  Using fast electron spin resonance spectroscopy of a single nitrogen-vacancy
defect in diamond, we demonstrate real-time readout of the Overhauser field
produced by its nuclear spin environment under ambient conditions. These
measurements enable narrowing the Overhauser field distribution by
post-selection, corresponding to a conditional preparation of the nuclear spin
bath. Correlations of the Overhauser field fluctuations are quantitatively
inferred by analysing the Allan deviation over consecutive measurements. This
method allows to extract the dynamics of weakly coupled nuclear spins of the
reservoir.
