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.
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