Physicists have talked about nuclear clocks for more than two decades. This week, two of them actually ran. Teams in Vienna and Beijing, working separately and with different equipment, each reported a functioning thorium-229 nuclear clock in back-to-back papers published in Nature on 7 October.
Ordinary atomic clocks, the kind that keep GPS satellites and stock exchanges in sync, count the oscillations of electrons jumping between energy levels. A nuclear clock goes deeper, using a shift inside the atom’s core. Tucked away beneath its electron cloud, the core is much better protected, so outside noise like heat or electric fields should barely nudge the beat.
There is a catch: almost every nucleus needs gamma rays to be nudged, which no laser can provide. Only one known isotope breaks that rule: thorium-229. Its transition sits at an unusually low energy, reachable with vacuum-ultraviolet light at about 148 nanometres. Both groups planted thorium atoms inside small calcium fluoride crystals that work without any cooling, on an ordinary optical table.
In Beijing, Ding Shiqian’s group at Tsinghua University first had to build a continuous laser in that awkward part of the spectrum, a feat they reported earlier this year. Their finished clock turned out to be about six-fold steadier than its Austrian counterpart, and two crystals grown independently gave matching readings. One of them needed just 1.4 micrograms of thorium.
The TU Wien team led by Thorsten Schumm, with partners at Germany’s PTB metrology institute, took a different route and immediately put their clock to work hunting for ultralight dark matter. They found no signal, but the limits they set reach into a corner of physics, interactions via the strong force, where electron-based clocks are simply blind. Pocket-sized nuclear timekeepers are still far off; the proof that the idea works is now in hand.
