Coupling of negative-positive trapped-ion pairs

Abstract

Direct motional coupling of opposite-charge trapped-ion pairs could provide a pathway to extend ultra-low temperatures and quantum control to negative ions that lack the suitable electronic energy structures required for direct laser cooling. Because positive and negative ions cannot be confined within a single electrostatic potential well, I investigate a configuration where single ions are trapped in close proximity within separate potential wells to couple their motion. I analytically and numerically evaluate the electrostatic trapping requirements. As a concrete implementation, I present an optimized segmented surface Paul trap design to couple an antimatter hydrogen molecular ion ($\bar{H}_2^-$) and a beryllium ion ($^9 Be^+$). A motional coupling frequency of 5 kHz can be achieved at an ion-ion separation of $35 \mu m$, with an ion height of $50 \mu m$, axial trap frequencies of 4 MHz, and static trap voltages with a magnitude of $\approx 20 V$. Finally, I outline three applications for this technique: quantum logic spectroscopy of $\bar{H}_2^-$ for matter-antimatter comparisons, the preparation of cold neutral deuterium atoms via near-threshold photo-detachment of $D^-$ for optical trapping, and quantum information processing using equal-mass opposite-charge ion pairs.