Characterizing gate operations near the sweet spot of an exchange-only qubit

TL;DR

This paper investigates how optimal working points, or 'sweet spots', in exchange-only qubits can significantly reduce charge noise effects, especially improving the fidelity of quantum gate operations in GaAs and Si devices.

Contribution

The study quantifies the effectiveness of sweet spots in mitigating charge noise for both X and Z rotations in exchange-only qubits, providing the first comprehensive analysis of their impact.

Findings

Sweet spots improve fidelity by at least 20 times in GaAs devices.

Fidelity improvements are even greater in silicon devices.

X rotations can be optimized to match the noise mitigation of Z rotations.

Abstract

Optimal working points or "sweet spots" have arisen as an important tool for mitigating charge noise in quantum dot logical spin qubits. The exchange-only qubit provides an ideal system for studying this effect because $Z$ rotations are performed directly at the sweet spot, while $X$ rotations are not. Here for the first time we quantify the ability of the sweet spot to mitigate charge noise by treating $X$ and $Z$ rotations on an equal footing. Specifically, we optimize $X$ rotations and determine an upper bound on their fidelity. We find that sweet spots offer a fidelity improvement factor of at least 20 for typical GaAs devices, and more for Si devices.