  Crystal defects can confine isolated electronic spins and are promising
candidates for solid-state quantum information. Alongside research focusing on
nitrogen vacancy centers in diamond, an alternative strategy seeks to identify
new spin systems with an expanded set of technological capabilities, a
materials driven approach that could ultimately lead to "designer" spins with
tailored properties. Here, we show that the 4H, 6H and 3C polytypes of SiC all
host coherent and optically addressable defect spin states, including spins in
all three with room-temperature quantum coherence. The prevalence of this spin
coherence shows that crystal polymorphism can be a degree of freedom for
engineering spin qubits. Long spin coherence times allow us to use double
electron-electron resonance to measure magnetic dipole interactions between
spin ensembles in inequivalent lattice sites of the same crystal. Together with
the distinct optical and spin transition energies of such inequivalent spins,
these interactions provide a route to dipole-coupled networks of separately
addressable spins.
