  Proposed silicon-based quantum-computer architectures have attracted
attention because of their promise for scalability and their potential for
synergetically utilizing the available resources associated with the existing
Si technology infrastructure. Electronic and nuclear spins of shallow donors
(e.g. phosphorus) in Si are ideally suited candidates for qubits in such
proposals, where shallow donor exchange gates are frequently invoked to perform
two-qubit operations. An important potential problem in this context is that
intervalley interference originating from the degeneracy in the Si
conduction-band edge causes fast oscillations in donor exchange coupling, which
imposes significant constraints on the Si quantum-computer architecture. We
discuss the theoretical origin of such oscillations. Considering two
substitutional donors in Si, we present a systematic statistical study of the
correlation between relative position distributions and the resulting exchange
distributions.
