@article{13460,
  author = {Chuankun Zhang and Tian Ooi and Jacob Higgins and Jack Doyle and Lars von der Wense and Kjeld Beeks and Adrian Leitner and Georgy Kazakov and Peng Li and Peter Thirolf and Thorsten Schumm and Jun Ye},
  title = {Dawn of a nuclear clock: frequency ratio of the 229m Th isomeric transition and the $^87$Sr atomic clock},
  abstract = {<p>Optical atomic clocks 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 the first nuclear clock. However, quantum state-resolved spectroscopy of the&nbsp;<sup>229m</sup>Th 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&nbsp;<sup>229</sup>Th nuclear clock transition in a solid-state CaF2&nbsp;host material and determine the absolute transition frequency. We stabilize the fundamental frequency comb to the JILA&nbsp;<sup>87</sup>Sr clock and coherently upconvert the fundamental to its 7th harmonic in the VUV range 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&nbsp;<sup>229</sup>Th nuclear clock transition and the&nbsp;<sup>87</sup>Sr 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 clock and demonstrate the first comparison 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.</p>
},
  year = {2024},
  journal = {Nature, in press},
  url = {https://arxiv.org/abs/2406.18719},
}