  Large-scale quantum computers must be built upon quantum bits that are both
highly coherent and locally controllable. We demonstrate the quantum control of
the electron and the nuclear spin of a single 31P atom in silicon, using a
continuous microwave magnetic field together with nanoscale electrostatic
gates. The qubits are tuned into resonance with the microwave field by a local
change in electric field, which induces a Stark shift of the qubit energies.
This method, known as A-gate control, preserves the excellent coherence times
and gate fidelities of isolated spins, and can be extended to arbitrarily many
qubits without requiring multiple microwave sources.
