Homogenization of Doppler broadening in spin noise spectroscopy

TL;DR

This study reveals that despite Doppler broadening, the spin noise spectrum of cesium vapor appears homogeneously broadened, with correlated or anticorrelated fluctuations depending on probe wavelengths, explained by a new theoretical model.

Contribution

It demonstrates effective homogenization of Doppler-broadened lines in spin noise spectroscopy and introduces a theoretical model accounting for atomic kinetics and spin dynamics.

Findings

Spin noise spectrum shows a homogeneously broadened line shape.

Correlations of Faraday rotation fluctuations depend on probe wavelength positioning.

Theoretical model explains the effective homogenization phenomenon.

Abstract

The spin noise of cesium atoms vapor with admixture of buffer gas is experimentally investigated by measuring the spin induced Faraday rotation fluctuations in the vicinity of D 2 line. The line, under these conditions, is known to be strongly inhomogeneously broadened due to the Doppler effect. Despite that, optical spectrum of the spin noise power, as we have found, has the characteristic shape of the homogeneously broadened line with the dip at the line center. This fact is in stark contrast with the results of previous studies of inhomogeneous quantum dot ensembles. In addition, the two-color experiments, where correlations of the Faraday rotation fluctuations for two probe wavelengths were measured, have shown, in a highly spectacular way, that these fluctuations are either correlated, or anticorrelated depending on whether the two wavelengths lie on the same side, or on different sides of the resonance. The experimental data are explained within the developed theoretical model which takes into account both kinetics and spin dynamics of Cs atoms. It is shown that the unexpected behavior of the optical Faraday rotation noise spectra and effective homogenization of the optical transition in the spin-noise measurements are related to smallness of the momentum relaxation time of the atoms as compared with their spin relaxation time.