  Entanglement is crucial to many quantum applications including quantum
information processing, simulation of quantum many-body systems, and
quantum-enhanced sensing. Molecules, because of their rich internal structure
and interactions, have been proposed as a promising platform for quantum
science. Deterministic entanglement of individually controlled molecules has
nevertheless been a long-standing experimental challenge. Here we demonstrate,
for the first time, on-demand entanglement of individually prepared molecules.
Using the electric dipolar interaction between pairs of molecules prepared
using a reconfigurable optical tweezer array, we realize an entangling
two-qubit gate, and use it to deterministically create Bell pairs. Our results
demonstrate the key building blocks needed for quantum information processing,
simulation of quantum spin models, and quantum-enhanced sensing. They also open
up new possibilities such as using trapped molecules for quantum-enhanced
fundamental physics tests and exploring collisions and chemical reactions with
entangled matter.
