The accuracy of atomic clocks, which measure time based on atomic transitions, is central to the function of systems as diverse as GPS navigation and radio astronomy. In theory, a nuclear clock based on an optical excitation of a nuclear transition, could be even better than atomic clocks in terms of stability and compactness. However, the only nuclear state with an excitation energy sufficiently low for this application is the first excited state of thorium-229. But this is arguably most exotic transition in the whole nuclear landscape, and has proven to be extremely hard to detect. Only some indirect evidence could be obtained previously. Here, based on low-energy microchannel plate detection, Lars von der Wense and colleagues achieve direct detection of the thorium-229 nuclear-clock transition, placing new limits on the transition energy and measuring the state’s half-life. As well as being a step towards a nuclear clock, these results also suggest that nuclear quantum optics and nuclear lasers based on this transition may be plausible possibilities.
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