Progress toward direct VUV frequency comb spectroscopy of the ²²⁹Th nuclear clock transition

Atomic nuclei are known to possess isomeric transitions that are typically in the X-ray energy range. The ²²⁹Th nucleus offers a uniquely low energy, narrow linewidth transition at 8.338(24) eV, corresponding to a wavelength of 148.71(47) nm in the vacuum ultraviolet (VUV). This ²²⁹Th transition is expected to have a high quality factor and a low sensitivity to external perturbations, and has been proposed as a nuclear-based optical frequency reference.

However, direct laser excitation of this transition has yet to be achieved. Based on recent nuclear physics experiments that significantly reduced the transition energy uncertainty, we are currently searching for this transition using a VUV optical frequency comb. We will present our recent attempt on optical excitation of the nuclear transition in a thin-film solid-state environment, aiming for building a nuclear-based clock. We will also discuss our efforts developing alternative spectroscopy target systems for the nuclear transition search.