  New magnetometry techniques based on Nitrogen Vacancy (NV) defects in diamond
have received much attention of late as a means to probe nanoscale magnetic
environments. The sensitivity of a single NV magnetometer is primarily
determined by the transverse spin relaxation time, $T_2$. Current approaches to
improving the sensitivity employ crystals with a high NV density at the cost of
spatial resolution, or extend $T_2$ via the manufacture of novel isotopically
pure diamond crystals. We adopt a complementary approach, in which optimal
dynamic decoupling techniques extend coherence times out to the
self-correlation time of the spin bath. This suggests single spin, room
temperature magnetometer sensitivities as low as 5\,pT\,Hz$^{-1/2}$ with
current technology.
