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Matt McEwen, Kevin C. Miao, Juan Atalaya, Alex Bilmes, Alex Crook, Jenna Bovaird, John Mark Kreikebaum, Nicholas Zobrist, Evan Jeffrey, Bicheng Ying, Andreas Bengtsson, Hung-Shen Chang, Andrew Dunsworth, Julian Kelly, Yaxing Zhang, Ebrahim Forati, Rajeev Acharya, Justin Iveland, Wayne Liu, Seon Kim, et al (7) (Feb 27 2024).

Abstract: Quantum error correction (QEC) provides a practical path to fault-tolerant quantum computing through scaling to large qubit numbers, assuming that physical errors are sufficiently uncorrelated in time and space. In superconducting qubit arrays, high-energy impact events produce correlated errors, violating this key assumption. Following such an event, phonons with energy above the superconducting gap propagate throughout the device substrate, which in turn generate a temporary surge in quasiparticle (QP) density throughout the array. When these QPs tunnel across the qubits’ Josephson junctions, they induce correlated errors. Engineering different superconducting gaps across the qubit’s Josephson junctions provides a method to resist this form of QP tunneling. By fabricating all-aluminum transmon qubits with both strong and weak gap engineering on the same substrate, we observe starkly different responses during high-energy impact events. Strongly gap engineered qubits do not show any degradation in T1 during impact events, while weakly gap engineered qubits show events of correlated degradation in T1. We also show that strongly gap engineered qubits are robust to QP poisoning from increasing optical illumination intensity, whereas weakly gap engineered qubits display rapid degradation in coherence. Based on these results, gap engineering removes the threat of high-energy impacts to QEC in superconducting qubit arrays.

Arxiv: https://arxiv.org/abs/2402.15644