Universal quantum logic from Zeeman and anisotropic exchange interactions

Some of the most promising proposals for scalable solid-state quantum computing, e.g., those using electron spins in quantum dots or donor electron or nuclear spins in Si, rely on a two-qubit quantum gate that is ideally generated by an isotropic exchange interaction. However, an anisotropic perturbation arising from spin-orbit coupling is inevitably present. Previous studies focused on removing the anisotropy. Here we introduce a new universal set of quantum logic gates that takes advantage of the anisotropic perturbation. The price is a constant but modest factor in additional pulses. The gain is a scheme that is compatible with the naturally available interactions in spin-based solid-state quantum computers.