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# The thorium-229 low-energy isomer and the nuclear clock

## Abstract

The 229Th nucleus has an isomeric state at an energy of about 8 eV above the ground state, several orders of magnitude lower than typical nuclear excitation energies. This has inspired the development of a field of low-energy nuclear physics in which nuclear transition rates are influenced by the electron shell. The low energy makes the 229Th isomer accessible to resonant laser excitation. Observed in laser-cooled trapped thorium ions or with thorium dopant ions in a transparent solid, the nuclear resonance may serve as the reference for an optical clock of very high accuracy. Precision frequency comparisons between such a nuclear clock and conventional atomic clocks will provide sensitivity to the effects of hypothetical new physics beyond the standard model. Although laser excitation of 229Th remains an unsolved challenge, recent experiments have provided essential information on the transition energy and relevant nuclear properties, advancing the field.

## Key points

• A nuclear clock, based on a radiative transition in the nucleus, is less sensitive to external perturbations and therefore potentially more precise than established atomic clocks that are based on transitions in the electron shell.

• The 229Th nucleus is the prime candidate for the realization of a nuclear clock because it possesses a low-energy (8 eV) excited state that is amenable to resonant laser excitation from the nuclear ground state, with an expected natural linewidth in the millihertz range.

• Recent experiments have provided essential information on the nuclear properties of 229Th (half-life 7,920 years), such as the nuclear moments, decay modes of the isomer and a more precise value of the isomer excitation energy, which is required to achieve laser excitation.

• Thorium-229 is studied as trapped atomic ions in vacuum or doped into transparent crystals such as CaF2. Because the nuclear transition energy is in the range of transitions of valence electrons, the electronic state may influence the nuclear excitation and decay rates.

• Because of a fine balance of contributions from the strong and electromagnetic interactions to the nuclear transition energy, a 229Th clock would be sensitive to predicted effects of physics beyond the standard model, such as temporal or spatial variations of fundamental constants.

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## Data availability

The raw data for Fig. 3 are available in the Zenodo repository121.

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## Acknowledgements

Our work on 229Th was supported by the European Union’s Horizon 2020 research and innovation programme under grant agreement no. 664732 “nuClock”, grant agreement no. 856415 “ThoriumNuclearClock” and grant agreement no. 882708 “CrystalClock”. The team has also received funding from the EMPIR project “CC4C”. This project has received funding from the EMPIR programme co-financed by the Participating States and from the European Unions Horizon 2020 research and innovation programme.

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## Glossary

Sternheimer antishielding

An external electric field gradient may be strongly enhanced at the position of the nucleus by the influence of the deformed electron shell, especially in heavy atoms.

Mössbauer spectroscopy

High-resolution, recoil-free gamma-ray spectroscopy performed with nuclei in solids, tuned via the Doppler shift between a moving source and stationary absorber.

Lamb–Dicke regime

When the motion of an absorber or emitter is constrained to a region that is smaller than the wavelength, the spectrum contains a resonance that is free from the first-order Doppler shift.

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Beeks, K., Sikorsky, T., Schumm, T. et al. The thorium-229 low-energy isomer and the nuclear clock. Nat Rev Phys 3, 238–248 (2021). https://doi.org/10.1038/s42254-021-00286-6

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• ### Nuclear clocks for testing fundamental physics

• E Peik
• , T Schumm
• , M S Safronova
• , A Pálffy
• , J Weitenberg
•  & P G Thirolf

Quantum Science and Technology (2021)