Researchers at Lawrence Livermore National Laboratory have directly linked cosmic radiation to performance degradation in superconducting qubits, observing up to a 30.5% reduction in both energy relaxation (T_1) and phase coherence (T_2) immediately after detecting radiation events. The team built a novel platform that sandwiches a superconducting qubit between two microwave kinetic inductance detector (MKID) arrays, allowing real-time correlation of radiation-induced phonon bursts with qubit decoherence.

This work demonstrates that decoherence isn’t random. It’s correlated with events attributed to penetrating muons, challenging assumptions used in most quantum error correction protocols. The platform provides a foundation for developing shielding and refining error-correction algorithms tailored to these correlated noise sources.

MKID Arrays Detect Radiation Coincident with Superconducting Qubits

Measurements reveal a quantifiable link between cosmic radiation and the performance of superconducting qubits, with coherence, both energy relaxation (T₁) and phase coherence (T₂), decreasing by as much as 30.5% immediately after the detection of penetrating muons by microwave kinetic inductance detector (MKID) arrays. This reduction in qubit performance occurs following simultaneous events registered by both MKID arrays, indicating a correlation beyond random decoherence and challenging assumptions within current quantum error correction protocols.

The platform’s design uses the sensitivity of MKIDs to quasiparticles generated when radiation interacts with superconducting films. An increase in kinetic inductance signals the arrival of these events. Each MKID array incorporates analog input lines with a bandwidth of ±350 MHz and a resolution of 12 bits, allowing for continuous monitoring of resonance frequency shifts.

This precise monitoring enables researchers to not only detect radiation but also to distinguish between penetrating and non-penetrating radiation based on the signal registered across both arrays. The entire three-layer assembly, two MKID arrays sandwiching the qubit, is housed within gold-coated copper boxes to ensure effective thermalization and microwave shielding during operation at dilution refrigerator temperatures. The observed correlation between radiation events and qubit decoherence is particularly noteworthy because most quantum error correction protocols assume that decoherence events are spatially and temporally uncorrelated.

This work demonstrates that radiation, specifically penetrating muons, introduces correlated errors that are not addressed by these standard protocols. The ability to detect radiation events in real-time, concurrent with qubit measurements, is important for establishing this link; previous studies relied on post-processing analysis, obscuring the immediate impact of radiation on qubit performance.

The research team’s methodology involved synchronizing MKID event detection with single-shot measurements of T₁ and T₂, allowing for a precise assessment of coherence loss following radiation strikes. The implications of these findings extend beyond simply identifying a source of qubit decoherence. The platform developed at Lawrence Livermore National Laboratory provides a foundation for systematic studies of radiation effects on quantum systems. The data gathered from this setup can be used to refine error-correction algorithms, specifically tailoring them to account for the correlated noise introduced by penetrating muons and other radiation sources.

The ability to predict and correct for these correlated errors is essential for building larger, more stable quantum computers capable of tackling complex computational problems. The sensitivity of the MKID arrays stems from their large kinetic inductance fraction, which amplifies the signal generated by quasiparticles created by impinging radiation.

These quasiparticles alter the MKID’s superconducting film, causing a measurable change in its kinetic inductance. The system’s design, with the qubit sandwiched between the two detectors, increases the likelihood of detecting radiation events that pass through the qubit layer on both the top and bottom MKID arrays. This geometry is critical for establishing the correlation between radiation and qubit decoherence, as it provides a more comprehensive picture of the radiation environment surrounding the qubit.

The research team’s approach represents an advancement in the field of quantum computing, moving beyond simply identifying sources of error to actively characterizing their impact on qubit performance in real-time. The ability to directly link radiation events to correlated qubit decoherence is a significant step towards building more robust and reliable quantum computers.

As quantum processors continue to scale up in size and complexity, the susceptibility to radiation-induced errors will only increase, making this type of research even more critical. The platform’s potential for developing targeted shielding and refining error-correction algorithms offers a promising path towards overcoming these challenges and realizing the full potential of quantum computation.

The work presented offers a new perspective on the challenges of maintaining qubit coherence in the face of environmental noise, and provides a pathway for developing more effective strategies for mitigating these effects. The experimental platform itself is a valuable tool for further research, enabling systematic studies of radiation effects and the development of innovative solutions for protecting qubits from external interference.

Platform Design Maximizes Muon Detection & Qubit Interaction

Symmetrical placement of the detector chips above and below the superconducting qubit was critical, maximizing the likelihood of muons traversing both detector substrates and registering as dual events. The solid angle achieved accounts for incident muons within a ± 57° range, beyond which the expected muon count falls below 100 counts per hour per square meter.

Analysis of single-detection events revealed a higher energy tail potentially originating from gamma or alpha particles with short stopping distances, or muons impacting the detectors at oblique angles, depositing more energy than perpendicular trajectories. Distinctions between top and bottom detector event distributions likely stem from minor variations in sensitivity and noise floor between the two detector arrays.

The experimental setup categorized events into three classes: ‘dual’ detections registering on both MKID arrays, and ‘top’ or ‘bottom’ detections registering on a single array, allowing for a detailed comparison of event characteristics and the identification of potential misclassifications. Single-shot measurements of excited state population were obtained by selecting a single delay time from the T₁ and T₂ decay curves, with a conditional reset pulse employed to minimize data acquisition latency.

Researchers accounted for single-event MKID triggers potentially caused by emissions from chip packaging or misclassified dual events, extracting normalized energy distributions for each event type to refine data analysis. Dual events consistently deposited less energy per detector, supporting the interpretation of a distinct event signature. The observed correlation between muon events and changes in qubit coherence times prompted an investigation into the fraction of single-event MKID triggers attributable to misclassified dual events.

A model was developed to estimate this fraction, revealing that approximately 18.4% of single-event T₁ measurements and 14.2% of single-event T₂ measurements were likely misclassified. Accounting for these misclassified events refined the estimated muon flux to 0.0154 events per second per square centimeter from the T₁ dataset and 0. The platform’s ability to closely monitor radiation effects on qubit performance offers a valuable tool for studying muon shielding and mitigation techniques.

These results have far-reaching implications for quantum error correction codes, as these events occur randomly at a rate of approximately one per minute, as the researchers state. The observed effects on qubit coherence are likely due to muons heating the substrate and creating quasiparticles near the Josephson junctions, disrupting the delicate superconducting state.

The variance in deposited muon energies, resulting from both absolute energy and trajectory, contributes to the observed average effect on qubit coherence. The top and bottom event curves exhibited symmetry and consistent recovery periods following detected events, further supporting the conclusion that deviations from baseline measurements correspond to dual events, even those initially misclassified as single events.

T_1 and T_2 Coherence Reduced by Penetrating Radiation Events

Penetrating muons directly correlate with reductions in qubit coherence, with measurements revealing up to a 30. The observed coherence loss occurs as a consequence of radiation-induced phonon bursts, detected in real-time through a novel platform designed to monitor qubit behavior. This configuration enables researchers to not only identify radiation events but also to quantify their impact on qubit performance during operation, a capability previously unavailable.

Analysis of MKID event energy distributions reveals that dual-detector events, indicative of penetrating particles, deposit less energy per detector on average compared to localized radiation sources. This distinction suggests that the observed coherence reductions are specifically tied to the passage of muons through the qubit substrate. The energy deposited by these muons heats the substrate and generates quasiparticles near the Josephson junctions of the qubits, a process believed to be the primary mechanism behind the observed decoherence.

The team’s methodology involves detecting correlated high-energy events using the three-layered device structure, with the qubit sample sandwiched between the two MKID arrays. This arrangement increases the probability of detecting penetrating radiative events by both the top and bottom detector layers. The observed effects differ from those reported in other studies, which have documented either catastrophic spontaneous energy decay over milliseconds or charge-parity jumps leading to bit flips.

This work focuses on the immediate impact of high-energy events on qubit coherence, providing a more direct assessment of radiation-induced decoherence. The team’s analysis of the detector response reveals that the MKID resonators experience larger frequency shifts and lower Q values during more energetic events, providing a quantifiable measure of the energy deposited by the radiation.

The observed reduction in $T_{1}$ and $T_{2}$ times following dual MKID events is statistically significant, indicating a strong correlation between radiation and qubit decoherence. The data supports the hypothesis that muon-induced heating and quasiparticle generation are key contributors to this effect. “We have demonstrated a new platform for measuring correlated qubit errors with radiation events originating outside of the device,” the researchers state, highlighting the potential for future investigations into the complex interplay between radiation and quantum coherence.

Correlated Errors Challenge Current Quantum Error Correction

The platform at Lawrence Livermore National Laboratory directly links reductions in qubit coherence to detected radiation, with coherence, both T1 and T2, decreasing by up to 30.5% following dual-detector events. This correlation challenges the assumption of uncorrelated decoherence underpinning most quantum error correction protocols, a finding with implications for scaling quantum computing architectures. The three-layer detection system, detailed in the work, allows researchers to map the phase response of detected events to temperature values between 100 and 300 millikelvin, a range dictated by the MKID’s sensitivity limits.

By recording data from all MKIDs when an event is detected on one, the team could investigate correlated events and differentiate between penetrating and non-penetrating radiation. This distinction is important because penetrating radiation, specifically muons, deposits a significant amount of energy into the substrate, triggering detectable events in multiple MKIDs simultaneously.

The team posits that muons heat the substrate and generate quasiparticles near the qubits’ Josephson junctions, disrupting their delicate quantum state. This mechanism differs from previously reported effects, which often involve catastrophic qubit resets or single-event upsets. The platform’s ability to isolate these correlated errors is a key advancement, as current error correction codes are designed to address random, uncorrelated decoherence. The calibrated temperature data derived from the MKIDs allows for a direct mapping of event phase response to temperature, though the usable range is constrained by detector limitations.

The team’s analysis focuses on both single and double chip “blowouts”, events triggering MKID responses and their subsequent effect on qubit decay (T1) and phase coherence (T2). The rate of these random events, approximately one per minute, highlights the need for robust error correction strategies tailored to address correlated noise sources.

Understanding how the qubit and readout resonator are affected by these events could lead to targeted error-corrective measures specifically designed for cosmic radiation. The team’s findings suggest that current quantum error correction codes may need refinement to account for these spatially correlated errors. While the platform currently measures the effects of radiation, future work could focus on implementing and testing new error correction algorithms designed to mitigate these correlated decoherence events. The platform’s design and data analysis techniques represent a step toward addressing a critical challenge in the field of quantum information science.

👉 More information
🗞 Superconducting-qubit decoherence correlated with detected radiation events
✍️ Alessandro R. Castelli et al.
🧠 DOI: http://link.aps.org/doi/10.1103/ddyy-t37l

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