  Electron and nuclear spins associated with point defects in insulators are
promising systems for solid state quantum technology. While the electron spin
usually is used for readout and addressing, nuclear spins are exquisite quantum
bits and memory systems. With these systems single-shot readout of nearby
nuclear spins as well as entanglement aided by the electron spin has been
shown. While the electron spin in this example is essential for readout it
usually limits nuclear spin coherence. This has set of the quest for defects
with spin-free ground states. Here, we isolate a hitherto unidentified defect
in diamond and use it at room temperature to demonstrate optical spin
polarization and readout with exceptionally high contrast (up to 45%), coherent
manipulation of an individual excited triplet state spin, and coherent nuclear
spin manipulation using the triplet electron spin as a meta-stable ancilla. By
this we demonstrate nuclear magnetic resonance and Rabi oscillations of the
uncoupled nuclear spin in the spin-free electronic ground state. Our study
demonstrates that nuclei coupled to single metastable electron spins are useful
quantum systems with long memory times despite electronic relaxation processes.
