Spin Damping in an RF Atomic Magnetometer

TL;DR

This paper demonstrates that negative feedback can significantly damp spin dynamics in RF atomic magnetometers, reducing noise and increasing bandwidth without sacrificing sensitivity, and distinguishes different noise sources through spectral analysis.

Contribution

It introduces a spin-damping technique that broadens noise spectra and enhances bandwidth while preserving sensitivity, and clarifies noise suppression mechanisms.

Findings

Bandwidth increased threefold with maintained sensitivity

Noise suppression varies with noise type and spectral response

Photon-shot noise suppression suggests polarization squeezing

Abstract

Under negative feedback, the quality factor Q of a radio-frequency magnetometer can be decreased by more than two orders of magnitude, so that any initial perturbation of the polarized spin system can be rapidly damped, preparing the magnetometer for detection of the desired signal. We find that noise is also suppressed under such spin-damping, with a characteristic spectral response corresponding to the type of noise; therefore magnetic, photon-shot, and spin-projection noise can be measured distinctly. While the suppression of resonant photon-shot noise implies the closed-loop production of polarization-squeezed light, the suppression of resonant spin-projection noise does not imply spin-squeezing, rather simply the broadening of the noise spectrum with Q. Furthermore, the application of spin-damping during phase-sensitive detection suppresses both signal and noise in such a way as to increase the sensitivity bandwidth. We demonstrate a three-fold increase in the magnetometer's bandwidth while maintaining 0.3 fT/\surdHz sensitivity.