Frequency reproducibility of solid-state Th-229 nuclear clocks

Solid-state $^{229}$Th nuclear clocks are set to provide new opportunities for precision metrology and fundamental physics. Taking advantage of a nuclear transition's inherent low sensitivity to its environment, orders of magnitude more emitters can be hosted in a solid-state crystal compared to current optical lattice atomic clocks. Furthermore, solid-state systems needing only simple thermal control are key to the development of field-deployable compact clocks. In this work, we explore and characterize the frequency reproducibility of the $^{229}$Th:CaF$_2$ nuclear clock transition, a key performance metric for all clocks. We measure the transition linewidth and center frequency as a function of the doping concentration, temperature, and time. We report the concentration-dependent inhomogeneous linewidth of the nuclear transition, limited by the intrinsic host crystal properties. We determine an optimal working temperature for the $^{229}$Th:CaF$_2$ nuclear clock at 195(5) K where the first-order thermal sensitivity vanishes. This would enable in-situ temperature co-sensing using different quadrupole-split lines, reducing the temperature-induced systematic shift below the 10$^{-18}$ fractional frequency uncertainty level. At 195 K, the reproducibility of the nuclear transition frequency is 280 Hz (fractionally $1.4\times10^{-13}$) for two differently doped $^{229}$Th:CaF$_2$ crystals over four months. These results form the foundation for understanding, controlling, and harnessing the coherent nuclear excitation of $^{229}$Th in solid-state hosts, and for their applications in constraining temporal variations of fundamental constants.