  The electronic and magnetic properties of a neutral substitutional nickel
(Ni$_s^0$) impurity in diamond are studied using density functional theory in
the generalized gradient approximation. The spin-one ground state consists of
two electrons with parallel spins, one located on the nickel ion in the $3d^9$
configuration and the other distributed among the nearest-neighbor carbons. The
exchange interaction between these spins is due to $p-d$ hybridization and is
controllable with compressive hydrostatic or uniaxial strain, and for
sufficient strain the antiparallel spin configuration becomes the ground state.
Hence, the Ni impurity forms a controllable two-electron exchange-coupled
system that should be a robust qubit for solid-state quantum information
processing.
