  We present two strategies for performing two-qubit operations on the electron
spins of an exchange-coupled pair of phosphorus donors in silicon, using the
ability to set the donor nuclear spins in arbitrary states. The effective
magnetic detuning of the two electron qubits is provided by the hyperfine
interaction when the $^{31}$P nuclei are prepared in opposite spin states. This
can be exploited to switch on and off SWAP operations with modest tuning of the
electron exchange interaction. Furthermore, the hyperfine detuning enables
high-fidelity conditional rotation gates based on selective resonant
excitation. The latter requires no dynamic tuning of the exchange interaction
at all, and offers a very attractive scheme to implement two-qubit logic gates
under realistic experimental conditions.
