Photon-Photon Entanglement with a Single Trapped Atom

An experiment is performed where a single rubidium atom trapped within a high-finesse optical cavity emits two independently triggered entangled photons. The entanglement is mediated by the atom and is characterized both by a Bell inequality violation of $S=2.5$, as well as full quantum-state tomography, resulting in a fidelity exceeding $F=90\%$. The combination of cavity-QED and trapped atom techniques makes our protocol inherently deterministic—an essential step for the generation of scalable entanglement between the nodes of a distributed quantum network.