Measuring the Hyperfine Structure of Radium-225 Ions

We discuss measurements of the hyperfine structure of the low-lying states in ²²⁵Ra⁺ (15 day half-life) made with laser-cooled and trapped ions. This isotope has nuclear spin I=1/2 which has favorable properties for a trapped ion qubit or for an optical clock. ²²⁵Ra⁺ is also sensitive to probes of time reversal symmetry due to its octupole-deformed nucleus. We use a thorium-229 (7900 year half-life) source to continuously generate ²²⁵Ra⁺ in a sealed vacuum system. The source is contained in a titanium crucible that is part of an effusive oven. The vapor pressure of thorium is more than a trillion times lower than radium, so it remains in the crucible while the radium is heated out. The neutral radium atoms are ionized in a two-step photoionization process and then laser-cooled and trapped. We measured the hyperfine splitting of the ²S_1/2 ground state with microwave Rabi flopping. We optically measured the ²P_1/2 excited state hyperfine splitting and are applying a similar technique to measure the hyperfine splitting of the ²D_3/2 excited state. These measurements will encompass all of the ion’s states that are used during Doppler cooling. Due to this ion’s favorable wavelengths and hyperfine structure we are working towards a ²²⁵Ra⁺ ion optical clock. We are also pursuing tests of fundamental symmetries with radium-bearing molecules, leveraging the enhanced sensitivity of the radium-225 nucleus to time reversal symmetry violation.