  A high-spin nucleus coupled to a color center can act as a long-lived memory
qudit in a spin-photon interface. The germanium vacancy (GeV) in diamond has
attracted recent attention due to its excellent spectral properties and
provides access to the ten-dimensional Hilbert space of the $I=9/2$ ${}^{73}$Ge
nucleus. Here, we observe the ${}^{73}$GeV hyperfine structure, perform nuclear
spin readout, and optically initialize the ${}^{73}$Ge spin into any eigenstate
on a $\mu$s timescale and with a fidelity of up to $\sim 84\%$. Our results
establish the ${}^{73}$GeV as an optically addressable high-spin quantum
platform for a high-efficiency spin-photon interface as well as for
foundational quantum physics and metrology.
