  Electric fields can be used to tune donor spins in silicon using the Stark
shift, whereby the donor electron wave function is displaced by an electric
field, modifying the hyperfine coupling between the electron spin and the donor
nuclear spin. We present a technique based on dynamic decoupling of the
electron spin to accurately determine the Stark shift, and illustrate this
using antimony donors in isotopically purified silicon-28. We then demonstrate
two different methods to use a DC electric field combined with an applied
resonant radio-frequency (RF) field to conditionally control donor nuclear
spins. The first method combines an electric-field induced conditional phase
gate with standard RF pulses, and the second one simply detunes the spins
off-resonance. Finally, we consider different strategies to reduce the effect
of electric field inhomogeneities and obtain above 90% process fidelities.
