  We report NMR experiments using high-power, RF decoupling techniques to show
that a 29-Si nuclear spin qubit in a solid silicon crystal at room temperature
can preserve quantum phase for 10^9 precessional periods. The coherence times
we report are longer than for any other observed solid-state qubit by more than
four orders of magnitude. In high quality crystals, these 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 1/f
noise. These results provide insight toward proposals for solid-state
nuclear-spin-based quantum memories and quantum computers based on silicon.
