  Single-qubit gates are essential components of a universal quantum computer.
Without selective addressing of individual qubits, scalable implementation of
quantum algorithms is not possible. When the qubits are discrete points or
regions on a lattice, the selective addressing of magnetic spin qubits at the
nanoscale remains a challenge due to the difficulty of localizing and confining
a classical divergence-free field to a small volume of space. Herein we propose
a new technique for addressing spin qubits using voltage-control of nanoscale
magnetism, exemplified by the use of voltage control of magnetic anisotropy
(VCMA). We show that by tuning the frequency of the nanomagnet's electric field
drive to the Larmor frequency of the spins confined to a nanoscale volume, and
by modulating the phase of the drive, single-qubit quantum gates with
fidelities approaching those for fault-tolerant quantum computing can be
implemented. Such single-qubit gate operations have the advantage of remarkable
energy efficiency, requiring only tens of femto-Joules per gate operation, and
lossless, purely magnetic field control (no E-field over the target volume).
Their physical realization is also straightforward using existing foundry
manufacturing techniques.
