  Quantum computation and quantum communication are expected to provide users
with capabilities inaccessible by classical physics. However, scalability to
larger systems with many qubits is challenging. One solution is to develop a
quantum network consisting of small-scale quantum registers containing
computation qubits that are reversibly interfaced to communication qubits. Here
we report on a register that uses both optical tweezers and optical lattices to
deterministically assemble a two-dimensional array of atoms in an optical
cavity. Harnessing a single-atom addressing beam, we stimulate the emission of
a photon from each atom and demonstrate multiplexed atom-photon entanglement
with a generation-to-detection efficiency approaching 90$\%$. Combined with
cavity-mediated quantum logic, our approach provides a possible route to
distributed quantum information processing.
