Coherence time of decoupled nuclear spins in silicon

We report NMR experiments using high-power rf decoupling techniques to show that a Si-29 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.