Comment by adrian_b

There are 2 Nature articles, for the 2 independent teams who have built such nuclear clocks with thorium 229.

This is the other article, from the Chinese team:

https://www.nature.com/articles/s41586-026-11122-1

While both teams have used fluorite crystals doped with Th229, which was the most obvious choice for the active medium, they have used different kinds of optical frequency multipliers in order to generate the ultraviolet light required to probe the thorium nuclei.

For now, the frequency multiplication method used by the Chinese team is superior, allowing a greater output power, which ensures a better signal-to-noise ratio.

The team from Austria intends to use a better frequency multiplier in the future, to increase their output power, and they are evaluating several methods, one of which is the 4-wave mixing in cadmium vapor, already demonstrated by the Chinese team.