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Volume 573 Issue 7773, 12 September 2019

A matter of time

The current standard for timekeeping is the atomic clock, which measures time based on transitions between two states of an atom. In this week’s issue, two papers edge closer to the realization of the first nuclear clock — one that would be based on transitions between states in a nucleus and so would compete with atomic timekeepers. The nucleus in question is that of thorium-229. This is the only nucleus known to have a first excited state (229mTh) in the energy range of a few electronvolts, which makes it the lowest of all known nucleii and, crucially, accessible by lasers. But thorium poses challenges, including the accurate determination of its excitation energy and the direct optical excitation of 229mTh. In one paper, Benedict Seiferle and his colleagues present the direct measurement of the transition energy from the ground state to the first excited state. And in a second paper, Noboru Sasao and his colleagues report the direct X-ray excitation of thorium-229 to the second excited state, which then quickly decays to 229mTh.

Cover image: Daria Bilous

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