Electron spin coherence in semiconductors:  Considerations for a spin-based solid-state quantum computer architecture

We theoretically consider coherence times for spins in two quantum computer architectures, where the qubit is the spin of an electron bound to a P donor impurity in Si or within a GaAs quantum dot. We show that low-temperature decoherence is dominated by spin-spin interactions, through spectral diffusion and dipolar flip-flop mechanisms. These contributions lead to $1–100-\mu \mathrm{s}$ calculated spin coherence times for a wide range of parameters, much higher than former estimates based on $T_2^{*}$ measurements.