical clocks agree within a reported fractional uncertainty of 4.8 × 10⁻¹⁸. Such demonstrations help establish the consistency needed for a possible optical redefinition of the SI second. They also expose the next challenge: at the frontier of timekeeping, understanding where a clock sits in Earth’s gravitational potential can be as consequential as understanding the clock itself.
