Spin noise spectroscopy beyond thermal equilibrium and linear response

TL;DR

This paper demonstrates that spin noise spectroscopy, when combined with weak radiofrequency fields, can probe non-equilibrium and nonlinear spin phenomena beyond the limits of linear response in unpolarized atomic ensembles.

Contribution

It introduces a method to study non-equilibrium and multiphoton spin phenomena using spin noise spectroscopy with weak RF fields, surpassing traditional linear response constraints.

Findings

Revealed Rabi splittings and Mollow triplets from spin fluctuations.

Observed Autler-Townes doublets and ac Zeeman shifts.

Detected nonlinear multiphoton coherences in atomic vapor.

Abstract

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.