Electron spin relaxation times of phosphorus donors in silicon

Donor electron spins in phosphorus-doped silicon (Si:P) are a candidate two-level system (qubit) for quantum information processing. Spin echo measurements of isotopically purified ²⁸Si:P are presented that show exceptionally long transverse relaxation (decoherence) times, T₂, at low temperature. Below ∼10 K the spin decoherence is shown to be controlled by instantaneous diffusion and at higher temperatures by an Orbach process. T₂ for small pulse turning angles is 14 ms at 7 K and extrapolates to ∼60 ms for an isolated spin, over 2 orders of magnitude longer than previously demonstrated.