New paradigm in time and frequency metrology: from the atomic to the nuclear clock

Abstract Thorium-229 with its 8.4 eV isomer is a unique system at the interface between nuclear physics and atomic physics, with a nuclear excitation energy in the range of transition energies of valence electrons. The magnetic dipole transition at 148 nm that connects the isomer with the ground state is accessible with the methods of precision laser spectroscopy. Among several possible applications, the development of a nuclear clock seems particularly attractive. This clock would offer high accuracy, high stability, and also high sensitivity in clock-based fundamental tests that involve the strong interaction in addition to electromagnetism. Laser excitation has been demonstrated with Th-229 dopant ions in crystals of calcium fluoride that are transparent in the vacuum-ultraviolet. These experiments can be seen as laser Mössbauer spectroscopy where interactions between the nucleus and the crystal lattice can be studied. We are presently searching for the nuclear excitation in atomic Th-229 ions in ion traps where interactions with the environment are well controlled and very high accuracy can be expected, opening the way towards a future primary clock.