Frequency ratio of the 229mTh nuclear isomeric transition and the 87Sr atomic clock

Chuankun Zhang (), Tian Ooi, Jacob S. Higgins, Jack F. Doyle, Lars Wense, Kjeld Beeks, Adrian Leitner, Georgy A. Kazakov, Peng Li, Peter G. Thirolf, Thorsten Schumm and Jun Ye ()
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Chuankun Zhang: University of Colorado Boulder
Tian Ooi: University of Colorado Boulder
Jacob S. Higgins: University of Colorado Boulder
Jack F. Doyle: University of Colorado Boulder
Lars Wense: University of Colorado Boulder
Kjeld Beeks: TU Wien
Adrian Leitner: TU Wien
Georgy A. Kazakov: TU Wien
Peng Li: IMRA America
Peter G. Thirolf: Ludwig-Maximilians-Universität München
Thorsten Schumm: TU Wien
Jun Ye: University of Colorado Boulder

Nature, 2024, vol. 633, issue 8028, 63-70

Abstract: Abstract Optical atomic clocks1,2 use electronic energy levels to precisely keep track of time. A clock based on nuclear energy levels promises a next-generation platform for precision metrology and fundamental physics studies. Thorium-229 nuclei exhibit a uniquely low-energy nuclear transition within reach of state-of-the-art vacuum ultraviolet (VUV) laser light sources and have, therefore, been proposed for construction of a nuclear clock3,4. However, quantum-state-resolved spectroscopy of the 229mTh isomer to determine the underlying nuclear structure and establish a direct frequency connection with existing atomic clocks has yet to be performed. Here, we use a VUV frequency comb to directly excite the narrow 229Th nuclear clock transition in a solid-state CaF2 host material and determine the absolute transition frequency. We stabilize the fundamental frequency comb to the JILA 87Sr clock2 and coherently upconvert the fundamental to its seventh harmonic in the VUV range by using a femtosecond enhancement cavity. This VUV comb establishes a frequency link between nuclear and electronic energy levels and allows us to directly measure the frequency ratio of the 229Th nuclear clock transition and the 87Sr atomic clock. We also precisely measure the nuclear quadrupole splittings and extract intrinsic properties of the isomer. These results mark the start of nuclear-based solid-state optical clocks and demonstrate the first comparison, to our knowledge, of nuclear and atomic clocks for fundamental physics studies. This work represents a confluence of precision metrology, ultrafast strong-field physics, nuclear physics and fundamental physics.

Date: 2024
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DOI: 10.1038/s41586-024-07839-6

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