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

The 229 Th 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 229 Th 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 229 Th remains an unsolved challenge, recent experiments have provided essential information on the transition energy and relevant nuclear properties, advancing the field.