Magnetic trapping of ultracold molecules at high density

Trapping ultracold molecules in conservative traps is essential for multiple applications, including quantum-state-controlled chemistry, quantum simulations and quantum information processing. The study of molecular collisions, in particular, requires samples at high densities, which have been challenging to achieve so far with established cooling and trapping techniques. Here we report the magnetic trapping of molecules in the triplet ground state at high density and ultralow temperature. We measure the inelastic loss rates in a single-spin sample and spin mixtures of fermionic molecules and spin-stretched atom–molecule mixtures. We demonstrate the sympathetic cooling of molecules in the magnetic trap by the radio-frequency evaporation of co-trapped atoms and observe an increase in the molecules’ phase-space density by a factor of 16. Magnetic trapping at these densities allows the study of both atom–molecule and molecule–molecule collisions in the ultracold regime in the absence of trapping light, which often leads to undesired photochemistry effects.