Designing Filter Functions of Frequency-Modulated Pulses for High-Fidelity Two-Qubit Gates in Ion Chains

TL;DR

This paper develops and experimentally validates filter functions for frequency-modulated laser pulses in ion-trap quantum computers, significantly improving two-qubit gate fidelity by suppressing parameter fluctuation noise.

Contribution

It introduces a method to design filter functions for frequency-modulated pulses that predict and mitigate noise effects in ion-trap quantum gates.

Findings

Gate fidelity improved from 99.23% to 99.55%

Filter functions accurately predict error due to parameter fluctuations

Method outperforms static offset robustness approaches

Abstract

High-fidelity two-qubit gates in quantum computers are often hampered by fluctuating experimental parameters. The effects of time-varying parameter fluctuations lead to coherent noise on the qubits, which can be suppressed by designing control signals with appropriate filter functions. Here, we develop filter functions for M{\o}lmer-S{\o}rensen gates of trapped-ion quantum computers that accurately predict the change in gate error due to small parameter fluctuations at any frequency. We then design the filter functions of frequency-modulated laser pulses, and compare this method with pulses that are robust to static offsets of the motional-mode frequencies. Experimentally, we measure the noise spectrum of the motional modes and use it for designing the filter functions, which improves the gate fidelity from 99.23(7)% to 99.55(7)% in a five-ion chain.