September 3, 2020
Journal Article

Impact of ionizing radiation on superconducting qubit coherence

Francisca Vasconcelos
David Kim
Alexander Melville
Bethany Niedzielski
Jonilyn Yoder
Simon Gustavsson
Brent VanDevender
Antti Vepsalainen
Amir Karamlou
John Orrell
Akshunna Dogra
Ben Loer
Joseph Formaggio
William Oliver

Abstract

The practical viability of any qubit technology stands on long coherence times and high-fidelity operations with the superconducting qubit modality being an auspicious example, However, superconducting qubit coherence is impacted by broken Cooper pairs referred to as quasiparticles, with a density that is empirically observed to be orders of magnitude greater than the value predicted for thermal equilibrium by the Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity. Previous work has shown that infrared photons significantly increase the quasiparticle density, yet even in the best isolated systems, it still remains higher than expected, suggesting that another generation mechanism exists. In this Letter, we provide evidence that ionizing radiation from environmental radioactive materials and cosmic rays contributes to this observed difference, leading to an elevated quasiparticle density that would ultimately limit superconducting qubits of the type measured here to coherence times in the millisecond regime. We further demonstrate that introducing radiation shielding reduces the flux of ionizing radiation and positively correlates with increased coherence time. Albeit a small effect for today’s qubits, reducing or otherwise mitigating the impact of ionizing radiation will be critical for realizing fault-tolerant superconducting quantum computers.

Revised: September 3, 2020 | Published: August 26, 2020

Citation

Vepsalainen A., A. Karamlou, J.L. Orrell, A. Dogra, B.M. Loer, F. Vasconcelos, and D.K. Kim, et al. 2020. "Impact of ionizing radiation on superconducting qubit coherence." Nature 584, no. 7822:551--556. PNNL-SA-150722. doi:10.1038/s41586-020-2619-8