  Harnessing the potential wide-ranging quantum science applications of
molecules will require control of their interactions. Here, we use microwave
radiation to directly engineer and tune the interaction potentials between
ultracold calcium monofluoride (CaF) molecules. By merging two optical
tweezers, each containing a single molecule, we probe collisions in three
dimensions. The correct combination of microwave frequency and power creates an
effective repulsive shield, which suppresses the inelastic loss rate by a
factor of six, in agreement with theoretical calculations. The demonstrated
microwave shielding shows a general route to the creation of long-lived, dense
samples of ultracold molecules and evaporative cooling.
