Observing random walks of atoms in buffer gas through resonant light absorption

TL;DR

This paper directly observes atomic random walks in buffer gas using resonant light absorption, analyzing spectral fluctuations to extract diffusion properties and atomic parameters with high precision.

Contribution

It introduces a novel spectroscopic method to measure atomic motion and related parameters by analyzing light absorption fluctuations caused by atomic random walks.

Findings

Spectral analysis reveals a $1/f^2$ behavior at high frequencies.

The method accurately determines diffusion constants and atomic parameters.

Theoretical spectra match experimental data exceptionally well.

Abstract

Using resonant light absorption, random walk motions of rubidium atoms in nitrogen buffer gas are observed directly. The transmitted light intensity through atomic vapor is measured and its spectrum is obtained, down to orders of magnitude below the shot noise level to detect fluctuations caused by atomic motions. To understand the measured spectra, the spectrum for atoms performing random walks in a gaussian light beam is computed and its analytical form is obtained. The spectrum has $1/f^2$ ($f$: frequency) behavior at higher frequencies, crossing over to a different, but well defined behavior at lower frequencies. The properties of this theoretical spectrum agree excellently with the measured spectrum. This understanding also enables us to obtain the diffusion constant, the photon cross section of atoms in buffer gas and the atomic number density, from a single spectral measurement. We further discuss other possible applications of our experimental method and analysis.