Hyperfine spectroscopy of optical-cycling transitions in singly ionized thulium

We present a spectroscopic investigation of [Formula: see text] that provides two key foundations for its use as a platform for advanced quantum applications. First, we establish the complete spectroscopic road map for optical cycling (including laser cooling) by performing high-resolution spectroscopy on [Formula: see text] ions in an ion trap. We characterize the primary 313 nm and complementary 448/453 nm cycling transitions, identify the essential near-infrared repumping frequencies, and determine the magnetic-dipole hyperfine A constants for all relevant levels. Second, we report a detailed characterization of a metastable state as a candidate for hosting a robust qubit, performing lifetime measurements and Zeeman-resolved microwave hyperfine spectroscopy with [Formula: see text] precision.