  Understanding the thermalization dynamics of quantum many-body systems at the
microscopic level is among the central challenges of modern statistical
physics. Here we experimentally investigate individual spin dynamics in a
two-dimensional ensemble of electron spins on the surface of a diamond crystal.
We use a near-surface NV center as a nanoscale magnetic sensor to probe
correlation dynamics of individual spins in a dipolar interacting surface spin
ensemble. We observe that the relaxation rate for each spin is significantly
slower than the naive expectation based on independently estimated dipolar
interaction strengths with nearest neighbors and is strongly correlated with
the timescale of the local magnetic field fluctuation. We show that this
anomalously slow relaxation rate is due to the presence of strong dynamical
disorder and present a quantitative explanation based on dynamic resonance
counting. Finally, we use resonant spin-lock driving to control the effective
strength of the local magnetic fields and reveal the role of the dynamical
disorder in different regimes. Our work paves the way towards microscopic study
and control of quantum thermalization in strongly interacting disordered spin
ensembles.
