Proposed silicon-based quantum-computer architectures have attractedattention because of their promise for scalability and their potential forsynergetically utilizing the available resources associated with the existingSi technology infrastructure. Electronic and nuclear spins of shallow donors(e.g. phosphorus) in Si are ideal candidates for qubits in such proposalsbecause of their long spin coherence times due to their limited interactionswith their environments. For these spin qubits, shallow donor exchange gatesare frequently invoked to perform two-qubit operations. We discuss in thisreview a particularly important spin decoherence channel, and bandstructureeffects on the exchange gate control. Specifically, we review our work on donorelectron spin spectral diffusion due to background nuclear spin flip-flops, andhow isotopic purification of silicon can significantly enhance the electronspin dephasing time. We then review our calculation of donor electron exchangecoupling in the presence of degenerate silicon conduction band valleys. We showthat valley interference leads to orders of magnitude variations in electronexchange coupling when donor configurations are changed on an atomic scale.These studies illustrate the substantial potential that donor electron/nuclearspins in silicon have as candidates for qubits and simultaneously theconsiderable challenges they pose. In particular, our work on spin decoherencethrough spectral diffusion points to the possible importance of isotopicpurification in the fabrication of scalable solid state quantum computerarchitectures. We also provide a critical comparison between the two mainproposed spin-based solid state quantum computer architectures, namely, shallowdonor bound states in Si and localized quantum dot states in GaAs.
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