Leveraging the exceptional environmental insensitivity of nuclear transitions, we demonstrate a solid-state clock with more than 1015 thorium-229 nuclei hosted in a 1 mm3 crystal, unlocking access to quantum devices with unprecedented large numbers of coherently controlled quantum emitters.

A major technological obstacle in realizing nuclear clocks is the lack of coherent laser light sources at nuclear transition wavelengths. Using a frequency comb laser in the vacuum-ultraviolet (VUV), we resolve the individual nuclear quantum states of the low-energy thorium-229 nuclear transition in a CaF2 crystal host and precisely determine their frequencies, referenced to the JILA strontium-87 optical lattice clock. We systematically characterize the state-dependent clock frequency reproducibility with respect to temperature, doping concentration and time.

This nuclear clock promises a scalable and portable platform for quantum science and metrology. Nuclear-atomic clock comparisons will also enable stringent tests for new physics beyond the Standard Model.