Reduced Crosstalk Errors during the Mølmer–Sørensen Gate through Choice of Vibrational Mode

Laser crosstalk is a major obstacle to the scaling of trapped ion quantum computers. Multi-qubit gates couple the ions' electronic states to a shared vibrational mode, often the center-of-mass (COM) mode where every ion has the same coupling to the mode. When crosstalk is present during the operation of the Mølmer–Sørensen (MS) entangling gate, the ions that are being targeted for entanglement become partially entangled with their neighbors, reducing the fidelity of the final state. We present numerical and analytical results showing that the final state fidelity can be increased using a non-COM vibrational mode for which the neighboring ions have lower coupling to the vibrational mode than the targeted ions. Though the efficacy of this strategy is highly dependent on the particular pair of ions being entangled, we find experimentally relevant scenarios in which fidelity can be increased from below 40% to above 90% for pairwise entanglement in a chain of 6 ions. We will present a method for quickly determining which vibrational mode of an ion string is most resilient to laser crosstalk when applying the MS gate on a given pair of ions. We are currently investigating ways to tailor vibrational modes to further improve the fidelity of multi-qubit gates in the presence of crosstalk.