  Paramagnetic defects in diamond and hexagonal boron nitride possess a unique
combination of spin and optical properties that make them prototypical
solid-state qubits. Despite the coherence of these spin qubits being critically
limited by spin-phonon relaxation, a full understanding of this process is not
yet available. Here we apply ab initio spin dynamics simulations to this
problem and quantitatively reproduce the experimental temperature dependence of
spin relaxation time and spin coherence time. We demonstrate that low-frequency
two-phonon modulations of the zero-field splitting are responsible for spin
relaxation and decoherence, and point to the nature of vibrations in
2-dimensional materials as the culprit for their shorter coherence time. These
results provide a novel interpretation to spin-phonon decoherence in
solid-state paramagnetic defects, offer a new strategy to correctly interpret
experimental results, and pave the way for the accelerated design of new spin
qubits.
