Laser-based measurement and control of atomic and molecular states form the foundation of modern quantum technology and provide deep insights into fundamental physics. Today’s most precise clocks are based on measurements of optical transitions in atoms. To this end, transitions with high quality factors, low sensitivities to external perturbations, and good signal-to-noise ratios are desired.

In this thesis, we achieve frequency-based laser spectroscopy of the ²²⁹Th nuclear clock transition using a vacuum ultraviolet (VUV) frequency comb. The high transition frequency of 2,020,407,384,335(2) kHz (in 150 K CaF₂ crystals) and a long excited state lifetime of 641(4) s show the high intrinsic quality factor of this nuclear transition. This transition frequency is predicted to be insensitive to external perturbations due to 1) the small electromagnetic moment of the atomic nucleus and 2) the shielding effect of the outer electronic shell. Further, the large number density of quantum emitters in a solid-state crystalline host promises a high signal-to-noise ratio. Moreover, based on the different fundamental interactions involved in nuclear versus electronic transitions, precise comparisons between nuclear and atomic clocks offer dramatically enhanced sensitivity to new physics. Resolving individual nuclear quantum states in its host crystal enables us to perform the first steps in characterizing the nuclear clock performance. 

Probing the ²²⁹Th nuclear transition required new tools. Building upon previous generations of extreme-ultraviolet (XUV) comb projects in our lab, we construct a VUV comb to perform direct frequency comb spectroscopy of the ²²⁹Th nuclear clock transition. We calibrate the absolute frequency by linking this comb to the JILA ⁸⁷Sr atomic clock. We also present our effort in making ²²⁹Th thin-film samples for reducing the cost and radioactivity of future nuclear clocks.