Laser Cooling of Radium-225 Ions

Radium-225 (nuclear spin $I=1\mathrm{/}2$) ions possess electronic hyperfine transitions that are first-order insensitive to magnetic field noise, which is advantageous for optical clocks and quantum information science. We report on laser cooling and trapping of radium-225 ions and hyperfine splitting measurements of the ion’s $7s$ ${}^2{S}_{1\mathrm{/}2}$, $7p$ ${}^2{P}_{1\mathrm{/}2}$, and $6d$ ${}^2{D}_{3\mathrm{/}2}$ states. We measured the ground state hyperfine constant, $A{(}^2{S}_{1\mathrm{/}2})=-27.684511056(9)\mathrm{G}\mathrm{H}\mathrm{z}$, and the quadratic Zeeman coefficient, $C_2=142.3(10){\mathrm{H}\mathrm{z}\mathrm{G}}^{-2}$, of the ${}^2{S}_{1\mathrm{/}2}(F=0,m_F=0)\leftrightarrow {}^2{S}_{1\mathrm{/}2}(F=1,m_F=0)$ transition. We also measured the hyperfine constants of the ${}^2{P}_{1\mathrm{/}2}$ state, $A{(}^2{P}_{1\mathrm{/}2})=-5.447(4)\mathrm{G}\mathrm{H}\mathrm{z}$, and the ${}^2{D}_{3\mathrm{/}2}$ state, $A{(}^2{D}_{3\mathrm{/}2})=-619.7(11)\mathrm{M}\mathrm{H}\mathrm{z}$.