Insider Brief
- A recent study shows that cosmic rays and gamma rays trigger correlated errors in superconducting qubits, posing a major challenge to fault-tolerant quantum computing.
- Researchers used a 63-qubit chip and in-fridge muon detectors to link over 80% of quasiparticle bursts to gamma rays and the remainder to cosmic-ray muons.
- The study raises concerns for architectures like Majorana qubits, which rely on stable charge-parity states, and suggests charge-parity monitoring could aid in both error mitigation and fundamental particle detection.
Cosmic rays are disrupting the fragile quantum bits inside superconducting processors, posing a serious obstacle to building reliable quantum computers. A recent study published in Nature Communications offers some of the most direct evidence yet that energetic particles from space are triggering correlated errors across entire arrays of superconducting qubits, undermining their ability to maintain the coherence necessary for quantum error correction.
Using a 63-qubit processor, researchers from the Beijing Academy of Quantum Information Sciences and collaborating Chinese institutions detected high-energy particle impacts by monitoring charge-parity changes and bit-flip events across multiple qubits. These changes occur when quasiparticles — which are broken pairs of superconducting electrons — tunnel through the quantum circuits, disrupting the delicate quantum states.
The team found that both cosmic-ray muons and terrestrial gamma rays are responsible for these bursts of quasiparticles. Crucially, the researchers were able to distinguish between the two types of radiation by installing muon detectors inside the dilution refrigerator housing the quantum chip. These detectors, positioned beneath the qubit sample, allowed them to confirm that some of the errors coincided precisely with muon impacts, while others did not — implicating gamma rays as the other main culprit.
Majorana qubits, long considered a holy grail for fault-tolerant quantum computing, promise built-in protection by encoding information in exotic, nonlocal quasiparticles. However, the findings raise concerns for these emerging architectures, which rely on stable charge-parity states to maintain their error-resilient properties. Cosmic-ray-induced charge-parity jumps could disrupt those states, meaning even rare events may compromise the integrity of systems once thought to be topologically protected.
On the bright side — a little cosmic ray pun — the team said the work may lead to some beneficial scientific applications.
“The proposed method, which monitors multiqubit simultaneous charge-parity jumps, has high sensitivity to QP bursts and may find applications for the detection of cosmic-ray particles, low-mass dark matter, and far-infrared photons,” the team writes in the paper.
Tracking These Cosmic Culprits
Quasiparticle bursts, or QP bursts, manifest as simultaneous disruptions in the charge parity of multiple qubits. These bursts are especially damaging because they produce correlated errors — multiple failures at once — which standard quantum error correction codes struggle to handle.
To measure these events, the researchers compared two detection methods: one based on charge-parity jumps (highly sensitive to quasiparticles), and one based on bit flips (which are more common during qubit relaxation). They showed that the charge-parity approach could detect disruptions at much lower energy thresholds, making it a more sensitive tool for identifying when and where cosmic particles strike.
Using this setup, the team observed QP bursts from muons occurring roughly once every 67 seconds and from gamma rays at a much higher rate. In total, 81.6% of the quasiparticle bursts were traced back to gamma rays, with the remaining 18.4% caused by muons.
Muons are tiny, fast-moving particles created when cosmic rays from deep space collide with atoms in Earth’s upper atmosphere, and they can pass through buildings and people unnoticed. Gamma rays are intense bursts of energy, often produced by radioactive decay on Earth or violent cosmic events like supernovas, and they can easily penetrate most materials.
Shielding the refrigerator with a layer of lead significantly reduced the gamma-ray-induced events but had little effect on muon-induced ones.
Why This Matters for Quantum Computing
Correlated errors from particle impacts threaten one of the core requirements for building useful quantum computers: fault tolerance. Current error correction strategies assume that errors are local and uncorrelated, affecting only one or a few qubits at a time. But if many qubits fail at once due to a single cosmic event, these strategies break down.
The problem becomes especially acute at larger scales. As superconducting chips grow to include hundreds or thousands of qubits, the odds of a particle hitting some part of the array obviously increase. Even a single burst could spoil long-running quantum computations.
The study also casts doubt on the idea that simply improving qubit coherence times or gate fidelities will be enough. Without addressing correlated errors, even the most advanced quantum processors may fall short of performing extended computations reliably.
Toward Better Detection — and Protection
All is not lost, the researchers suggest and — interestingly — the team’s detection method may itself offer a solution. By closely monitoring charge-parity jumps across multiple qubits, researchers can not only identify cosmic events after they happen but may eventually be able to anticipate them and adapt in real-time.
For instance, if a cosmic-ray strike is detected mid-computation, quantum error correction circuits could be re-routed around the affected qubits. This could allow the system to “drop out” sections of the chip where correlated errors are likely to propagate, preserving the rest of the calculation.
Another potential path is to operate quantum computers underground or in shielded environments, reducing the flux of cosmic rays. But such strategies increase costs and complexity, and still wouldn’t eliminate gamma-ray-induced errors, which can originate from materials within the laboratory itself.
Materials Matter
One reason this study observed faster decay of quasiparticles than previous experiments (including Google’s Sycamore processor) is the material design of the qubits. The aluminum films used in the Beijing team’s chip serve as quasiparticle traps because of their lower superconducting gap compared to surrounding tantalum layers. This causes quasiparticles to accumulate and recombine more quickly, reducing the duration of disruption.
By engineering such traps deliberately and reducing areas where quasiparticles can linger, chip designers might further mitigate damage from particle strikes. Additionally, using materials with carefully designed energy gaps could help absorb or neutralize phonons — energy waves generated during particle impacts — before they reach sensitive areas.
The researchers believe that their charge-parity monitoring approach could go beyond quantum computing. Because it can detect tiny energy transfers with high spatial and temporal resolution, the method could potentially be used to search for exotic particles such as dark matter or detect far-infrared photons. By tuning the device sensitivity and adjusting its materials, this quantum chip might one day serve as a tool for fundamental physics, not just computation.
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