Thorium nuclear clocks for fundamental tests of physics
Th-229 has an exceptionally low-energy excited nuclear isomer state with an excitation energy of only a few electron volts, making it accessible to laser manipulation. With a predicted relative radiative linewidth of 1e-19, constructing a Thorium nuclear clock becomes possible that could rival todays most advanced optical atomic clocks.
The few-eV transition emerges from a fortunate near-degeneracy of the two lowest nuclear energy levels. However, the Coulomb and strong-force contributions to these level energies differ on the MeV level. This makes the Th-229 nuclear level structure uniquely sensitive to variations of fundamental constants and ultralight dark matter.
Very recently, the applicants have proven the long-sought existence of the low-energy isomer, determined the lifetime in different electronic environments, quantified the nuclear moments and charge radius based on the hyperfine splitting, and constrained the isomer energy. However, knowledge on the electronic and nuclear properties is still insufficient to exploit the Th-229 system for fundamental tests.
This project aims to close this gap and realize three prototype nuclear Thorium clocks using complementary approaches in trapped ions and solids. We will develop customized VUV laser systems and perform precision spectroscopy of the Th-229 nuclear transition. Comparing these clocks among each other and with state-of-the-art optical clocks will allow us to benchmark the new frequency standard before ultimately applying it to test fundamental physics. 
This project requires a unique combination of experimental and theoretical expertise in atomic and nuclear physics, high precision metrology and fundamental symmetries. Furthermore, special infrastructure is required for (distributed) clock comparison, precision spectroscopy as well as processing of Th-229. The synergy team is composed to optimally respond to these challenges while being rooted in established and successful collaborations.