Constraining fifth forces with Sr₂ molecules

Our Sr₂ molecular lattice clock complements current frequency references by operating in the THz range. Additionally, being based on molecular vibrations rather than on electronic states in an atom, it can be sensitive to different physics and serve as a sensor for ultrashort-range forces between the Sr nuclei. Comparing hertz-level molecular isotope-shift spectroscopy of Sr₂ to quantum chemistry calculations can allow us to characterize these forces precisely and potentially rule out new mass-dependent “fifth” forces. We present progress toward measuring isotope shifts between the ⁸⁸Sr₂ and ⁸⁶Sr₂ isotopologues. To address the challenges posed by the lower natural abundance of ⁸⁶Sr and ⁸⁴Sr, we have designed a second-generation molecular clock experiment, featuring a higher atomic flux and an in-vacuum lattice cavity to increase the trapped-molecule coherence time.