Transit Noise in Spin Squeezing Experiments with Coated Rubidium Vapor Cell

TL;DR

This study investigates how transit noise affects spin squeezing in coated rubidium vapor cells, demonstrating its dependence on probe beam size and atomic Larmor frequency through combined theoretical and experimental analysis.

Contribution

It provides a detailed analysis of transit noise effects and offers practical guidance for optimizing probe beam parameters to improve spin squeezing performance.

Findings

Transit noise increases as probe beam spot area decreases.

Lower Larmor frequencies lead to larger transit noise contributions.

Experimental and theoretical results show a 2.7 dB difference in squeezing between beam sizes.

Abstract

Spin squeezing can suppress quantum projection noise via interparticle entanglement, therefore enabling measurement sensitivities beyond the standard quantum limit. In practice, however, the Gaussian and finite intensity profiles of the optical probe beam induce spatially inhomogeneous atom-light interactions. As polarized atoms move within a vapor cell, they experience position-dependent optical intensities, generating transit noise that limits spin squeezing performance. Here, we investigate the transit noise in a coated rubidium vapor cell through combined theoretical analysis and experimental measurements. By varying the probe beam diameter, we quantify the dependence of transit noise on beam size and atomic Larmor frequency. Our results show that, for a vapor cell with fixed dimensions, the transit noise increases as the probe beam spot area decreases. Moreover, when the Larmor frequency is below the characteristic linewidth of the transit noise, the noise contribution becomes larger. We further calculated and measured spin squeezing for different beam sizes and found an experimental difference of $2.7 \pm 0.2$ dB between 2~mm and 0.6~mm, similar to the theoretical prediction of $3.0 \pm 0.3$ dB. Theoretical analysis under conditions of stronger squeezing shows that transit noise becomes an even more critical limiting factor. These results provide practical guidance for optimizing probe beam parameters and suppressing transit noise in spin squeezing experiments.