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

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) = $217105181895(20)$ kHz; R(2) = $219042856621(28)$ kHz; R(3) = $220704304951(28)$ 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.

5 pages, 4 figures