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Quasiparticle Poisoning of Superconducting Fluxonium Qubits

Kesavan Manivannan

Published May 10, 2026• 0 citations
OpenAlex

Abstract

Superconducting qubits based on Josephson junctions have emerged as a leading hardware platform in the quest to realize large-scale quantum computers. However, qubit decoherence remains a major hurdle on the path toward fault-tolerant quantum computing. In particular, excess quasiparticle (QP) excitations constitute a significant source of dissipation in superconducting quantum devices. The empirically observed QP density in these systems is orders of magnitude larger than that predicted by Bardeen-Cooper-Schrieffer (BCS) theory. Moreover, QP poisoning due to high-energy particle impacts can cause correlated errors that simultaneously affect multiple qubits, thereby impeding quantum error correction. Although QP-induced decoherence has been extensively studied in widely implemented transmon qubits, its impact on fluxonium qubits, a leading alternative architecture, remains far less explored. In this thesis, we investigate energy relaxation in fluxonium qubits induced by controlled QP poisoning using an on-chip Josephson injector junction that generates Cooper-pair-breaking phonons and photons. We separately measure the relaxation and excitation rates, instead of the conventional energy relaxation time. For specific injection parameters, we observe an apparent reversal of the rates, with the excitation rate exceeding the relaxation rate. We further measure the rates as a function of external flux to disentangle the effects of QP tunneling across the small junction and the junction array superinductor. Across all measured samples, differing in qubit spectra and injection conditions, we infer roughly an order of magnitude higher QP density at the small junction than in the array junctions. Our findings advance the understanding of QP-induced decoherence in fluxonium qubits and provide insight for reconciling discrepancies in reported bounds on QP densities, potentially enabling the development of QP mitigation strategies tailored to fluxonium qubits.

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Quasiparticle (QP) effects play a significant role in the coherence and fidelity of superconducting quantum circuits.The Andreev bound states of high-transparency Josephson junctions can act as low-energy traps for QPs, providing a mechanism for studying the dynamics and properties of both the QPs and the junction.Using locally injected and thermal QPs, we study QP loss and QP poisoning in epitaxial Al-InAs Josephson junctions incorporated in a superconducting quantum interference device (SQUID) galvanically shorting a superconducting resonator to ground.We observe changes in the resonance line shape and frequency shifts consistent with QP trapping into and clearing out of the ABSs of the junctions when the junctions are phase biased.By monitoring the QP trapping and clearing mechanisms in time, we find a time scale of O(1 µs) for these QP dynamics, consistent with the presence of phonon-mediated QP-QP interactions.Our measurements suggest that electron-phonon interactions play a significant role in the relaxation mechanisms of our system, while electron-photon interactions and electron-phonon interactions govern the clearing mechanisms.Our results highlight the QP-induced dissipation and complex QP dynamics in superconducting quantum circuits fabricated on superconductor-semiconductor heterostructures.

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Superconducting qubits based on Josephson junctions have emerged as a leading hardware platform in the quest to realize large-scale quantum computers. However, qubit decoherence remains a major hurdle on the path toward fault-tolerant quantum computing. In particular, excess quasiparticle (QP) excitations constitute a significant source of dissipation in superconducting quantum devices. The empirically observed QP density in these systems is orders of magnitude larger than that predicted by Bardeen-Cooper-Schrieffer (BCS) theory. Moreover, QP poisoning due to high-energy particle impacts can cause correlated errors that simultaneously affect multiple qubits, thereby impeding quantum error correction. Although QP-induced decoherence has been extensively studied in widely implemented transmon qubits, its impact on fluxonium qubits, a leading alternative architecture, remains far less explored. In this thesis, we investigate energy relaxation in fluxonium qubits induced by controlled QP poisoning using an on-chip Josephson injector junction that generates Cooper-pair-breaking phonons and photons. We separately measure the relaxation and excitation rates, instead of the conventional energy relaxation time. For specific injection parameters, we observe an apparent reversal of the rates, with the excitation rate exceeding the relaxation rate. We further measure the rates as a function of external flux to disentangle the effects of QP tunneling across the small junction and the junction array superinductor. Across all measured samples, differing in qubit spectra and injection conditions, we infer roughly an order of magnitude higher QP density at the small junction than in the array junctions. Our findings advance the understanding of QP-induced decoherence in fluxonium qubits and provide insight for reconciling discrepancies in reported bounds on QP densities, potentially enabling the development of QP mitigation strategies tailored to fluxonium qubits.
Quasiparticles are a prominent decoherence source in superconducting qubits, but their effects are notoriously difficult to isolate in fluxonium. Unlike the transmon, fluxonium is insensitive to offset charge, precluding charge-parity detection of quasiparticle tunneling. We address this challenge by measuring the excitation and de-excitation rates in a fluxonium qubit under controlled on-chip quasiparticle injection and isolating the quasiparticle-induced contribution by subtracting the background transition rates. We show that to accurately model the external magnetic flux dependence of the quasiparticle-induced transition rates, it is necessary to account for the superconducting gap asymmetry across the Josephson junctions. A comparison between theory and experiment constrains the relative quasiparticle densities in the small junction and the junction array and helps explain previously reported discrepancies between the bounds on the quasiparticle densities inferred for these two circuit elements.
Nonequilibrium quasiparticles (QPs) generated by stray infrared and ionizing radiation can limit the performance of superconducting quantum processors and present challenges for quantum error correction schemes. Models of QP-induced energy relaxation commonly assume that the characteristic energy of the QPs and the qubit transition energy are both small relative to the superconducting gap. Under these assumptions, certain qubits such as the fluxonium would exhibit protection against QP-induced dissipation at specific bias points. Here, we show that this is not necessarily the case, numerically analyzing the predicted rate of QP-induced dissipation in fluxonium qubits for different QP energy distributions and for QPs created via photon-assisted tunneling processes. We find that accounting for small numerical factors, existing theoretical models predict sensitivity to QP-induced errors at bias points previously thought to be protected. We find that inclusion of asymmetry in the superconducting gap energy across the junction can reintroduce suppression of QP-induced relaxation, as expected. Additionally, for QPs created by photon-assisted tunneling, we predict that T1T_1T1​ protection will only occur for a specific energy of pair-breaking radiation. This understanding of fluxonium sensitivity to QP-induced dissipation informs the development of fluxonium-based processors and future QP-mitigation strategies.
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.
When a high-energy particle, such as a γ ray or muon, impacts the substrate of a superconducting qubit chip, large numbers of electron-hole pairs and phonons are created. The ensuing dynamics of the electrons and holes changes the local offset-charge environment for qubits near the impact site. The phonons that are produced have energy above the superconducting gap in the films that compose the qubits, leading to quasiparticle excitations above the superconducting ground state when the phonons impinge on the qubit electrodes. An elevated density of quasiparticles degrades qubit coherence, leading to errors in qubit arrays. Because these pair-breaking phonons spread throughout much of the chip, the errors can be correlated across a large portion of the array, posing a significant challenge for quantum error correction. In order to study the dynamics of γ -ray impacts on superconducting qubit arrays, we use a γ -ray source outside the dilution refrigerator to controllably irradiate our devices. By using charge-sensitive transmon qubits, we can measure both the offset-charge shifts and quasiparticle poisoning due to the γ irradiation at different doses. We study correlations between offset-charge shifts and quasiparticle poisoning for different qubits in the array and compare these with numerical modeling of charge and phonon dynamics following a γ -ray impact. We thus characterize the poisoning footprint of these impacts and quantify the performance of structures for mitigating phonon-mediated quasiparticle poisoning.
The performance of various superconducting devices operating at ultra-low temperatures is impaired by the presence of non-equilibrium quasiparticles. Inelastic quasiparticle (QP) tunneling across Josephson junctions in superconducting qubits results in decoherence and spurious excitations and, notably, can trigger correlated errors that severely impede quantum error correction. In this work, we use "gap engineering" to suppress the tunneling of low-energy quasiparticles in Al-based transmon qubits, a leading building block for superconducting quantum processors. By implementing potential barriers for QP, we strongly suppress QP tunneling across the junction and preserve charge parity for over 10310^3103 seconds. The suppression of QP tunneling also results in a reduction in the qubit energy relaxation rates. The demonstrated approach to gap engineering can be easily implemented in all Al-based circuits with Josephson junctions.
Quasiparticle (QP) effects play a significant role in the coherence and fidelity of superconducting quantum circuits.The Andreev bound states of high-transparency Josephson junctions can act as low-energy traps for QPs, providing a mechanism for studying the dynamics and properties of both the QPs and the junction.Using locally injected and thermal QPs, we study QP loss and QP poisoning in epitaxial Al-InAs Josephson junctions incorporated in a superconducting quantum interference device (SQUID) galvanically shorting a superconducting resonator to ground.We observe changes in the resonance line shape and frequency shifts consistent with QP trapping into and clearing out of the ABSs of the junctions when the junctions are phase biased.By monitoring the QP trapping and clearing mechanisms in time, we find a time scale of O(1 µs) for these QP dynamics, consistent with the presence of phonon-mediated QP-QP interactions.Our measurements suggest that electron-phonon interactions play a significant role in the relaxation mechanisms of our system, while electron-photon interactions and electron-phonon interactions govern the clearing mechanisms.Our results highlight the QP-induced dissipation and complex QP dynamics in superconducting quantum circuits fabricated on superconductor-semiconductor heterostructures.