Laser cooling of optically trapped molecules

Ultracold molecules are ideal platforms for many important applications, ranging from quantum simulation^1–5 and quantum information processing ^6,7 to precision tests of fundamental physics^2,8–11. Producing trapped, dense samples of ultracold molecules is a challenging task. One promising approach is direct laser cooling, which can be applied to several classes of molecules not easily assembled from ultracold atoms^12,13. Here, we report the production of trapped samples of laser-cooled CaF molecules with densities of 8 × 10⁷ cm^‑3 and at phase-space densities of 2 × 10^‑9, 35 times higher than for sub-Doppler-cooled samples in free space¹⁴. These advances are made possible by efficient laser cooling of optically trapped molecules to well below the Doppler limit, a key step towards many future applications. These range from ultracold chemistry to quantum simulation, where conservative trapping of cold and dense samples is desirable. In addition, the ability to cool optically trapped molecules opens up new paths towards quantum degeneracy.