Sensitivity, quantum limits, and quantum enhancement of noise spectroscopies

TL;DR

This paper investigates the fundamental quantum limits of noise spectroscopy, deriving a general Fisher information expression, confirming limits through experiments, and demonstrating sensitivity enhancement with squeezed light.

Contribution

It introduces a universal Fisher information formula for noise spectroscopy, confirms quantum limits experimentally, and shows how squeezed light can surpass these limits.

Findings

Shot noise sets local and global quantum limits for spin noise spectroscopy.

Experimental validation of theoretical quantum limits using Faraday rotation in Rb vapor.

Squeezed light enables sensitivity beyond standard quantum limits.

Abstract

We study the fundamental limits of noise spectroscopy using estimation theory, Faraday rotation probing of an atomic spin system, and squeezed light. We find a simple and general expression for the Fisher information, which quantifies the sensitivity to spectral parameters such as resonance frequency and linewidth. For optically-detected spin noise spectroscopy, we find that shot noise imposes "local" standard quantum limits for any given probe power and atom number, and also "global" standard quantum limits when probe power and atom number are taken as free parameters. We confirm these estimation theory results using non-destructive Faraday rotation probing of hot Rb vapor, observing the predicted optima and finding good quantitative agreement with a first-principles calculation of the spin noise spectra. Finally, we show sensitivity beyond the atom- and photon-number-optimized global standard quantum limit using squeezed light.