Researchers Measure Clock Difference at New Accuracy

An international atomic clock comparison has achieved uncertainty below 5 imes 10^{-18}. Single-ion ytterbium optical clocks, one located in the United Kingdom and one in Germany, compared measurements remotely over a 2220\,km fibre link. Atomic clocks in separate countries agreed to within 1.0(4.8)times 10 -18 with unprecedented accuracy. This international comparison used existing fibre optic cables connecting independently developed systems.

These precise measurements are key as work moves towards redefining how we measure time itself, potentially shifting from current definitions based on atoms to those reliant on light waves. Modern atomic clocks rely on incredibly precise frequencies, think of choosing a particular tuning fork’s vibration as your baseline sound, and these new measurements push that precision further than ever before.

The team used this long distance link not only for clock comparisons but also to search for subtle distortions in spacetime, known as Lorentz invariance, which dictates the laws of physics should remain constant regardless of motion.

Record ytterbium clock comparison surpasses prior uncertainty thresholds enabling refined tests

A fractional frequency difference of 1.0(4.8)times 10^-18 was achieved between two independently developed ytterbium optical clocks, one in the UK and another in Germany. The measurement exceeds an uncertainty threshold of 5times 10^-18, surpassing all previous international comparisons. Previously, a feat like this had been limited by maintaining coherent light signals over vast distances and accurately accounting for relativistic effects such as gravity variations; no other comparison reached this level of precision before now.

This breakthrough enables improved constraints on fundamental physics parameters related to Lorentz invariance, specifically refining the Robertson-Mansouri-Sexl parameter α by approximately a factor of five. Over 10^6 seconds of data revealed an exceptionally stable frequency ratio between the clocks, daily averages showed fluctuations within just 1.6times 10^-18.

Careful correction for signal degradation and phase coherence was required across a fibre optic link spanning 2220 kilometers to achieve this precision. Uncertainty in correcting differing gravitational potentials contributed most significantly to the overall error budget, requiring calculations based on height differences equivalent to under three centimeters between sites.

However, these measurements still rely on carefully controlled laboratory conditions and do not yet demonstrate stability over extended periods or in less ideal environments needed for real-world timekeeping applications. The successful comparison of optical clocks in the UK and Germany marks a step towards redefining our fundamental unit of time; however, achieving even greater precision isn’t simply about building better clocks but tackling subtle distortions in spacetime itself. Accurately modelling gravitational differences between sites currently limits uncertainty, this requires accounting for height variations smaller than three centimetres.

This approach relies on increasingly complex theoretical corrections which may eventually reach their limits. Still, this international comparison remains vital as scientists push for ever-finer time measurements, demonstrating the feasibility of linking optical clocks across considerable distances using fibre optic cables, a key requirement for future global timescales. This successful link begins a new era of precision measurement and opens avenues to test fundamental physics by searching for subtle violations in Einstein’s theories of relativity through precise clock comparisons over large baselines.

Future networks could redefine global timescales while simultaneously offering stringent tests of general relativity; establishing coherent signal maintenance demanded careful correction for factors affecting how quickly time passes along the fibre optic link. Building on previous attempts, this work demonstrates that maintaining coherence is possible even with significant distance separating the devices.

Researchers successfully compared two single-ion optical clocks, one in the United Kingdom and one in Germany, over a 2220 kilometer fiber optic link. The study highlights challenges related to accounting for relativistic effects caused by gravity potential differences between locations, which currently limits measurement precision. By analyzing over 10⁶ seconds of data, they also improved constraints on parameters used to test fundamental physics theories like Lorentz invariance.

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