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Nature Communications

A stress-induced source of phonon bursts and quasiparticle poisoning

Robin Anthony-Petersen, A. Biekert, R. Bunker, C. L. Chang, Y.-Y. Chang, Luke Chaplinsky, E. Fascione, Caleb W. Fink, M. Garcia-Sciveres, R. Germond, Wei Guo, S. A. Hertel, Z. Hong, Noah Kurinsky, Xinran Li, J. Lin, M. Lisovenko, Rupak Mahapatra, Adam Mayer, Daniel N. McKinsey, Siddhant Mehrotra, N. Mirabolfathi, Brian Neblosky, W. A. Page, P. K. Patel, Bjoern Penning, H. D. Pinckney, Mark Platt, M. Pyle, Maggie Reed, R. K. Romani, Hadley Santana Queiroz, B. Sadoulet, B. Serfass, Ryan Smith, Peter Sørensen, B. Suerfu, Aritoki Suzuki, Ryan Underwood, V. Senthil Velan, Gensheng Wang, Y. Wang, S. L. Watkins, Michael R. C. Williams, Volodymyr G. Yefremenko, Jianjie Zhang

Published Jul 31, 2024• 31 citations
Publisher

Abstract

The performance of superconducting qubits is degraded by a poorly characterized set of energy sources breaking the Cooper pairs responsible for superconductivity, creating a condition often called "quasiparticle poisoning". Both superconducting qubits and low threshold dark matter calorimeters have observed excess bursts of quasiparticles or phonons that decrease in rate with time. Here, we show that a silicon crystal glued to its holder exhibits a rate of low-energy phonon events that is more than two orders of magnitude larger than in a functionally identical crystal suspended from its holder in a low-stress state. The excess phonon event rate in the glued crystal decreases with time since cooldown, consistent with a source of phonon bursts which contributes to quasiparticle poisoning in quantum circuits and the low-energy events observed in cryogenic calorimeters. We argue that relaxation of thermally induced stress between the glue and crystal is the source of these events.

DOI
10.1038/s41467-024-50173-8

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The performance of superconducting qubits is degraded by a poorly characterized set of energy sources breaking the Cooper pairs responsible for superconductivity, creating a condition often called "quasiparticle poisoning". Both superconducting qubits and low threshold dark matter calorimeters have observed excess bursts of quasiparticles or phonons that decrease in rate with time. Here, we show that a silicon crystal glued to its holder exhibits a rate of low-energy phonon events that is more than two orders of magnitude larger than in a functionally identical crystal suspended from its holder in a low-stress state. The excess phonon event rate in the glued crystal decreases with time since cooldown, consistent with a source of phonon bursts which contributes to quasiparticle poisoning in quantum circuits and the low-energy events observed in cryogenic calorimeters. We argue that relaxation of thermally induced stress between the glue and crystal is the source of these events.
The performance of superconducting qubits is degraded by a poorly characterized set of energy sources breaking the Cooper pairs responsible for superconductivity, creating a condition often called ``quasiparticle poisoning". Both superconducting qubits and low threshold dark matter calorimeters have observed excess bursts of quasiparticles or phonons that decrease in rate with time. Here, we show that a silicon crystal glued to its holder exhibits a rate of low-energy phonon events that is more than two orders of magnitude larger than in a functionally identical crystal suspended from its holder in a low-stress state. The excess phonon event rate in the glued crystal decreases with time since cooldown, consistent with a source of phonon bursts which contributes to quasiparticle poisoning in quantum circuits and the low-energy events observed in cryogenic calorimeters. We argue that relaxation of thermally induced stress between the glue and crystal is the source of these events.
Errors that are correlated across a qubit array pose an obstacle to quantum error correction. We observe elevated correlated poisoning rates in superconducting qubit arrays at the start of a cooldown followed by power-law reductions in time, while the rate of offset charge shifts remains constant, evidence of nonionizing sources of pair-breaking phonon bursts. We also investigate different sample packages, with some exhibiting elevated poisoning due to mechanical impacts from the cryocooler. Possible sources include relaxation of built-up stress in device thin films, substrates, or packaging.
Suppression of Quasiparticle Poisoning to 10−1110^{-11}10−11 Levels in Superconducting Qubits via Infrared Shielding
Quasiparticle poisoning bottlenecks superconducting qubits, limiting coherence and the scalability of quantum processors. In this work, we systematically investigate quasiparticle poisoning in superconducting qubits under three infrared (IR) shielding configurations, ranging from a dedicated multi-layer design to a simplified implementation. By measuring quasiparticle-induced parity switching, we demonstrate a suppression of the switching rate by over four orders of magnitude via the implementation of improved shielding. In the best configuration, the rate decreases over time following cooldown and reaches 0.069 \,Hz on day 34, corresponding to an anticipated quasiparticle density per Cooper pair of 1.88×10−111.88\times10^{-11}1.88×10−11. To our knowledge, this represents the lowest quasiparticle density reported in the literature to date. The remaining quasiparticle population is likely dominated by sporadic phonon bursts stemming from mechanical stress release in the on-chip films, as well as from the surrounding environment. The effective qubit temperature follows the phonon bath down to 17 \,mK, enabling initialization errors of ∼0.01%\sim 0.01\%∼0.01% for 3 \,GHz qubits. These results demonstrate that proper IR shielding and thermalization are essential for suppressing quasiparticle poisoning and enabling high-coherence, scalable superconducting qubit systems.
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.
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.
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.