Silicon-based spin quantum computation and the shallow donor exchange   gate
  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. 
Belita Koiller, R. B. Capaz, X. Hu and S. Das Sarma