Squeezed-light enhancement and backaction evasion in a high sensitivity optically pumped magnetometer

TL;DR

This paper demonstrates that polarization squeezing enhances the sensitivity and bandwidth of a quantum-noise-limited optically pumped magnetometer, effectively evading measurement backaction noise and improving performance at high frequencies.

Contribution

It provides a theoretical and experimental analysis of how polarization squeezing improves magnetometer sensitivity and bandwidth, including backaction noise evasion in a high-density rubidium vapor system.

Findings

Polarization squeezing improves high-frequency sensitivity.

Measurement backaction noise is evaded through ellipticity anti-squeezing.

The model includes spin projection noise, probe noise, and backaction effects.

Abstract

We study the effect of optical polarization squeezing on the performance of a sensitive, quantum-noise-limited optically pumped magnetometer. We use Bell-Bloom (BB) optical pumping to excite a $^{87}$Rb vapor containing $8.2 \cdot 10^{12} \mathrm{atoms/cm^3}$ and Faraday rotation to detect spin precession. The sub-$\mathrm{pT}/\sqrt{\mathrm{Hz}}$ sensitivity is limited by spin projection noise (photon shot noise) at low (high) frequencies. Probe polarization squeezing both improves high-frequency sensitivity and increases measurement bandwidth, with no loss of sensitivity at any frequency, a direct demonstration of the evasion of measurement backaction noise. We provide a model for the quantum noise dynamics of the BB magnetometer, including spin projection noise, probe polarization noise, and measurement backaction effects. The theory shows how polarization squeezing reduces optical noise, while measurement backaction due to the accompanying ellipticity anti-squeezing is shunted into the unmeasured spin component. The method is compatible with high-density and multi-pass techniques that reach extreme sensitivity.