  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.
