Effects of Spin-Exchange Collisions on the Fluctuation Spectra of Hot Alkali Vapors

We present a first-principles analysis of the noise spectra of alkali-metal-metal vapors in and out of the spin-exchange-relaxation-free (SERF) regime, and we predict non-intuitive features with a potential to further improve the sensitivity of SERF media, and which must be taken into account in their use in quantum optical applications. Studying the process of spin-noise spectroscopy (SNS), we derive analytic formulas for the observable noise spectra, and for the correlation functions among different hyperfine components, which give additional insight into the spin dynamics. The analytic results indicate a variety of distortions of the spin-noise spectrum relative to simpler models, including a broad spectral background that mimics optical shot noise, interference of noise contributions from the two ground-state hyperfine levels, noise reduction at the spin-precession frequency, and ``hiding'' of spin-noise power that can introduce a systematic error in noise-based calibrations, e.g., for spin-squeezing experiments.