Ion–photon entanglement and Bell inequality violation with 138 Ba +

We report on the demonstration of ion–photon entanglement and Bell inequality violation in a system of trapped Ba138+ ions. Entanglement between the Zeeman sublevels of the ground state of a single Ba138+ ion and the polarization state of a single 493 nm photon emitted by the ion with a fidelity of 0.84±0.01 was achieved, along with a Bell signal of 2.3, exceeding the classical limit of 2 by more than eight standard deviations. This system is a promising candidate for a loophole-free Bell inequality violation test using long-range ion–ion entanglement as the wavelengths of the transitions of Ba138+ are in the visible region and thus suitable for long-range transmission over fiber optic cable.