Working toward a nuclear clock

New Scientist : Atomic clocks, the most accurate clocks to date, use the visible light or microwave signals that atoms predictably emit when one of their electrons drops from a high-energy state to a lower-energy state. The signals are so regular that atomic clocks are used to define the length of a second. In principle, the radiation emitted when an atomic nucleus changes from a high-energy state to a lower-energy state could also form the basis of a clock. Most such changes in nuclei yield gamma rays or high-energy x rays, but thorium-229 nuclei are predicted to absorb and emit UV light. The narrowness of the UV transition, which separates two low-lying states, implies even greater accuracy than that of the best atomic clocks. But before anyone can exploit that property, it’s necessary to cool and trap thorium-229 ions—which is what Alex Kuzmich of the Georgia Institute of Technology and colleagues have now accomplished . Nuclear clocks could be used to test whether the strength of the fundamental forces of nature changes over time; if a nuclear clock were paired with an atomic clock, a change in the strength of the strong nuclear force relative to the strength of the electromagnetic force would be revealed as the time kept by the two clocks diverged. Having trapped thorium-229 ions, Kuzmich and his colleagues are pursuing the next step toward making a nuclear clock: finding the UV transition’s precise frequency.