Ultimate limits of exoplanet spectroscopy: a quantum approach

TL;DR

This paper establishes the fundamental quantum limits for detecting atmospheric spectral features of exoplanets amidst bright stars, proposing a measurement strategy that significantly improves detection accuracy over traditional methods.

Contribution

It introduces the ultimate quantum limit for exoplanet spectral detection and proposes a structured measurement approach that outperforms direct spectroscopy.

Findings

Structured measurement based on spatial demultiplexing achieves quantum limits.

Significant improvement in error exponent over direct spectroscopy for faint planets.

Optimal measurement combines interferometry and spectrum analysis.

Abstract

One of the big challenges in exoplanet science is to determine the atmospheric makeup of extrasolar planets, and to find biosignatures that hint at the existence of biochemical processes on another world. The biomarkers we are trying to detect are gases in the exoplanet atmosphere like oxygen or methane, which have deep absorption features in the visible and near-infrared spectrum. Here we establish the ultimate quantum limit for determining the presence or absence of a spectral absorption line, for a dim source in the presence of a much brighter stellar source. We characterise the associated error exponent in both the frameworks of symmetric and asymmetric hypothesis testing. We found that a structured measurement based on spatial demultiplexing allows us to decouple the light coming from the planet and achieve the ultimate quantum limits. If the planet has intensity $\epsilon \ll 1$ relative to the star, we show that this approach significantly outperforms direct spectroscopy yielding an improvement of the error exponent by a factor $1/\epsilon$. We find the optimal measurement, which is a combination of interferometric techniques and spectrum analysis.