Coherence time of decoupled nuclear spins in silicon

We report NMR experiments using high-power rf decoupling techniques to show that a ^29Si nuclear spin in a solid silicon crystal at room temperature can preserve quantum phase for 10^9 precessional periods. The coherence times we report are more than four orders of magnitude longer than for any other observed solid-state qubit. We also examine coherence times using magic-angle-spinning techniques and in isotopically altered samples. In high-quality crystals, coherence times are limited by residual dipolar couplings and can be further improved by isotopic depletion. In defect-heavy samples, we provide evidence for decoherence limited by a noise process unrelated to the dipolar coupling. The nonexponential character of these data is compared to a theoretical model for decoherence due to the same charge trapping mechanisms responsible for 1∕f noise. These results provide insight into proposals for solid-state nuclear-spin-based quantum memories and quantum computers based on silicon.