Optical clock spectroscopy in weakly bound molecules

With relative accuracies reaching 10⁻¹⁸, optical atomic clocks are currently the most sensitive physical instruments known to man. Weakly bound ultracold molecules enable the study of fundamental physics through their sensitivity to e.g. the proton-to-electron mass ratio or hypothetical Yukawa-type fifth forces predicted by several extensions of the Standard Model. These applications, however, require precision measurements of molecular transitions that are beyond current experimental capabilities. Here we propose to construct an 'optical molecular clock' that would solve this problem by bringing the experimental techniques used in optical atomic clocks to the realm of cold molecules. We show that such a clock could utilize ultracold ytterbium molecules and we predict the positions and properties of Yb₂ clock lines. A successful experimental realization of this proposal could pave the way towards sub-Hz level molecular spectroscopy. (paper)