The 229Th nucleus possesses an exceptionally low-lying isomeric state of about 8eV, about four orders of magnitude lower than common nuclear excitation energies. This state is coupled to the ground state by a magnetic dipole transition with an estimated linewidth of less than 1mHz. It has been proposed to use this unique transition as a reference for a new type of optical clock. Such a nuclear clock is expected to be highly immune to perturbations from external electromagnetic fields, as well as a sensitive probe for temporal variations of fundamental constants.

PTB Opt-88