Sub-Doppler frequency metrology in HD for test of fundamental physics

TL;DR

This paper reports highly precise measurements of weak transitions in HD molecules using advanced spectroscopy techniques, providing critical data for testing quantum electrodynamics and exploring fundamental physics questions.

Contribution

The study achieves a three-order-of-magnitude improvement in the accuracy of HD transition frequencies using noise-immune cavity-enhanced spectroscopy and frequency comb referencing.

Findings

Transition frequencies measured with unprecedented precision

Benchmark data for testing QED in molecules

Potential to address proton radius puzzle and fundamental forces

Abstract

Weak transitions in the (2,0) overtone band of the HD molecule at $\lambda = 1.38 \, \mu$m were measured in saturated absorption using the technique of noise-immune cavity-enhanced optical heterodyne molecular spectroscopy. Narrow Doppler-free lines were interrogated with a spectroscopy laser locked to a frequency comb laser referenced to an atomic clock to yield transition frequencies [R(1) = $217\,105\,181\,895\,(20)$ kHz; R(2) = $219\,042\,856\,621\,(28)$ kHz; R(3) = $220\,704\,304\,951\,(28)$ kHz] at three orders of magnitude improved accuracy. These benchmark values provide a test of QED in the smallest neutral molecule, and open up an avenue to resolve the proton radius puzzle, as well as constrain putative fifth forces and extra dimensions.