Topic
Clocks referenced to a nuclear transition rather than an electron transition, currently at prototype stage using thorium-229.
Teams at PTB-TU Wien and Tsinghua report the first working nuclear clocks, locking UV lasers to thorium-229 nuclei after more than 20 years of effort. With two labs running one crystal design, results can now be reproduced and checked against atomic clocks.
Reality
High-harmonic X-ray sources normally emit only at integer multiples of their driving laser. A TU Wien and UC San Diego method detunes the driver instead, filling in the frequencies between two neighbouring harmonics.
Reality
Physicists reporting in Nature drove the 8.4-electronvolt nuclear transition in a thorium-doped calcium fluoride crystal and read the resonance as a dip in the transmitted beam, a detection route that works without waiting for the excited nucleus to decay.
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After more than 15 years of crystal work, Thorsten Schumm's group found the one lattice position that answers on a single wavelength. The two precision figures in the account sit a factor of 100 apart.
Reality
A Science paper maps the four places a thorium-229 atom can sit in a calcium fluoride crystal and finds only one with a clean nuclear transition. That turns a materials guess into a spec.
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