  Transition metal ions provide a rich set of optically active defect spins in
wide bandgap semiconductors. Chromium (Cr4+) in silicon-carbide (SiC) produces
a spin-1 ground state with a narrow, spectrally isolated, spin-selective,
near-telecom optical interface. However, previous studies were hindered by
material quality resulting in limited coherent control. In this work, we
implant Cr into commercial 4H-SiC and show optimal defect activation after
annealing above 1600 C. We measure an ensemble optical hole linewidth of 31
MHz, an order of magnitude improvement compared to as-grown samples. An
in-depth exploration of optical and spin dynamics reveals efficient spin
polarization, coherent control, and readout with high fidelity (79%). We report
T1 times greater than 1 second at cryogenic temperatures (15 K) with a T2* =
317 nanoseconds and a T2 = 81 microseconds, where spin dephasing times are
currently limited by spin-spin interactions within the defect ensemble. Our
results demonstrate the potential of Cr4+ in SiC as an extrinsic, optically
active spin qubit.
