Researchers from the Massachusetts Institute of Technology and the Johns Hopkins Applied Physics Laboratory observed reduced errors in qubit arrays with a greater difference in superconducting gap energy, suggesting a path toward mitigating a vulnerability in quantum computing technology. The work demonstrates that high-energy particles increase quasiparticle density near Josephson junctions, creating correlated errors that are difficult to correct with standard methods. The researchers found that increasing the degree of gap engineering at the Josephson junction lessened the severity of correlated errors.

Radiation-Induced Quasiparticle Generation in Superconducting Circuits

The severity of errors in transmon qubit arrays diminished with increased differences in superconducting gap energy at the Josephson junction, a finding that addresses a persistent challenge in building stable quantum computers. Researchers exposed arrays to alpha particles from an americium-241 source and electrons from a pulsed linear accelerator to assess the impact of radiation on qubit performance. Observations of temporally and spatially correlated errors supported the hypothesis that hadronic cosmic rays contribute to the 10 to the power of negative 10 error floor previously observed in quantum systems.

Correlated errors arise from the generation of quasiparticles near Josephson junctions, a phenomenon that conventional error correction methods struggle to address. Alpha particle irradiation revealed time and space-correlated T1 errors, while electron irradiation showed temporally correlated T1 and T2 errors, though the latter measurement was not sensitive to spatial correlations.

Increasing δΔM1 hastened the recovery time for both T1 and T2 errors, a result attributed to the trapping of quasiparticles within the capacitor and ground-plane. This suggests a pathway for managing quasiparticle density and improving qubit coherence. The team constructed a model of quasiparticle dynamics that qualitatively aligns with their experimental observations, providing a framework for future investigations into the interplay between phonons, quasiparticles, and superconducting circuits.

“We observed qualitative agreement between this model and our data,” the researchers wrote, noting the potential for incorporating the model into packages such as G4CMP. This work reinforces the many-sided influence of radiation on superconducting qubits and provides strategies for improving radiation resilience.

Correlated Errors in Transmon Qubits from Alpha Particles

The influence of hadronic cosmic rays on superconducting qubit performance is increasingly understood through experiments exposing arrays to alpha particles. This finding suggests that shielding against such radiation may become a critical component of future quantum computer design, beyond simply maintaining cryogenic temperatures. Investigations extended to electron irradiation, using the Controlled Linac Irradiation of Quantum Experiments facility at the Johns Hopkins Applied Physics Laboratory. This contrasts with the alpha particle results, indicating differing mechanisms of error generation depending on the radiation source.

The team’s work highlights the many-sided influence of radiation on qubit stability, demonstrating that even sub-MeV radiation, like terrestrial gamma rays and cosmic ray muons, can induce state transitions in qubits engineered to mitigate errors. Efforts to counteract these radiation-induced errors focused on “gap engineering”, manipulating the superconducting gap energy at both the Josephson junction (δΔJJ) and the capacitor/ground-plane interface (δΔM1).

Impact of δΔ_(M1) on Reducing Relaxation Errors

Following radiation exposure, the dynamics of quasiparticles differ between the qubit capacitor and the ground plane, a distinction linked to varying trapping efficiencies, according to recent experiments. The ground plane, containing features like air bridges and galvanic contact, appears to more effectively remove quasiparticles from the system than the qubit capacitor itself, influencing error recovery timescales.

The faster recovery time observed in qubit arrays incorporating both Josephson junction and capacitor/ground-plane gap engineering suggests that a difference in superconducting gap energy at the M1 interface actively traps quasiparticles away from the Josephson junction. This trapping mechanism reinforces the importance of optimizing both δΔJJ and δΔM1 for enhanced radiation resilience, as the team found that a larger gap difference at the Josephson junction lessened the severity of correlated T1 errors.

Electron Radiation Measurement of T_1 and T_2 Errors

Measurements using a pulsed linear accelerator revealed temporally correlated errors affecting both qubit relaxation and excitation, but showed no sensitivity to spatial correlations between qubits. The degree to which these T1 errors were mitigated depended on the design of the qubit arrays, specifically the difference in superconducting gap energy across the Josephson junction (δΔJJ). Arrays with a larger δΔJJ exhibited fewer correlated errors, indicating a protective effect from this gap engineering strategy.

Further enhancing this protection, an increased δΔM1 hastened the recovery time following radiation exposure. The experimental setup at CLIQUE, the facility used for these measurements, provided two key advantages: triggered timing and tunable beam current.

Varying the mean number of electrons incident on the qubit substrate, from 0 to a monitored level, allowed for precise investigation of the superconducting gap profile’s influence on radiation response. Control arrays, fabricated alongside those with engineered gaps, had a δΔJJ less than the qubit energy and a δΔM1/h designed value of 0.2 GHz, while the array with the most extensive gap engineering featured a δΔJJ/h of 10 GHz and a δΔM1/h of 0.1 GHz. Analysis removed ambiguous qubit trajectories following excited-state measurements to ensure data accuracy.

δΔ_(M1) Enhancement Hastens Quasiparticle Recovery

Analysis of qubit relaxation and excitation demonstrated that the degree to which these errors diminished depended on the energy deposited in the substrate, with larger δΔ(M1) values consistently shortening recovery times. The observed effect extends beyond simply suppressing initial error rates; increasing δΔ(M1) reduced qubit frequency recovery times by an order of magnitude. Examining qubit relaxation in arrays with low δΔ(M1) revealed a marked difference in response based on Josephson junction orientation, a phenomenon attributable to the influence of quasiparticles within the capacitor layer.

This suggests that quasiparticle dynamics are not uniform across layers, with the capacitor layer exhibiting a distinct sensitivity to junction orientation. The team’s data, presented in panels showing state probability changes and peak shifts, supports a model where a larger δΔ(M1) effectively transforms the capacitor layer into a quasiparticle trap.

While a greater δΔJJ suppresses the initial increase in qubit transition rate, it is the enhancement of δΔM1 that accelerates recovery from errors. The researchers write, highlighting the complementary roles of these gap engineering strategies. Future work, they suggest, could focus on maximizing both gap differences and incorporating additional quasiparticle traps and phonon sinks to further enhance radiation resilience in superconducting qubits.

Gap Engineering Strategies for Radiation Resilience

A model of quasiparticle dynamics constructed by the team aligns qualitatively with observed experimental data, reinforcing the complex interplay between radiation and superconducting qubits. This work demonstrates that increasing the superconducting gap difference at the capacitor/ground-plane interface, denoted δΔM1, actively traps quasiparticles, accelerating error recovery following radiation exposure. The research highlights a detailed approach to radiation resilience, moving beyond simply suppressing initial error rates to actively managing quasiparticle behavior.

The team notes that the observed error rates are comparable to those expected from cosmic ray impacts above 1 MeV, suggesting that operation within a shallow underground facility, between 15 and 30 meters, could provide sufficient shielding against these specific radiation sources. The many-sided influence of radiation on qubits highlights the need for continued innovation in both device design and operational environments.

Model of Quasiparticle Dynamics Validates Observations

The model builds upon previous work, specifically referencing studies of quasiparticle densities in the metal layers adjacent to Josephson junctions, and incorporates the impact of varying superconducting gap profiles. This approach allowed for a qualitative agreement between simulated and observed qubit behavior following exposure to radiation sources. This subtle approach to modeling acknowledges that quasiparticles with energies below the gap difference behave differently than those above it, influencing their distribution and subsequent impact on qubit performance.

The model also accounts for the creation of phonons, which are vibrations within the material, as a result of both electron-hole recombination and direct nuclear displacement caused by radiation. These phonons then interact with the superconducting material, further contributing to the generation of quasiparticles.

Hadronic Cosmic Rays and the 10^(-10) Error Floor

Measurements using an americium-241 source at the Massachusetts Institute of Technology revealed temporally and spatially correlated T1 errors, bolstering the connection between hadronic cosmic rays and the persistent 10 to the power of negative 10 error floor plaguing superconducting qubits. The team exposed arrays of transmon qubits to alpha-particles emitted from the source, finding that these MeV-scale energy depositions mimic the effects expected from cosmic ray impacts.

This observation supports the hypothesis that these high-energy particles, rather than previously suspected factors, are a primary driver of qubit errors. The observed recovery times from these radiation-induced errors align with those reported in earlier studies, further strengthening the link between cosmic ray events and the observed error rate. The rate of cosmic rays above 1 MeV, the team notes, is consistent with the one-per-hour error rate previously documented, suggesting a direct correlation between external radiation and qubit performance.

While junction gap engineering, increasing the difference in superconducting gap energy, effectively mitigates some radiation-induced errors, the qubits remain vulnerable to energy deposits on the scale of a few MeV. This sensitivity suggests that simply suppressing initial quasiparticle creation is insufficient, as the energy scale of these events bypasses the protective effects of gap engineering.

This work reinforces the assertion that Josephson junction gap engineered superconducting qubits are still negatively affected by radiation, emphasizing the ongoing challenge of building robust quantum processors. Additional error mitigation strategies will be important for achieving the desired performance levels in future quantum computers, even with advancements in gap engineering.

👉 More information
🗞 Characterization of Radiation-Induced Errors in Superconducting Qubits Protected with Various Gap-Engineering Strategies
✍️ H. D. Pinckney et al.
🧠 DOI: http://link.aps.org/doi/10.1103/plgq-byn7

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