Digital Predistortion of Optical Fields for Fast and High-Fidelity Entangling Gates in Trapped-Ion Qubits

TL;DR

This paper presents a digital predistortion technique to correct nonlinear distortions in control hardware, significantly improving the fidelity of entangling gates in trapped-ion quantum processors.

Contribution

The authors develop a device-agnostic predistortion method that extends linear operation range and suppresses intermodulation, enhancing quantum gate performance.

Findings

3-5 dB suppression of intermodulation tones

Approximately doubling the diffraction efficiency at a $10^{-3}$ error threshold

Confirmed fidelity improvements in two-qubit Bell states

Abstract

High-fidelity quantum gates require precise classical control signals, yet the analog hardware delivering these signals introduces nonlinear distortions that degrade gate performance. We demonstrate digital predistortion of an acousto-optic modulator used to generate multi-tone entangling-gate waveforms in a trapped-ion processor based on $^{88}$Sr$^+$. By measuring and inverting the static nonlinear amplitude response of the modulator, we apply a feed-forward correction that extends its linear operating range and suppresses spurious intermodulation products. Spectral analysis of the gate beam shows 3--5 dB suppression of the dominant intermodulation tones, approximately doubling the usable diffraction efficiency at a $10^{-3}$ estimated gate-error threshold. Direct two-qubit Bell-state fidelity measurements confirm that predistortion consistently improves entangling-gate performance. The calibrate-and-invert methodology is device and platform agnostic, applicable to any nonlinear element in the classical control chain of a quantum processor.