We describe our work on trapping, cooling and detecting mixed ion species for a scalable ion trap quantum information processing architecture. These mixed species chains in linear RF traps 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 sympathetic cooling of normal modes that couple well to the coolant ions (Ba in our case). Average motional occupation numbers are estimated to be 10 to 20 quanta per mode for these well cooled modes for chains with small numbers of ions, consistent with the Doppler limit temperature. For normal modes that do not couple significantly to the coolant atoms, the occupation numbers are significantly higher, of order several thousand. Strategies for better cooling of these modes are discussed. Further, we are working to implement these techniques in microfabricated surface traps in order to exercise greater control over ion chain ordering and positioning.
We present work toward remote entanglement of barium ions in traps separated by a few meters. A new version of an ion trap specialized for remote entanglement is introduced. The new trap allows for highly efficient collection of ion fluorescence while simultaneously minimizing ion micromotion and aligning the trap position precisely to the focus of an in-vacuum parabolic mirror by using a set of bias electrodes and a piezoelectric micro-positioning system. The success rate of the remote entanglement procedure depends strongly on the efficiency with which ion fluorescence can be coupled into an optical fiber. Characterization of our system in terms of ion fluorescence collection and fiber coupling efficiency is presented. Results demonstrating entanglement between a single barium ion and single spontaneously emitted photons are shown. The entanglement fidelity of the ion-photon state is measured to be 0.84(1) and a CHSH Bell signal of 2.303(36) finds violation of the CHSH version of the Bell inequality by over eight standard deviations. Barium’s relatively long wavelength transitions make it an ideal candidate for our longer term goal of remote entanglement of ions separated by a kilometer or more. Such long distance remote entanglement should allow for a loophole-free verification of the violation of the Bell inequality.