  We report on room-temperature coherent manipulation of the spin of a single
nitrogen-vacancy center in diamond and a study of its coherence as a function
of magnetic field. We use magnetic resonance to induce Rabi nutations, and
apply a Hahn spin echo to remove the effect of low-frequency dephasing. A sharp
rise in the decoherence rate is observed at magnetic fields where the
nitrogen-vacancy center spin couples resonantly to substitutional nitrogen
spins via the magnetic dipolar coupling. Finally, we find evidence that away
from these energy resonances spin flips of nitrogen electrons are the main
source of decoherence.
