Two Thorium-229 Nuclear Clocks in Nature, 7 October 2026: TU Wien and PTB Run One Unattended for a Day, a Tsinghua-Led Team Reaches 5e-13 Instability, for GNSS-Denied Timing, TRL 4

Nature published two independent thorium-229 nuclear clocks on October 7, 2026: a TU Wien and PTB clock that ran unattended for a day and measured within-run instability in the low 1e-14 range, and a Tsinghua-led clock at 5e-13 whose two home-grown crystals agree to one part in ten trillion. A solid-state reference at room temperature is the shape of holdover timing a GNSS-denied force eventually needs, and both papers state the barriers plainly: day-to-day reproducibility of 5e-13, crystal strain and the ultraviolet laser power a field-deployable nuclear clock would have to carry. TRL 4.