Hyperfine spectroscopy and microwave characterization of a trapped ¹⁶⁹Tm⁺ qubit candidate

We report the experimental realization of a complete spectroscopic roadmap for the laser cooling and state control of trapped singly ionized thulium (¹⁶⁹Tm⁺). While the complex electronic structure of rare-earth ions offers unique advantages for quantum information, it also presents challenges in identifying closed optical cycles. We identify a primary cooling transition at 313 nm and a complementary pair at 448/453 nm, along with the necessary near-infrared repumping frequencies. By performing hyperfine-resolved fluorescence spectroscopy in a linear quadrupole trap, we determine magnetic-dipole hyperfine A constants for relevant levels and demonstrate Zeeman-resolved microwave spectroscopy of a long-lived metastable state as a candidate for qubit encoding. These findings lower the technical barrier for utilizing thulium as a scalable platform for quantum sensing and "omg" qubit protocols.