  We present pulsed electron-nuclear double resonance (ENDOR) experiments which
enable us to characterize the coupling between bismuth donor spin qubits in Si
and the surrounding spin bath of 29Si impurities which provides the dominant
decoherence mechanism (nuclear spin diffusion) at low temperatures (< 16 K).
Decoupling from the spin bath is predicted and cluster correlation expansion
simulations show near-complete suppression of spin diffusion, at optimal
working points. The suppression takes the form of sharply peaked divergences of
the spin diffusion coherence time, in contrast with previously identified
broader regions of insensitivity to classical fluctuations. ENDOR data suggest
that anisotropic contributions are comparatively weak, so the form of the
divergences is largely independent of crystal orientation.
