⚛️ quantum physics

A Cryogenic Muon Tagging System Based on Kinetic Inductance Detectors for Superconducting Quantum Processors

This paper presents the design, simulation, and successful operation of a cryogenic Kinetic Inductance Detector (KID) system that achieves approximately 90% efficiency in tagging atmospheric muons at millikelvin temperatures, offering a viable solution for mitigating radiation-induced errors in superconducting quantum processors.

Original authors: Ambra Mariani, Laura Cardani, Mustafa Bal, Nicola Casali, Ivan Colantoni, Angelo Cruciani, Giorgio Del Castello, Daniele Delicato, Francesco De Dominicis, Matteo del Gallo Raccagiovine, Matteo Folcare

Published 2026-03-25

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Ambra Mariani, Laura Cardani, Mustafa Bal, Nicola Casali, Ivan Colantoni, Angelo Cruciani, Giorgio Del Castello, Daniele Delicato, Francesco De Dominicis, Matteo del Gallo Raccagiovine, Matteo Folcarelli, Sabrina Garattoni, Anna Grassellino, Mehmood Khan Yasir Raja, Valerio Pettinacci, Alberto Ressa, Tanay Roy, Marco Vignati, David van Zanten

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you are trying to build a super-fast, super-precise computer that uses the laws of quantum physics to solve problems. This is a Superconducting Quantum Processor. It's incredibly powerful, but it's also incredibly fragile. Think of it like a house of cards built on a shaking table. If even a tiny breeze hits it, the whole thing collapses.

In the real world, that "breeze" is ionizing radiation. Specifically, high-energy particles from space called muons (which are like heavy, super-fast electrons) are constantly raining down on us from the atmosphere. When a muon hits the quantum computer, it creates a tiny burst of energy that scrambles the computer's calculations, causing errors.

The problem? These muons are like ghosts. They can pass through lead walls, concrete, and even the entire Earth without stopping. You can't just build a thicker shield to stop them; it's impossible.

The Solution: A "Bouncer" for the Quantum Computer

The paper you shared describes a clever new solution: a Cryogenic Muon Tagging System.

Think of this system as a high-tech bouncer standing at the door of the quantum computer's club. Its job isn't to stop the muons (because it can't), but to spot them the moment they walk in and yell, "Hey! A muon is here! Pause the party!"

Here is how they built this bouncer, explained simply:

1. The Sensors: "Super-Conductive Trampolines"

The team built two special sensors called Kinetic Inductance Detectors (KIDs).

The Analogy: Imagine a trampoline made of a special metal that only works when it's freezing cold (colder than outer space, about -273°C).
How it works: When a muon zips through the trampoline, it doesn't just bounce; it creates a tiny ripple (a sound wave) in the metal. Because the metal is so sensitive, this ripple changes the trampoline's "vibe" (its electrical frequency). The system hears this change instantly.
The Setup: They stacked two of these trampolines, one on top and one on the bottom of a "fake" quantum chip (a silicon wafer).

2. The "Coincidence" Trick: Catching the Ghost

How do you know it's a muon and not just a random bump or a stray spark?

The Analogy: Imagine you are trying to catch a fly flying through a room. If you see it hit the top window, it might be a bird. If you see it hit the bottom window, it might be a bug. But if you see it hit the top window and the bottom window at the exact same split-second, you know it's a fast flyer passing straight through.
The Science: The system looks for signals in the top sensor and the bottom sensor happening within a tiny fraction of a second (340 microseconds). If both sensors go "ding!" at the same time, it's almost certainly a muon passing straight through the middle.

3. The "Freeze" Factor

Usually, electronics break if you put them in a freezer. But quantum computers need to be in a freezer to work. The genius of this paper is that they built the bouncer inside the freezer with the computer.

They operated these sensors at 20 millikelvin (that's 0.02 degrees above absolute zero).
They proved that the sensors work perfectly in this extreme cold and don't interfere with the delicate quantum computer sitting in the middle.

The Results: A Perfect Score

The team tested their prototype and found:

Efficiency: They caught about 90% of the muons that passed through. That's like a bouncer spotting 9 out of 10 troublemakers.
False Alarms: They almost never got tricked by background noise (like random gamma rays from the walls). The "dead time" (the time the computer has to pause while the bouncer checks) is so small it's practically zero.
Agreement: Their real-world results matched their computer simulations perfectly.

Why This Matters

This is a huge step forward. For a long time, scientists thought muons were an unsolvable problem for quantum computers built on the surface of the Earth.

This system proves we can build a real-time alarm system that sits right next to the quantum processor.

The Future: In the future, when a muon is detected, the computer won't just crash. It will instantly say, "Oh, a muon just hit us. Let's throw away the data from the last few milliseconds and try again." This is called a veto.
The Goal: This allows quantum computers to run longer, more complex calculations without being ruined by cosmic rays, bringing us closer to the "fault-tolerant" quantum computers that could revolutionize medicine, finance, and science.

In short: They built a super-sensitive, ultra-cold "muon radar" that fits inside a quantum computer's fridge. It spots cosmic rays instantly, allowing the computer to ignore the damage and keep working. It's a small sensor with a massive impact on the future of computing.

1. Problem Statement

Superconducting quantum processors are increasingly limited by ionizing radiation, which acts as a source of decoherence.

Mechanism: Environmental $\gamma$ -rays and cosmic-ray muons deposit energy in the chip substrate, generating free charges. Their recombination produces high-energy phonons that propagate across the device, breaking Cooper pairs and creating bursts of quasiparticles.
Impact: This degrades qubit relaxation times ( $T_1$ ) and, critically, induces correlated errors across multiple qubits. These errors violate the spatial and temporal independence assumptions required by most quantum error correction algorithms.
The Muon Challenge: While $\gamma$ -rays are more frequent, atmospheric muons are more problematic due to their high energy and penetration power. They cannot be effectively mitigated by passive shielding (e.g., lead or underground facilities are complex or impractical for many setups).
Need: A real-time monitoring system (veto) capable of identifying muon events to guide error-correction strategies or reject corrupted data is essential for fault-tolerant quantum computing.

2. Methodology

The authors designed, simulated, and experimentally validated a cryogenic muon-tagging system using Kinetic Inductance Detectors (KIDs).

System Design

Architecture: A vertical stack of three detectors arranged with $\sim$ $\sim$ 4.5 mm spacing:
- Top and Bottom Detectors: Act as the muon-tagging system (veto).
- Central Detector: Acts as a proxy for a superconducting quantum chip (qubit).
Detector Technology:
- Substrate: High-resistivity silicon wafers (525 $\mu$ m thick, 20.25 cm $^2$ area).
- Sensor: Aluminum Kinetic Inductance Detectors (KIDs) patterned on the silicon.
- Principle: When a particle traverses the silicon, it generates athermal phonons. These phonons are absorbed by the superconducting KID film, breaking Cooper pairs and changing the kinetic inductance. This shifts the resonant frequency of the KID, which is read out via RF tones.
- Advantage: Using the silicon substrate as a large absorber allows for high geometrical efficiency ( $\sim$ 90%) with only two detectors, whereas direct absorption would require hundreds of KIDs.
Cryogenic Operation: The system operates at $\sim$ 20 mK inside a dilution refrigerator, compatible with standard qubit environments.
Readout: Multiplexed RF readout using a daisy-chain configuration. The system shares electronics compatible with qubit control (e.g., Ettus X310 or RFSoC platforms).

Simulation (Monte Carlo)

Tool: Geant4 toolkit.
Scope: Modeled the full 3D geometry of the detector stack, copper holders, and shielding.
Inputs: Simulated atmospheric muon flux (sea level) and environmental $\gamma$ -ray flux (measured in the lab).
Goal: Predict muon-tagging efficiency and estimate "dead time" caused by accidental coincidences from background $\gamma$ -rays.

3. Key Contributions

First Cryogenic Prototype: Demonstrated the first operational muon-tagging system based on KIDs specifically designed for integration with superconducting quantum processors at millikelvin temperatures.
Phonon-Mediated Detection: Validated a detection scheme where the silicon substrate acts as the primary absorber, achieving high efficiency with minimal hardware footprint.
Compatibility Proof: Showed that the system can operate in parallel with qubit readout without introducing electromagnetic crosstalk or degrading qubit coherence ( $T_1$ ).
Quantitative Validation: Provided a rigorous comparison between experimental data and Geant4 simulations, confirming the system's ability to distinguish true muon events from background noise.

4. Results

The prototype was tested for approximately one hour at the Sapienza University of Rome.

Muon-Induced Coincidence Rate:
- Measured: $(192 \pm 9) \times 10^{-3}$ events/s.
- Simulated (Geant4): $(195 \pm 12) \times 10^{-3}$ events/s.
- Conclusion: Excellent agreement between experiment and simulation.
Tagging Efficiency:
- The system achieved a muon-tagging efficiency of $\sim$ 90%.
- This was determined by the ratio of tagged events (coincidences in top/bottom detectors) to total muon events passing through the central chip.
Background and Dead Time:
- Accidental Coincidences: The rate of random coincidences caused by environmental $\gamma$ -rays was measured at $\sim(3.5 \pm 0.1) \times 10^{-3}$ events/s, consistent with predictions.
- Dead Time: With a veto window of 1–5 ms, the fractional dead time due to background coincidences is $< 10^{-4}$ (0.01%). Even with a conservative 25 ms window, dead time remains negligible (0.05%).
Detector Performance:
- Internal quality factors ( $Q_i$ ) reached $10^5 - 10^6$ .
- Pulse decay times were several hundred microseconds, enabling sub-millisecond time resolution for coincidence analysis.

5. Significance and Outlook

Feasibility of Active Veto Systems: The results prove that active muon veto systems can be integrated directly into the cryogenic environment of quantum processors without significant performance penalties (dead time or crosstalk).
Error Mitigation Strategy: This technology enables real-time identification of muon-induced correlated errors. Future systems can use this data to:
- Veto: Discard quantum operations occurring during a muon event.
- Correct: Apply specific error-correction algorithms tailored to the timing and nature of the radiation event.
Scalability: While the prototype used a central chip proxy of $\sim$ 8 cm $^2$ (representing a 80–100 qubit chip), the authors note that for smaller chips (typical current sizes $<1$ cm $^2$ ), the tagging efficiency would be even higher as the external detectors cover a larger fraction of the active area.
Future Work: The next steps involve coupling the system to a real multi-qubit chip, implementing real-time veto algorithms, and optimizing KID geometries and materials to further enhance detection efficiency.

In summary, this work establishes a critical technological prerequisite for scaling superconducting quantum computers above ground by providing a robust, low-dead-time method to mitigate one of the most insidious sources of correlated errors: cosmic-ray muons.