The digital revolution relies on the miniaturization of semiconductor-based transistors, significantly accelerating data processing speeds. However, future demands for even faster processing face limitations due to the physical constraints of miniaturization, as quantum effects will increasingly influence performance. In response, scientists are exploring innovative technologies that harness quantum effects to transform information processing. One promising area is spintronics, which focuses on controlling the spin state of charge carriers like electrons and holes. This manipulation creates nonequilibrium spin states that eventually relax back to equilibrium due to environmental interactions. Spintronics encounters two main challenges: creating environments for long-lived nonequilibrium spin states and developing reliable measurement techniques for relaxation dynamics. Optical spectroscopy of carrier spin noise—statistical spin fluctuations in thermal equilibrium—emerges as a novel method to study spin dynamics. The power spectrum of spin noise serves as the Fourier transform of nonequilibrium relaxation dynamics. This work reviews recent experimental advancements in carrier spin noise spectroscopy across atomic gases, bulk semiconductors, and semiconductor nanostructures, highlighting its potential for future developments in the field.
