Thorium Nuclear Clocks: A New Frontier of Precision express
Ordinary atomic clocks rely on spectroscopy — counting electron oscillations. Thorium-229 nuclear clocks address the nucleus itself. Its excited state lives for 641 seconds: for nuclear physics, this is a pendulum in thick syrup, where one swing lasts an eternity, but is impeccably stable.
Placing such nuclei in a crystal, physicists face a problem: lattice defects act like an uneven floor, disrupting the rhythm of each pendulum in its own way. The task is to level this floor. Then the clocks will become more accurate: an error of one second over the entire age of the Universe. They will notice the difference in the passage of time between floors and test the Standard Model of physics for strength.
Original abstract
as written by the authors arXiv:2511.13017
Motivated by recent experimental breakthroughs toward a realization of a solid-state Thorium-229 nuclear clock, we review the technology, basic physics motivation, and limitations of the present generation of atomic clocks. We then discuss prospects for a new generation of clocks based on an anomalous low-energy 8.4 eV nuclear transition in Th-229, with an extremely long lifetime of 641 seconds when doped into CaF crystals. To realize such solid-state nuclear clocks one must confront basic nuclear, AMO, and solid state physics questions. Key challenges are understanding and minimizing the effects of inhomogeneous broadening, associated with strains and electric field gradients due to both the Th dopants and intrinsic crystal defects.
Thorium-229 is the only nucleus whose transition can be triggered by a regular ultraviolet laser, rather than hazardous gamma radiation. This makes the experiments tabletop and safe.