Physicists from the University of Cambridge, Ghent University and the University of Oxford have shown that a quantum particle crossing a boundary between order and disorder is transmitted rather than reflected, but emerges in a changed, spread-out form. The result, published in Nature Physics, may help address the magnetic monopole paradox, a physics puzzle discussed since the 1980s.
Key facts
- The team studied quantum wave packets crossing a duality defect in a spin-chain model.
- In the model, particles passed through the defect every time rather than bouncing back.
- After crossing, the particle became a nonlocal object attached to an invisible string leading back to the defect.
- The researchers found that a previously virtual mathematical space represents the defect’s internal quantum state.
What does the result say about particles crossing a duality defect?
The result says a particle crossing a duality defect is perfectly transmitted, yet does not remain an ordinary local particle. It re-emerges as a faint, spread-out excitation connected back to the defect by an invisible string. The researchers liken this transformed state to a “wisp of fog.”
A duality defect is a boundary between order and disorder. The team used a spin chain—a simple model of quantum matter—and sent quantum wave packets at such a boundary. Wave packets are localised bundles of quantum waves used to identify a particle’s probable location and motion.
According to the researchers, the particle has no boundary from which to reflect because the defect can be moved freely. Its transmission is therefore perfect, but its identity changes in the process. The outgoing state is nonlocal: it is associated with a string that extends back to the defect.
Why does this matter for the magnetic mystery?
The work offers a route toward clarifying the magnetic monopole paradox. Since the 1980s, physicists have considered what happens when a charged particle scatters from a magnetic monopole, an isolated magnetic charge. Earlier work appeared to show that the outgoing state disappeared, while later work proposed that missing states were concealed in twisted sectors of the monopole.
The new study identifies an analogous destination for the transformed particle in its spin-chain setting. Rather than being reflected or simply vanishing, it crosses the defect and becomes an extended excitation whose string remains connected to that boundary. The authors say this result may help solve the long-running monopole puzzle.
The study also gives a physical interpretation to a mathematical component of the framework used to describe the defect. The virtual space in the tensor-network description is not merely bookkeeping, the researchers found: it is the defect’s internal quantum state. Its size determines the number of internal degrees of freedom carried by the defect.
What this means for physics students and researchers
For students and researchers working on quantum matter, the study provides a concrete example of how entanglement-based tools and tensor networks can describe experimentally meaningful processes. It also connects abstract ideas about non-invertible symmetries and duality defects to a direct question: what happens when a quantum excitation encounters one?
The use of a simple spin-chain model means the effect could be tested on existing quantum-simulation platforms, including cold atoms, trapped ions and superconducting processors. The researchers say such experiments could produce the first direct observation of a particle changing identity while crossing a topological interface.
How did the team reach the finding?
The researchers modelled quantum matter with a spin chain and used quantum wave packets to probe a duality defect. Their analysis builds on tensor-network methods, which describe quantum matter through entanglement rather than solely through particles.
The paper, “Perfect particle transmission through duality defects”, was authored by Atsushi Ueda and colleagues and published in Nature Physics in 2026. The work involved researchers from Cambridge, Ghent and Oxford.
Frequently asked questions
What is a duality defect?
A duality defect is a boundary between order and disorder in the quantum system studied by the researchers. When a quantum particle crosses it, the study found that the particle passes through but changes into a nonlocal, spread-out excitation linked back to the defect.
Can the effect be tested experimentally?
Yes. Because the underlying model is a simple spin chain, the researchers say the effect could be tested on existing quantum-simulation platforms, including cold atoms, trapped ions and superconducting processors.
