Trapping and sympathetic cooling of single thorium ions for spectroscopy

Abstract

Precision optical spectroscopy of exotic ions reveals accurate information about nuclear properties such as charge radii and magnetic and quadrupole moments. Thorium ions exhibit unique nuclear properties with high relevance for testing symmetries of nature. We report loading and trapping of single $^{232}\mathrm{Th}^{+}$ ions in a linear Paul trap, embedded into and sympathetically cooled by small crystals of trapped $^{40}\mathrm{Ca}^{+}$ ions. Trapped thorium ions are identified in a nondestructive manner from the voids in the laser-induced calcium fluorescence pattern emitted by the crystal, and alternatively, by means of a time-of-flight signal when extracting ions from the Paul trap and steering them into an external detector. We have loaded and handled a total of 231 individual thorium ions. We reach a time-of-flight detection efficiency of $\ensuremath{\gtrsim}95%$, consistent with the quantum efficiency of the detector. The sympathetic cooling technique is expected to be applicable for other isotopes and various charge states of thorium, e.g., for future studies of $^{229\mathrm{m}}\mathrm{Th}$.