New Techniques for Fast and High-Fidelity Trapped Ion Photonic Interconnects

Trapped atomic ions are a leading candidate platform for quantum simulation and computing but system sizes are limited by motional mode crowding and transport overhead. Multiple reasonably-sized, well-controlled modules can be connected into one universal system using photonic interconnects, in which photons entangled with ions in each trap are collected into and detected in a Bell-state analyzer. The speed of these interconnects has heretofore been limited by the use of 0.6 NA objectives and the need to periodically pause entanglement attempts for recooling. In this work, we characterize a system with two in-vacuo 0.8 NA lenses on either side of an ion trap and use it to demonstrate the most efficient free-space ion trap photonic interconnect to date. In addition, we introduce an ytterbium ion as a sympathetic coolant during the entangling attempts cycle to remove the need for recooling, enabling a record photon-mediated entanglement rate of 250 cps between two trapped ions. We collect 493 nm photons from barium ions, which allows us to make use of an in-fiber beamsplitter, thus eliminating the spatial mismatch error that has contributed significant infidelity to all previous ion trap photonic interconnect demonstrations. The major remaining error source is imperfections in the photon polarization encoding, so we also develop a new protocol for remotely entangling two ions using time-bin encoded photons and present preliminary results of an experimental implementation. Finally, we prepare the first remote entangled state involving two barium ions in separate vacuum chambers.