Spectral Lineshape Measurements with Shot-Noise Limited Accuracy

TL;DR

This paper demonstrates laser absorption spectroscopy at the quantum shot-noise limit, achieving high precision measurements of spectral lineshapes in cesium vapor without sacrificing accuracy, revealing subtle spectral features.

Contribution

It introduces a method to perform shot-noise limited spectroscopy with high accuracy, enabling detailed observation of spectral lineshapes beyond Doppler broadening effects.

Findings

Achieved shot-noise limited absorption measurements at 2 ppm accuracy.

Observed homogeneous lineshape components despite Doppler broadening.

Enabled direct measurement of low-intensity optically-induced broadening.

Abstract

Spectroscopy has played the key role in revealing, and thereby understanding, the structure of atoms and molecules. A central drive in this field is the pursuit of higher precision and accuracy so that ever more subtle effects might be discovered. Here, we report on laser absorption spectroscopy that operates at the conventional quantum limit imposed by photon shot-noise. Furthermore, we achieve this limit without compromising the accuracy of the measurement. We demonstrate these properties by recording an absorption profile of cesium vapor at the 2 parts-per-million level. The extremely high signal-to-noise ratio allows us to directly observe the homogeneous lineshape component of the spectral profile, even while in the presence of Doppler broadening that is a factor of 100 times wider. We can do this because we can precisely measure the spectral profile at a frequency detuning more than 200 natural linewidths from the line center. We use the power of this tool to demonstrate direct measurements of a low-intensity optically-induced broadening process that is quite distinct from the well-known power broadening phenomenon.