Sympathetic Cooling ofHe+4Ions in a Radio-Frequency Trap

We have generated Coulomb crystals of ultracold ${}^4\mathrm{H}\mathrm{e}^{+}$ ions in a linear radio-frequency trap, by sympathetic cooling via laser-cooled ${}^9\mathrm{B}\mathrm{e}^{+}$. Stable crystals containing up to $150$ localized ${\mathrm{H}\mathrm{e}}^{+}$ ions at $\sim 20\mathrm{m}\mathrm{K}$ were obtained. Ensembles or single ultracold ${\mathrm{H}\mathrm{e}}^{+}$ ions open up interesting perspectives for performing precision tests of QED and measurements of nuclear radii. This Letter also indicates the feasibility of cooling and crystallizing highly charged atomic ions using ${}^9\mathrm{B}\mathrm{e}^{+}$ as coolant.