SUB-DOPPLER FREQUENCY METROLOGY IN HD FOR TEST OF FUNDAMENTAL PHYSICS

Molecular hydrogen has evolved into a benchmark quantum test system for fundamental physics. Accurate results on the vibrational splitting in hydrogen isotopologues can be exploited to 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. We will present the first sub-Doppler determination of weak dipole transitions in the (2,0) overtone band of HD at $\lambda\sim{\rm 1.38\,\mu m}$. To saturate and detect the weak absorption we have implemented a technique called Noise-Immune Cavity-Enhanced Optical Heterodyne Molecular Spectroscopy (NICE-OHMS). To obtain an absolute frequency during the measurements, the spectroscopy laser is simultaniously locked onto a Cs-clock referenced optical frequency comb. The obtained Doppler-free linewidth of $\sim{\rm 300 kHz}$ (FWHM) could give access and insight into the underlying hyperfine structure. Our current determination of the obtained transition frequencies is around 30 kHz; a 1000-fold improvement on the previous Doppler-broadened determination