Maximising Precision in Saturation-Limited Absorption Measurements

TL;DR

This paper analyzes how saturation affects measurement precision in absorption spectroscopy and proposes an optimization strategy that leverages quantum and classical probes, including amplitude-squeezed light, to approach fundamental limits.

Contribution

It introduces a generalised model accounting for saturation effects and develops an optimization method for probe power and state to maximize measurement precision.

Findings

Optimal probe powers are within the saturation regime.

Amplitude-squeezed light can achieve >85% of the quantum limit.

The strategy is applicable to high-precision thermometry and chlorophyll spectroscopy.

Abstract

Quantum fluctuations in the intensity of an optical probe is noise which limits measurement precision in absorption spectroscopy. Increased probe power can offer greater precision, however, this strategy is often constrained by sample saturation. Here, we analyse measurement precision for a generalised absorption model in which we account for saturation and explore its effect on both classical and quantum probe performance. We present a classical probe-sample optimisation strategy to maximise precision and find that optimal probe powers always fall within the saturation regime. We apply our optimisation strategy to two examples, high-precision Doppler broadened thermometry and an absorption spectroscopy measurement of Chlorophyll A. We derive a limit on the maximum precision gained from using a non-classical probe and find a strategy capable of saturating this bound. We evaluate amplitude-squeezed light as a viable experimental probe state and find it capable of providing precision that reaches to within > 85% of the ultimate quantum limit with currently available technology.