NPL and PTB Compare Two Ytterbium-Ion Optical Clocks Over a 2220 km Fiber Link at 4.8e-18 Uncertainty, arXiv 21 October 2026: The First International Comparison Under the CCTF's 5e-18 Bar, TRL 5

NPL and PTB report that two independently built ytterbium-ion optical clocks agree at 1.0 × 10⁻¹⁸ with an uncertainty of 4.8 × 10⁻¹⁸ across a 2220 km fiber link, the first international comparison under the 5e-18 bar the CCTF listed as unmet in 2025. The Defense Monitor reads what that means for GNSS-independent timing, the UK's PNT resilience framework, and the three gaps between a 25-day campaign and a timing service.