Comment Summary misses the coolest details (Score 5, Interesting) 72
Normally you need a serious punch to get a nucleus into an excited state: x-rays, gamma rays, or bombardment with energic particles. It so happens that thorium-229 requires the lowest energy to get to an excited state, and that can be achieved with deep-UV (148 nm) light. Laser light at that wavelength has only recently become possible. Also: Th-229 does not really exist in nature, you have to extract it from the decay of U-235, which itself has low natural abundance.
So it's pretty cool stuff, and this first demonstration hints at what could be possible in the future. Atomic clocks use light in the microwave range; this nuclear clock uses light in a frequency band five orders or magnitude higher, and so could theoretically have that much greater precision. And because the fundamental phenomenon occurs in the nucleus rather than the electron cloud, it's much less susceptible to external interference.