Observation of Collisions between Two Ultracold Ground-State CaF Molecules

We measure inelastic collisions between ultracold CaF molecules by combining two optical tweezers, each containing a single molecule. We observe collisions between (2)Sigma CaF molecules in the absolute ground state vertical bar X, v = 0, N = 0, F = 0 >, and in excited hyperfine and rotational states. In the absolute ground state, we find a two-body loss rate of 7(4) x 10(-11) cm(3)/s, which is below, but close to, the predicted universal loss rate.