An exciting new development in the quest for ever greater precision in measurement of time and frequency is the possibility of replacing current atomic clocks with a nuclear clock based on anomalously low-energy nuclear isomeric excitation lines in Th-229. Typical electromagnetic excitations of nuclei correspond to x-ray and γ-ray energies (104–106 eV). The exceptionally low isomeric excitation energy of∼ 8.4 eV in Th-229 results from an accidental cancellation between two very large energy scales: the Coulomb and the strong-force energy. The large values of these energies and their near cancellation make the Th-229 nucleus an excellent testing ground to look for time variation in the fine structure constant (which would affect the Coulomb energy) or to search for interactions with dark matter (which might affect the strong-force energy).

Recent breakthrough experiments have located the narrow resonance of the nucleus and directly excited it with laser light. Indeed, a recent experiment based on a precision optical comb has directly connected the frequency of UV laser light near 8.4 eV to an optical atomic clock and has determined the Th-229 excitation frequency with 12 significant digits. This nuclear isomeric transition has numerous advantages for use in precision timekeeping and quantum sensing, building on the possibility of employing a huge number of nuclei embedded in a solid-state host. However, this platform has an array of challenges such as inhomogeneous line broadening and systematic frequency shifts. Overcoming these challenges will require coordinated and synergistic efforts among disparate fields of nuclear, atomic/optical and condensed matter physics. Motivated by these goals, we will explore the following topics: 

• Implications for fundamental physics
• Defects and ionic configurations–theory and experiment
• Linewidth broadening mechanisms
• Trapped ion vs. solid-state platforms
• Fast readout of nuclear states
• Solid state hosts with gaps < 8.4 eV
• Optical resonators at 8.4 eV
• New highly coherent, higher power VUV laser