Radiation Impacts in Superconducting Qubits 2026 (RISQ26)

15 Jun 2026, 08:00 → 18 Jun 2026, 18:00 America/Chicago

University of Wisconsin, Madison

See the conference website for more information on lodging and venue details, as well as to register for the event.

Monday 15 June

08:45 → 10:35 G4CMP Workshop: Morning Session

08:45 Intro/Welcome

Speaker: Jared Yamaoka

09:00 Current and Near-Term G4CMP physics capabilities

Speaker: Ryan Linehan

09:30 G4CMP Consortium, Github Organization, and Project Planning

Speaker: Birgit Zatschler

09:50 Lightning Talk

Speaker: Israel Hernandez (Illinois Institute of Technology)

10:05 Lightning Talk

Speaker: David Sadek

10:20 Analysis of Event Position Dependence in HVeV Detectors with G4CMP and MEMS

Speaker: Caitlyn Stone-Whitehead (Colorado School of Mines)

10:35 → 10:55 Break: Coffee

10:55 → 12:10 G4CMP Workshop: Morning Session II

10:55 Design Insights for Direct Absorber Phonon-Mediated Detectors from G4CMP

Speaker: Selby Dang

11:10 Modeling of Radiation Impact Physics and Phonon Dynamics at PNNL

Speaker: Jake Inman

11:25 Modeling the scrambling of two-level systems due to radiation impacts

Speaker: Eric Mascot

11:40 A Physics-based, AI-accelerated Codesign Approach for the Optimization of Superconducting Quantum Circuits

Speaker: Paul Baity (Brookhaven National Laboratory)

11:55 Lightning Talk

Speaker: David Kealhofer

12:10 → 13:40 Break: Lunch

13:40 → 15:10 G4CMP Workshop: Afternoon Session

13:40 Advanced G4CMP Features Tutorial

Speaker: Ryan Linehan (Fermi National Accelerator Laboratory)

13:40 Introductory Tutorial

Speakers: Jake Inman, Jesse Lutz

15:10 → 15:30 Break: Coffee

15:30 → 17:30 G4CMP Workshop: Afternoon Session II

15:30 S3DF Demonstration

Speaker: Stefan Zatschler

16:15 Discussion

Tuesday 16 June

09:00 → 10:15 Oral Presentations: Kick-Off

09:00 Kick-Off & Welcome

Speaker: Britton Plourde

09:15 Plenary Overview - TBD

10:15 → 10:45 Break: Coffee

10:45 → 12:00 Oral Presentations: Industry

10:45 Correlated Error Bursts in a Gap-Engineered Superconducting Qubit Array

One of the roadblocks towards the implementation of a fault-tolerant superconducting quantum processor is impacts of ionizing radiation with the qubit substrate. Such impacts temporarily elevate the density of quasiparticles (QPs) across the device, leading to correlated qubit error bursts. The most damaging errors—T1 errors—stem from QP tunneling across the qubit Josephson junctions (JJs). Recently, we demonstrated [Phys. Rev. Lett. 133, 240601 (2024)] that this type of error can be strongly suppressed by engineering the profile of superconducting gap at the JJs in a way that prevents QP tunneling. In this work, we identify a new type of impact-induced correlated error that persists in the presence of gap engineering. We observe that impacts shift the frequencies of the affected qubits, and thus lead to correlated phase errors. The frequency shifts are systematically negative, reach values up to 3 MHz, and last for ∼1 ms. We provide evidence that the shifts originate from QP-qubit interactions in the JJ region. Further, we demonstrate that the shift-induced phase errors can be detrimental to the performance of quantum error correction protocols.

Speaker: Alex Opremcak (Google Quantum AI)

11:15 Fast Measurements of the Impacts of Ionizing Radiation on Superconducting Qubits

We will discuss devices that have microwave kinetic inductance detectors (MKIDs) fabricated on the same substrate as qubits to assessing the impact of ionizing radiation. We will present our results using the MKIDs as a radiation sensor to look for correlations between detected events and two-level system dynamics: we observe no correlation in our data. We will also compare the recovery time of our devices to those in the literature, which suggests a link with the proximity of niobium to the junction. Finally, we will discuss on-going studies aimed at providing an off-the-shelf option to enhance the radiation event rate.

Speaker: Bradley Christensen (Northrop Grumman)

11:45 Discussion

12:00 → 13:30 Break: Lunch

13:30 → 15:15 Oral Presentations: Afternoon Session

13:30 Recovery Dynamics of a Gap-Engineered Transmon after a Quasiparticle Burst

Ionizing radiation impacts create bursts of quasiparticle density in superconducting qubits. These bursts temporarily degrade qubit coherence, which can be detrimental for quantum error correction. Here, we experimentally resolve quasiparticle bursts in 3D gap-engineered transmon qubits by continuously monitoring qubit transitions. Gap engineering allows us to reduce the burst detection rate by a factor of 5. This reduction falls 4 orders of magnitude short of that expected if the quasiparticles were to quickly thermalize to the cryostat temperature. We associate the limited effect of gap engineering with the slow thermalization of the phonons in our chips after the burst.

Speaker: Heekun Nho (Yale University)

14:00 Detection of Spatiotemporally Correlated Errors in OCS Transmons and Fluxonium

Spatiotemporally correlated error bursts, arising from quasiparticles generated by ionizing radiation and mechanical noise, pose significant challenges to implementing quantum error correction in superconducting quantum processors. In this work, we explore methods for detecting these errors in two superconducting circuits: offset charge sensitive (OCS) transmons and Fluxonium. In OCS transmons, a circuit ideal for sensing quasiparticle tunneling, we demonstrate a novel approach to directly measure charge variations across the junction using a direct-dispersive shift of the readout resonator using voltage modulation. This technique enables continuous measurement of charge parity and offset charge, key indicators of correlated errors, without requiring recalibration. In Fluxonium, a promising circuit for high performance processors, we investigate spatiotemporally correlated errors and analyze their behavior under varying applied flux to gain deeper insights into the effects of non-equilibrium quasiparticles on device performance. These findings contribute to advancing error detection and mitigation strategies for next-generation quantum processors.

Speaker: Jeffrey Gertler (MIT Lincoln Laboratory)

14:30 Phonon-Only Events and Correlated Quasiparticle Poisoning

In addition to radiation impacts, superconducting qubit arrays can be subject to correlated quasiparticle poisoning from nonionizing sources that also result in bursts of pair-breaking phonons. We observe elevated correlated and single-qubit 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, a characteristic associated with ionizing impacts, remains constant. In addition, for qubit arrays suspended by wirebonds in their sample package, we observe elevated poisoning due to mechanical impacts from the cryocooler. We discuss possible sources of these phonon-only events, including relaxation of built-up stress in the device thin films, substrates, or packaging, and potential mitigation strategies.

Speaker: Britton Plourde (University of Wisconsin, Madison)

15:00 Discussion

15:15 → 15:45 Break: Coffee

15:45 → 17:30 Oral Presentations: Backgrounds

15:45 Using Classical Athermal Phonon Sensor Technology as a Complementary Tool to Understand Quasiparticle Poisoning in Qubits

In the last decade, the sensitivity of superconducting athermal phonon detectors has improved by nearly two orders of magnitude, driven largely by the scientific need to search for dark matter with smaller masses and thus smaller energy depositions. Though these detectors are fabricated using nearly identical materials and fabrication techniques as those used for superconducting qubits, they have been purposefully optimized to maximize sensitivity to athermal phonons within the substrate; the exact opposite design goal to that of computation qubits, where one tries to absolutely minimize environmental coupling. This orthogonal optimization makes superconducting athermal phonon detectors an extremely valuable complementary tool to understand the source of quasi-particle poisoning in qubits. In this talk, we'll present studies trying to characterize the source of non-ionizing phonon burst backgrounds seen in these detectors and compare and contrast these results to studies characterizing non-ionizing burst backgrounds in computational qubits.

Speaker: Matt Pyle (University of California, Berkeley)

16:15 Qubit error bursts in superconducting quantum processors of Quantum Inspire: quasiparticle pumping and anomalous time dependence

We investigate qubit error bursts in 5- and 7-transmon processors of similar design, fabrication and packaging, but with different types of qubit Josephson junctions. The duration and rate of bursts are device specific but within the range of prior experiments and consistent with ionizing radiation. We observe two unforeseen signatures specifically in the processor with Dolan junctions. First, increasing the rate of π pulsing in the detection scheme shortens the recovery time to equilibrium, which is explained by a quasiparticle pumping mechanism. The second signature is an anomalous time dependence in the burst rate: a surge happens days or weeks after cooldown, followed by a strong suppression that persists until thermal cycling.

Speaker: Marios Samiotis (QuTech & Kavli Institute of Nanoscience, Delft University of Technology)

16:45 QUALIPHIDE-FIR dark matter experiment and LEE

The QUAntum LImited PHotons In the Dark Experiment (QUALIPHIDE) is a cryogenic broadband search for light dark matter candidates, primarily looking for signatures of massive hidden photons kinetically mixing with Standard Model photons. We report results from an experimental campaign coupling a focusing metallic dish to a pixelated array of energy-resolving $\sim$3 to $\sim$120THz sensitive photon counting microwave kinetic inductance detectors. Using data from 20-hour exposure, we set world leading constraint of the kinetic mixing parameter $\chi$ below the 10$^{-11}$ level for the majority of hidden photon masses between 13 and 100 meV. Simultaneously, we also report on a search for nuclear and electronic scattering events originating from dark matter capable of exciting single phonons within the small volume superconducting sensors, yielding the first ever terrestrial constraints on the cross-sections of certain light (10 to 1000 keV) dark matter species. We will also discuss the potential connection between our measured background and Low-Energy Excess in quantum sensors.

Speaker: Lanqing Yuan (Washington University in St. Louis)

17:15 Discussion

17:30 → 19:30 Poster Session

Wednesday 17 June

09:00 → 10:15 Oral Presentations: Theory Primer

09:00 Effect of quasiparticles on the parameters of a gap-engineered transmon

We evaluate the quasiparticle contribution to the frequency shift and relaxation rates of a transmon with the Josephson junctions connecting superconductors that have unequal energy gaps. The gap difference substantially affects the transmon characteristics. We investigate their dependence on the density and effective temperature of the quasiparticles, and on the nominal (unperturbed by the quasiparticles) transmon frequency. At temperatures low compared to the qubit frequency, the gap difference can induce an anomalous positive frequency shift, resulting in a non-monotonic temperature dependence of the transmon frequency. The qubit relaxation rate exhibits a resonance when the qubit frequency matches the gap difference; the shape of the resonance is strongly temperature-dependent.

Speaker: Leonid I. Glazman (Yale University)

09:30 Correlated Error Bursts in a Gap-Engineered Superconducting Qubit Array

Quantum error correction fundamentally requires that physical errors are sufficiently uncorrelated in time and space. In superconducting qubit processors, impacts from ionizing radiation violate this assumption by elevating quasiparticle density across the substrate, triggering correlated qubit error bursts. Previously, we demonstrated that the most damaging of these—correlated T1 errors originating from quasiparticle tunneling—can be strongly suppressed by engineering the superconducting gap profile at the Josephson junctions. Here, we present our group's recent findings on a new mechanism of impact-induced correlated errors that persists despite gap engineering. We observe that radiation impacts systematically shift the frequencies of affected qubits by up to 3 MHz for ~1 ms, resulting in correlated phase errors. We provide evidence that these shifts stem from quasiparticle-qubit interactions in the junction region, and we demonstrate that these shift-induced phase errors can be detrimental to the performance of QEC protocols.

Speaker: Gabrielle Roberts (Google Quantum AI)

10:00 Discussion

10:15 → 10:45 Break: Coffee

10:45 → 12:30 Oral Presentations: Co-located Sensors

10:45 The LLNL Cosmic Sandwich – an experimental and theoretical approach towards understanding non-equilibrium energy deposition in superconducting devices

Quantum error correction (QEC) protocols for superconducting qubits assume spatially and temporally uncorrelated decoherence events. However, recent evidence has seen large chip disruption due to cosmic rays. Some of these events are mitigated by gap engineering the qubits, but some effects still remain. We present a platform that sandwiches a superconducting transmon qubit between two microwave kinetic inductance detector (MKID) arrays, enabling real-time detection of radiation-induced phonon bursts. By synchronizing MKID event detection with single-shot measurements of qubit energy relaxation and phase coherence, we observe statistically significant reductions in coherence parameters immediately following dual MKID events which we attribute to penetrating muons. Interestingly, our timescales for recovery are much shorter than other events seen in the literature. We will discuss the recovery dynamics and simulations that connect different types of events with qubit and MKID response, as well as future experiments being planned using our platform.
This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344

Speaker: Dr Yaniv Rosen (LLNL)

11:15 A Cryogenic Muon Tagging System Integrated with a Superconducting Qubit Device for Radiation-Induced Error Mitigation

Superconducting qubits are highly sensitive to ionizing radiation, which can induce correlated errors and limit scalable fault-tolerant quantum computing. In particular, cosmic-ray muons can deposit energy in the substrate, generating phonon bursts that break Cooper pairs and produce quasiparticles, leading to correlated decoherence events across multiple qubits. We present the development of a cryogenic muon tagging system based on Kinetic Inductance Detectors (KIDs) and its integration with superconducting quantum hardware. Originally developed within the ACE-SuperQ project and validated as a standalone detector, the system demonstrated a muon tagging efficiency of approximately 90% and excellent agreement with Monte Carlo simulations. Building on this validation, the tagging system has been integrated with a multi-qubit superconducting chip operated in a dilution refrigerator. The detector configuration consists of a multi-layer KID stack arranged above and below the quantum device, enabling time-coincident identification of muon-induced events within the same cryogenic environment. The integrated setup has been successfully commissioned, enabling simultaneous operation of the qubit chip and the muon tagging system. A first measurement campaign has been carried out, and preliminary data show time-correlated events between the muon tagging detectors and the qubit readout. A quantitative analysis of radiation-induced effects on qubit performance is currently ongoing. This work represents a step toward the implementation of event-level radiation tagging as a tool for characterizing and potentially mitigating correlated errors in superconducting quantum processors, while establishing a modular platform for future studies at the interface between particle physics and quantum information science.

Speaker: Tanay Roy (FNAL)

11:45 Combining qubit arrays and TES sensors to understand and mitigate the effects of background radiation

In this presentation we describe ongoing work at NIST with circuits that combine qubit arrays and superconducting transition-edge sensors (TESs) on the same silicon substrate. The TESs are intended to serve two purposes. Because the TESs are fabricated directly on the silicon, they are sensitive to phonons created in the substrate by background radiation and thus they provide an on-chip, spectroscopic detection capability for background events. In addition, the bias power in the TESs is automatically reduced by negative electrothermal feedback when they are excited, providing a virtual thermal conductance between the TES and the thermal bath. The circuits are designed so that this virtual conductance is significant compared to the physical conductance between the silicon substate and the bath. As a result, the TESs are expected to stabilize the thermal environment of the qubit array after a background event and mature implementations of the circuit could provide protection from disruptive correlated decoherence events. Having recently fabricated devices with both working TESs and qubits, we are now characterizing them. Ongoing measurements include studies of the electromagnetic interactions between TESs and qubits, extraction of the thermal conductances between the TES, chip, and bath, and demonstrations of radiation detection and energy removal by the TESs. We report progress on these varied measurements.

Speaker: Joel Ullom (NIST)

12:15 Discussion

12:30 → 14:00 Break: Lunch

14:00 → 15:45 Oral Presentations: Chip Response - Calibration & Modeling

14:00 Progress at the CLIQUE Facility

The CLIQUE (Controlled Linac Irradiation of Quantum Experiments) Facility at Johns Hopkins Applied Physics Laboratory is an experimental user facility that contains an electron linear accelerator (linac) used as an on-demand high-energy particle source to study deleterious effects on quantum systems. The linac provides a pulsed, microsecond burst of ~20 MeV electrons that are redirected at a modified dilution refrigerator where they deposit energy in the system of interest (e.g. superconducting qubits, quantum dot spin qubits, or detectors). The error dynamics of individual qubits and the system as a whole can be easily and quickly extracted due to the on-demand nature of our radiation source. We present extensive results on radiation-induced qubit errors in both fixed-frequency and flux-tunable transmons. We additionally highlight joint efforts with external researchers, including a gap-engineering study with MIT and quantum dot measurements with UW-Madison. The CLIQUE facility, open to future collaborations, provides a testbed for novel quantum device design and packaging techniques for radiation-induced error mitigation.

Speaker: Tom McJunkin (Johns Hopkins Applied Physics Laboratory)

14:30 Measuring and modeling the impact of radiation on superconducting qubits protected through gap engineering

Impacts from high-energy particles have been demonstrated to cause correlated errors in superconducting qubits by increasing the quasiparticle density in the Josephson junction (JJ) leads. These correlated errors are particularly harmful as they cannot be remedied via conventional error correcting codes. It was recently demonstrated that these correlated errors can be reduced or eliminated by engineering the difference in superconducting gap across the JJ to be larger than the qubit frequency. In order to test the efficacy of this strategy we have exposed arrays of this type of “gap engineered” qubits to a variety of radioactive sources, scanning both particle type and energy deposited in the substrate. We also characterize the effect of another layer of gap engineering away from the JJ to help suppress QP-induced dephasing errors. We will describe both the measurements performed and a quasiparticle model consistent with these measurements, discussing the implications for the future of preventing correlated errors.

Speaker: Doug Pinckney (MIT)

15:00 Radioassay of Materials in Superconducting Qubit Systems and Development of Alpha-Decay Calibration Sources for Radiation Upset Studies

Environmental radioactivity has been increasingly recognized as a limiting factor in the performance of superconducting quantum devices. Trace-level contamination in materials comprising dilution refrigerators and qubit packaging can produce ionizing radiation that generates quasiparticles, contributing to correlated error bursts and decoherence. Understanding and mitigating these effects requires both a thorough characterization of the radioactive content of common cryogenic materials, finding lower radioactivity alternatives, and controlled methods for studying the response of quantum devices to specific forms of radiation.

We present results from a systematic radioassay campaign leveraging Pacific Northwest National Laboratory's extensive ultralow-background measurement capability. A broad survey of materials commonly found in dilution refrigerators and superconducting qubit assemblies was conducted using inductively coupled plasma mass spectrometry (ICP-MS) or high-purity germanium gamma-ray spectroscopy. Drawing on decades of experience in radiopure materials selection for rare-event physics, we triaged the material inventory to prioritize components of highest radiological concern, which we will discuss in this work.

In parallel, we discuss the design and development of well-characterized alpha-emitting calibration sources suitable for deployment inside dilution refrigerators. These sources enable controlled exposure of superconducting devices to alpha-radiation, allowing the systematic study of upset events and the disentanglement of contributions from alpha particles versus from sources of beta radiation and gamma rays to qubit error rates. Together, these efforts support the broader community goal of understanding and mitigating radiation-induced decoherence in next-generation quantum processors.

Speaker: Isaac Arnquist (PNNL)

15:15 Alphas as a Source of Chip-Wide Correlated Errors

Correlated error bursts causing decoherence in superconducting qubits have been detrimental to quantum error correction schemes, with recent work by Google showing approximately once per hour correlated bursts in their qubits. Over the past decade, it has been shown that ionizing radiation contributes to this effect, with cosmic rays being commonly identified. However, with advent of bandgap-engineering, the reduced sensitivity to energy depositions in a substrate implies that only the highest energy events should be the problem, but it has not yet been definitively shown which type of ionizing radiation is the culprit.

At PNNL, we have fabricated custom alpha sources from the Po-210 isotope with activities of 10 or less decays per minute. These sources have been placed with direct line of sight to a 4 cm$^2$ sapphire substrate patterned with an array of nine superconducting detectors (MKIDs) operating at 10 mK. We have measured the multiplexed response of these resonators to different sources of ionizing radiation, for which alphas demonstrate a specific signature when interacting with the substrate via high-amplitude, highly correlated events. In this talk, we will discuss these results as they pertain to superconducting qubits, as well as outline the next steps in fully disentangling signatures due to different types of ionizing radiation.

Speaker: Samuel Watkins (Pacific Northwest National Laboratory)

15:30 Discussion

15:45 → 16:15 Break: Coffee

16:15 → 17:30 Oral Presentations: Spin Qubits - Overlapping Communities

16:15 Radiation-induced offset charge jumps in Si/SiGe quantum dot qubits

In this talk we report the observation of offset charge jumps induced by external radiation in Si/SiGe quantum dots that serve as spin qubits. Such charge jumps are important for quantum dot qubits, because they directly alter the operating point of the qubit in gate voltage space, and such uncontrolled shifts can induce errors in qubit initialization, readout, and manipulation. Using the linear accelerator at the Johns Hopkins University Applied Physics Lab (JHU-APL), we observe offset charge shifts in a Si/SiGe Tunnel Falls chip [1,2] with two 3-dot, 1-sensor devices inside a dilution refrigerator. The accelerator allows for the precise injection of few-to-single high energy electrons (approximately 16 MeV) into the device with energy deposition similar to that of cosmic-ray muons. We observe both correlated and uncorrelated jumps across three separate quantum dots whose offset charge is monitored simultaneously. In a separate experiment, we also observe offset charge jumps arising from e-h pairs generated optically at the backside of the handle wafer on which the SiGe heterostructure is grown. These photon bursts emulate the effects of environmental radiation when energy is deposited in the substrate, whose thickness far exceeds that of the SiGe heterostructure itself. We find that such absorption at the back of the wafer also results in discrete jumps in the offset charge, and we discuss possible locations for the trapping of such charge. An important difference between superconducting qubits and Si/SiGe qubits is that the heterostructure hosting the latter has a lattice constant larger than that of bulk silicon, necessitating the removal of atomic density and leading to layers with defects caused by this mismatch [3]. An important result from measurements of photon-induced e-h pair generation at the back of the chip is that carrier can and do cross these layers and become trapped very close to the quantum dots at the top of the heterostructure.
[1] Neyens, S. et al. Probing single electrons across 300-mm spin qubit wafers. Nature 629, 80–85 (2024).
[2] George, H. C. et al. 12-Spin-Qubit Arrays Fabricated on a 300 mm Semiconductor Manufacturing Line. Nano Letters 25, 793–799 (2025).
[3] F. Schäffler, High-mobility Si and Ge structures, Semiconductor Science and Technology 12, 1515 (1997).

Speaker: Mark Eriksson (University of Wisconsin-Madison)

16:45 Investigating Effects of Gamma Radiation on Spin Qubits in HRL SLEDGE Devices

Spin qubits have seen much progress over recent years, proving to be an appealing candidate for scalable quantum computing with small footprints, electrical control, promising coherence times, and the industry-compatible silicon material platform. As advances are made in the spaces of control and scaling, addressing noise from various origins becomes increasingly pertinent. It has been shown that cosmic rays can cause bit flips and correlated errors in superconducting qubits [1]; however, similar effects in spin qubits have not been widely reported or studied in literature. To characterize the effects of ionizing radiation, we use a Cs-137 gamma source to irradiate Si/SiGe SLEDGE devices from HRL, obtained through the Qubits for Computing Foundry. We investigate the effects of ionizing radiation on the SETs using Coulomb-blockade peaks. We record significant drifts in peak positions that eventually saturate and persist after the source is removed. This behavior is attributed to filling of charge traps near the SETs after gamma rays Compton scatter and produce tracks of electrons and holes. A charge transport-based framework combining Monte Carlo simulations with Geant4/G4CMP, trapping dynamics, and a simple device model is developed to qualitatively explain this phenomenon. We note that discrete jumps in the peaks can also be observed, which we attribute to single radiation events. These jumps are also highly correlated between the two SETs. Preliminary measurements on the effects of gamma radiation on single exchange-only qubits are also performed, showing fidelity loss over time. These experiments should provide insights into the effects of environmental radiation on spin qubit devices.

[1] Li, X., Wang, J., Jiang, YY. et al. Cosmic-ray-induced correlated errors in superconducting qubit array. Nat Commun 16, 4677 (2025)

Speaker: Joshua Lou (University of Maryland)

17:15 Discussion

17:30 → 19:30 Poster Session

Thursday 18 June

09:00 → 10:45 Oral Presentations: Chip Response - Event Reconstruction

09:00 Response of qubits to particles and thermal phonons

Superconducting qubits are susceptible to transient energy deposition arising from cosmic rays and environmental radioactivity. High-energy phonons generated by particle interactions in the qubit chip substrate can create quasiparticles that temporarily degrade qubit coherence.
We investigate the qubit response under controlled irradiation using a proximal Radium-224 source. To identify radiation-induced events, we develop and implement a robust data-selection protocol capable of discriminating signal events from background noise. The observed event rate scales with source activity and exhibits an exponential decay consistent with the radioactive decay of Radium-224.
In addition, we expose the qubit to purely thermal pulses generated by a silicon heater and to optical pulses from an LED source, with the aim of characterizing the device response to different types of energy deposition.
Furthermore, we explore the use of multiple qubits as phonon-mediated detectors, aiming to maximize the signal-to-noise ratio through simultaneous readout. Finally, we correlate the qubit response with that of a well-established cryogenic sensor, namely a neutron transmutation doped (NTD) germanium thermistor, capable of reconstructing the energy deposited in the substrate.
We present recent results and outline future prospects for the development of radiation-resilient quantum devices and phonon-based detection techniques.

Speaker: Alberto Ressa (Istituto Nazionale di Fisica Nucleare)

09:30 Particle impact energy reconstruction using transmons

When an ionizing particle interacts with the substrate of a superconducting qubit chip, it generates high-energy athermal phonons that propagate through the material, breaking Cooper pairs in the superconducting films and inducing quasiparticle poisoning. These non-equilibrium quasiparticles limit qubit coherence times and introduce correlated errors across large qubit arrays, posing a major challenge for the development of fault-tolerant quantum computers. At the same time, the potential sensitivity of superconducting qubits to Cooper pair breaking makes them promising detectors for dark matter and coherent elastic neutrino–nucleus scattering, given the meV-scale energy required for quasiparticle generation in most superconductors. In this work, we present a detailed statistical analysis of radiation-induced relaxation errors aimed at modeling the time evolution of quasiparticle density dynamics. From experimental data on five ground-plane transmon qubits, we extract the quasiparticle recombination constant with a precision of ≤10%. Furthermore, we investigate the correlation between the linear loss rate and the energy deposited in the qubit island. Finally, we introduce a statistical reconstruction method based on position localization to estimate the total energy deposited on the chip, providing a pathway toward using superconducting qubits as sensitive particle detectors.

Speaker: Emanuela Celi (Northwestern University)

10:00 SQUATs: Qubit-Based Sensors for Dark Matter

The Superconducting Quasiparticle-Amplifying Transmon (SQUAT) is a sensor architecture targeting meV (THz) detection based on a weakly charge-sensitive transmon qubit directly coupled to a transmission line.  Energy depositions in the qubit capacitor generate quasiparticles that tunnel across the Josephson junction.  Each tunnel changes the qubit parity and produces a measurable signal in CW transmission.  SQUATs use junctions with lower Tc than their capacitors to induce quasiparticle trapping near the junction and amplify the tunneling rate for a given energy deposition.  Their enhanced sensitivity to quasiparticle events and intrinsic multiplexability make SQUATs a promising detector foundation for low-energy-threshold rare-event searches. In this talk, I will give an update on the SQUAT development program.  I will present the design, characterization, and background sources influencing parity-switching rates.  I will also discuss progress towards a position-dependent energy calibration using MEMS-based cryogenic optical beam steering.

Speaker: Hannah Magoon (Stanford University)

10:30 Discussion

10:45 → 11:15 Break: Coffee

11:15 → 12:45 Oral Presentations: Wrap-Up

11:15 G4CMP Workshop Summary

11:45 Close-Out

12:15 Discussion