Source data for: Raman sideband cooling of molecules in an optical tweezer array

Ultracold molecules, because of their rich internal structures and interactions, have been proposed as a promising platform for quantum science and precision measurement. Direct laser-cooling promises to be a rapid and efficient way to bring molecules to ultracold temperatures. For trapped molecules, laser-cooling to the quantum motional ground state remains an outstanding challenge. A technique capable of reaching the motional ground state is Raman sideband cooling, first demonstrated in trapped ions and atoms. In this work, we demonstrate for the first time Raman sideband cooling of molecules. Specifically, we demonstrate 3D Raman cooling for single CaF molecules trapped in an optical tweezer array, achieving average radial (axial) motional occupation as low as $\bar{n}_r=0.27(7)$ ($\bar{n}_z=7.0(10)$). Notably, we measure a 1D ground state fraction as high as 0.79(4), and a motional entropy per particle of $s = 4.9(3)$, the lowest reported for laser-cooled molecules to date. These lo...