Trapping and sympathetic cooling of single thorium ions for spectroscopy

TL;DR

This paper demonstrates the successful trapping, identification, and sympathetic cooling of single thorium ions, enabling precise spectroscopy for nuclear property investigations and future quantum experiments.

Contribution

It introduces a method for trapping and sympathetically cooling single thorium ions using calcium ion crystals, advancing nuclear spectroscopy capabilities.

Findings

Trapped and identified 231 individual Th ions.

Achieved over 95% detection efficiency.

Demonstrated sympathetic cooling applicable to other isotopes.

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 $^{232}$Th$^+$ ions in a linear Paul trap, embedded into and sympathetically cooled by small crystals of trapped $^{40}$Ca$^+$ ions. Trapped Th ions are identified in a non-destructive manner from the voids in the laser-induced Ca 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 Th ions. We reach a time-of-flight detection efficiency of $\gtrsim 95\, \%$, 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 Th e.g., for future studies of $^{229m}$Th.