The 229Th isomer: prospects for a nuclear optical clock
Dr. Lars von der Wense (JILA, University of Colorado, Boulder, USA)

A nuclear optical clock based on ^229Th ions is expected to achieve a higher accuracy than the best atomic clocks operational today [1]. Although proposed back in 2003 [2], such a nuclear frequency standard has not yet become reality. The main obstacle that has so far hindered the development of a nuclear clock was an imprecise knowledge of the energy value of a nuclear excited state of the ^229Th nucleus, generally known as the ^229Th isomer. This metastable nuclear excited state is the one of lowest energy in the whole nuclear landscape and - with an energy of less than 10 eV - offers the potential for nuclear laser spectroscopy, which poses a central requirement for the development of a nuclear clock [3].

Recently, a couple of new experiments have led to an improved knowledge about the isomer’s excitation energy [4, 5, 6], thereby constraining the isomeric energy to a value of 8.12 0.11 eV. This new knowledge offers great potential for future laser spectroscopy experiments and the development of a nuclear optical clock. With a wavelength equivalent of 152.7 2.1 nm, the energy is very well accessible by the 7th harmonic of a high-power Yb:doped-fiber frequency-comb [7] and a corresponding spectroscopy experiment is already in preparation [8]. If successful, the experiment would provide the first laser spectroscopy of a nuclear transition, thereby improving our current constraints of the isomer’s energy by six orders of magnitude. In addition, the stabilization of an individual comb-mode to the nuclear transition would result in the immediate development of a nuclear frequency standard. In this presentation I will give an overview over the current status of the nuclear clock development, with a particular focus on the most recent progress. Also the next required steps will be detailed and future perspectives will be given.

References
[1] C.J. Campbell et al., Phys. Rev. Lett. 108, 120802 (2012).
[2] E. Peik and C. Tamm, Eur. Phys. Lett. 61, 181 (2003).
[3] L. von der Wense and B. Seiferle, arXiv:2009.13633 (2020).
[4] B. Seiferle et al., Nature 573, 243 (2019).
[5] A. Yamaguchi et al., Phys. Rev. Lett. 123, 222501 (2019).
[6] T. Sikorsky et al., Phys. Rev. Lett. 125, 142503 (2020).
[7] C. Zhang et al., Phys. Rev. Lett. 125, 093902 (2020).
[8] L. von der Wense and C. Zhang, Eur. Phys. J D 74, 126 (2020).