We present a measurement of the branching fractions for decay from the long-lived 5D 5/2 level in 138 Ba + . The branching fraction for decay into the 6S 1/2 ground state was found to be 0.846(25)stat (4)sys. We also report an improved measurement of the 5D 5/2 lifetime, τ5D 5/2 = 31.2(0.9) s. Together these measurements provide the first experimental determination of transition rates for decay out of the 5D 5/2 level. The low (< 7 × 10 −12 Torr) pressure in the ion trap in which these measurements were made simplified data acquisition and analysis. Comparison of the experimental results with theoretical predictions of the transition rates shows good agreement.

We report on the demonstration of ion-photon entanglement and Bell inequality violation in a system of trapped 138 Ba + ions. Entanglement between the Zeeman sublevels of the ground state of a single 138 Ba + 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 over eight standard deviations. This system is a promising candidate for a loophole-free Bell inequality violation test as the wavelengths of the transitions of 138 Ba + are in the visible region and thus suitable for long range transmission over fiber optic cable.

We report on progress toward implementing mixed ion species quantum information processing for a scalable ion-trap architecture. Mixed species chains may help solve several problems with scaling ion-trap quantum computation to large numbers of qubits. Initial temperature measurements of linear Coulomb crystals containing barium and ytterbium ions indicate that the mass difference does not significantly impede cooling at low ion numbers. Average motional occupation numbers are estimated to ben ≈ 130 quanta per mode for chains with small numbers of ions, which is within a factor of three of the Doppler limit for barium ions in our trap. We also discuss generation of ion-photon entanglement with barium ions with a fidelity of F ≥ 0.84, which is an initial step towards remote ion-ion coupling in a more scalable quantum information architecture. Further, we are working to implement these techniques in surface traps in order to exercise greater control over ion chain ordering and positioning.