  Per the fluctuation-dissipation theorem, the information obtained from spin
fluctuation studies in thermal equilibrium is necessarily constrained by the
system's linear response functions. However, by including weak radiofrequency
magnetic fields, we demonstrate that intrinsic and random spin fluctuations
even in strictly unpolarized ensembles \emph{can} reveal underlying patterns of
correlation and coupling beyond linear response, and can be used to study
non-equilibrium and even multiphoton coherent spin phenomena. We demonstrate
this capability in a classical vapor of $^{41}$K alkali atoms, where spin
fluctuations alone directly reveal Rabi splittings, the formation of Mollow
triplets and Autler-Townes doublets, ac Zeeman shifts, and even nonlinear
multiphoton coherences.
