  Dressed spin states, a spin coupling to continuous radiation fields, can
fundamentally change how a spin responds to magnetic fluctuations. Using
dressed spin states, we were able to protect an electron spin in diamond from
decoherence. Dressing a spin with resonant microwaves at a coupling rate near 1
MHz leads to a 50 times reduction in the linewidth of the spin transition,
limited by transit-time broadening. The spin decoherence and the energy level
structure of the dressed states were probed with optical
coherent-population-trapping processes. Compared with dynamical decoupling,
where effects of the bath are averaged out at specific times, the dressed state
provides a continuous protection from decoherence.
