Spin-noise spectroscopy of a noise-squeezed atomic state

Spin-noise spectroscopy is emerging as a powerful technique for studying the dynamics of various spin systems also beyond their thermal equilibrium and linear response. In this context, we demonstrate a nonstandard mode of the spin-noise analysis applied to an out-of-equilibrium nonlinear atomic system realized by a Bell-Bloom atomic magnetometer. Driven by an external pump and undergoing a parametric excitation, this system is known to produce noise squeezing. Our measurements not only reveal a strong asymmetry in the noise distribution of the atomic signal quadratures at the magnetic resonance, but also provide insight into the mechanism behind its generation and evolution. In particular, a structure in the spectrum is identified which allows to investigate the main dependencies and the characteristic timescales of the noise process. The results obtained are compatible with parametrically induced noise squeezing. Notably, the noise spectrum provides information on the spin dynamics even in regimes where the macroscopic atomic coherence is lost, effectively enhancing the sensitivity of the measurements. Our Letter promotes spin-noise spectroscopy as a versatile technique for the study of noise squeezing in a wide range of spin-based magnetic sensors.