PTB reports interspecies optical‑clock comparison below 5×10⁻¹⁸ using transportable clock

Researchers at Germany’s national metrology institute say they have cleared an important benchmark for next-generation timekeeping, reporting an optical-clock comparison between two different atomic species with a fractional uncertainty of 4.3 × 10⁻¹⁸ using a transportable clock. The result, described in an arXiv preprint posted Aug. 3, is not yet peer-reviewed.

The milestone matters because the international roadmap for a possible redefinition of the SI second calls for high-accuracy comparisons between different kinds of optical clocks at or below about 5 × 10⁻¹⁸. PTB, formally Physikalisch-Technische Bundesanstalt in Braunschweig, Germany, said its measurement reaches that level while also showing that an ultra-precise clock can be moved and still reproduce 10⁻¹⁸-scale performance.

In the preprint, “Interspecies clock comparison below 5 × 10−18 uncertainty with a transportable clock,” all of the authors are from PTB. They report an optical frequency ratio between a stationary ytterbium-ion clock and a transportable strontium optical lattice clock of νYb+ / νSr = 1.4959916185449005881(65), with what the abstract calls “a fractional uncertainty of 4.3 × 10−18.”

The comparison paired PTB’s ytterbium-ion clock, based on the E3 transition, with a transportable strontium lattice clock. The measurement campaign stretched across nearly two years, from April 2023 to February 2025, and was carried out in four separate runs. According to the authors, the four results were consistent within their statistical uncertainties, supporting the claim that the transportable clock could be relocated and still agree with the stationary system at the 10⁻¹⁸ level. They also say the result improves on the previous best by more than a factor of three and is among the few interspecies comparisons to meet the sub-5 × 10⁻¹⁸ threshold in the metrology roadmap.

The basic comparison method was designed to keep the two clocks phase-coherent, meaning their ticking could be matched precisely over time. PTB said it used a common ultrastable laser reference and one branch of an optical frequency comb, a device that links optical frequencies for precision measurement. The strontium clock’s systematic uncertainty had previously been reduced to “a total systematic uncertainty of 2.1 × 10−18” in related PTB work published in 2025, while the ytterbium-ion clock’s systematic uncertainty in this comparison was 2.7 × 10⁻¹⁸.

A key part of the story is that the strontium clock was not confined to a permanent lab setup. PTB said the system is housed in an air-conditioned car trailer and was operated intermittently off-campus during the study. That matters because the best long-distance clock comparisons often rely on stabilized optical-fiber links, which are not available everywhere. A transportable optical clock can instead be brought to another location and used as a transfer standard.

Because clocks this precise are sensitive to gravity, the team also had to account for Einstein’s relativity. A clock at a different height runs at a slightly different rate, so PTB applied a relativistic redshift correction based on geometric leveling. The uncertainty from that correction was 5.0 × 10⁻¹⁹, and the researchers remeasured the transportable clock’s height after each move.

That sensitivity is also why optical clocks are of interest for chronometric geodesy, a field that uses timekeeping to measure Earth’s gravitational potential and height differences. At about this level of precision, a change in height of roughly 1 centimeter shifts a clock’s frequency by about 1.1 × 10⁻¹⁸. For now, though, PTB’s latest result should be viewed as a preprint claim awaiting peer review, even as it marks what the authors describe as the most accurate interspecies frequency comparison involving a transportable clock to date.