We demonstrate precise control of the coupling of each of two trapped ions to the mode of
an optical resonator. When both ions are coupled with near-maximum strength, we generate
ion-ion entanglement heralded by the detection of two orthogonally polarized cavity
photons. The entanglement fidelity with respect to the Bell state Ψ+ reaches F≥(91.9±2.5)%.
This result represents an important step toward distributed quantum computing with cavities
linking remote atom-based registers.

Optical cavities constitute a coherent interface between light and matter, while strings of
addressable ions enable local quantum information processing. Coupling multiple,
individually addressable ions to the mode of an optical cavity not only provides a framework
for quantum network protocols but also can enhance the strength of the ion-cavity interface.
Here, we demonstrate precise control of the coupling of each of two ions to the mode of an
optical resonator. We then show ion-ion entanglement heralded by the detection of two …