  We show that electric field noise from surface charge fluctuations can be a
significant source of spin decoherence for near-surface nitrogen-vacancy (NV)
centers in diamond. This conclusion is based on the increase in spin coherence
observed when the diamond surface is covered with high-dielectric-constant
liquids, such as glycerol. Double resonance experiments show that improved
coherence occurs even though the coupling to nearby electron spins is unchanged
when the liquid is applied. Multipulse spin echo experiments reveal the effect
of glycerol on the spectrum of NV frequency noise.
