Correlated Error Bursts in a Gap-Engineered Superconducting Qubit Array

TL;DR

This paper investigates impact-induced correlated errors in gap-engineered superconducting qubits, revealing frequency shifts caused by quasiparticle interactions that can impair quantum error correction.

Contribution

It identifies a new type of correlated error due to impact-induced frequency shifts in gap-engineered superconducting qubits, despite suppression of quasiparticle tunneling errors.

Findings

Impact events cause negative frequency shifts up to 3 MHz.

Frequency shifts last approximately 1 ms.

Shift-induced phase errors can hinder quantum error correction.

Abstract

One of the roadblocks towards the implementation of a fault-tolerant superconducting quantum processor is impacts of ionizing radiation with the qubit substrate. Such impacts temporarily elevate the density of quasiparticles (QPs) across the device, leading to correlated qubit error bursts. The most damaging errors, $T_1$ errors, stem from QP tunneling across the qubit Josephson junctions (JJs). Recently, we demonstrated that this type of error can be strongly suppressed by engineering the profile of superconducting gap at the JJs in a way that prevents QP tunneling. In this work, we identify a new type of impact-induced correlated error that persists in the presence of gap engineering. We observe that impacts shift the frequencies of the affected qubits, and thus lead to correlated phase errors. The frequency shifts are systematically negative, reach values up to $3\,{\rm MHz}$, and last for $\sim 1\,{\rm ms}$. We provide evidence that the shifts originate from QP-qubit interactions in the JJ region. Further, we demonstrate that the shift-induced phase errors can be detrimental to the performance of quantum error correction protocols.