Sympathetic cooling of trapped Th3+ alpha-recoil ions for laser spectroscopy

The authors experimentally demonstrate the sympathetic cooling of Th³⁺ ions, captured from uranium alpha-recoil sources, in a linear Paul trap using laser-cooled ⁸⁸Sr⁺ ions. They show the efficacy of their method via high-resolution laser spectroscopy. This demonstration is an important step in enabling the use of these ions as the reference of a highly accurate nuclear optical clock.