Measuring quasiparticle dynamics for particle impact reconstruction in a superconducting qubit chip

TL;DR

This paper models quasiparticle dynamics in superconducting qubits caused by particle impacts, providing a framework for using qubits as energy-resolving detectors to improve fault tolerance.

Contribution

It introduces a statistical analysis method to distinguish quasiparticle loss channels and links relaxation events to ballistic phonon propagation, enabling particle detection.

Findings

Measured quasiparticle recombination in multiple transmon qubits.

Discovered energy-dependent qubit relaxation dynamics.

Linked relaxation events to substrate phonon propagation.

Abstract

Quasiparticle poisoning following particle impacts poses a significant challenge to the development of fault-tolerant superconducting quantum computers, as a sudden excess of quasiparticles can simultaneously degrade the coherence of multiple qubits across large device arrays. In this work, we present a statistical analysis that models the time evolution of radiation-induced qubit energy relaxation through quasiparticle density dynamics. This study provides insight into quasiparticle loss processes by distinguishing between recombination and trapping decay channels and assessing their respective impact on qubit performance. We precisely measure quasiparticle recombination in multiple transmon qubits and uncover an unexpected dependence of qubit relaxation dynamics on deposited energy. By linking correlated relaxation events across qubits to ballistic phonon propagation, we introduce a statistical localization approach to extract the energy deposited in the substrate, which is in good agreement with Monte Carlo simulation. This work establishes the quantitative framework for using an arbitrary subset of superconducting transmon qubits in a QPU as energy-resolving witness particle detectors.