Quantum-enhanced magnetometry at optimal number density

TL;DR

This paper demonstrates that using squeezed light and back-action evasion techniques can significantly improve the sensitivity of optically-pumped magnetometers at their optimal atom density, surpassing classical limits.

Contribution

It introduces a method combining squeezed probe light and measurement back-action evasion to enhance magnetometer sensitivity at optimal atom density.

Findings

Squeezed light improves sensitivity beyond classical limits.

Optimal sensitivity is density-dependent and limited by quantum noise.

Back-action evasion enables surpassing the laser-light sensitivity threshold.

Abstract

We study the use of squeezed probe light and evasion of measurement back-action to enhance the sensitivity and measurement bandwidth of an optically-pumped magnetometer (OPM) at sensitivity-optimal atom number density. By experimental observation, and in agreement with quantum noise modeling, a spin-exchange-limited OPM probed with off-resonance laser light is shown to have an optimal sensitivity determined by density-dependent quantum noise contributions. Application of squeezed probe light boosts the OPM sensitivity beyond this laser-light optimum, allowing the OPM to achieve sensitivities that it cannot reach with coherent-state probing at any density. The observed quantum sensitivity enhancement at optimal number density is enabled by measurement back-action evasion.