Th nuclear clock*

The ^229mTh isomeric nuclear transition, with an energy of 8.4 eV (λ=148 nm), is a prime candidate for a nuclear optical frequency reference.  The transition is expected to have a high quality factor and enhanced sensitivity for ultra-light dark matter searches. Last year, our team achieved quantum-state-resolved laser spectroscopy of the nucleus in a ²²⁹Th:CaF₂ crystal using a vacuum-ultraviolet frequency comb.

To build a clock, state-dependent systematic shifts of the nuclear transition frequency must be well-characterized and understood. We present our study of the temperature dependence of the transition frequency and the ²²⁹Th concentration dependence of the transition linewidth. We also present our ongoing ‘time record’ of the nuclear transition frequency referenced to the JILA Sr atomic clock, providing a unique monitor for temporal variations in fundamental constants.