Illustration of quantum states in a strange metal: a deep blue crystal lattice receding into the distance with a glowing central node and neon green and cyan energy beams threading through the structure. Illustration: Harald Ritsch / TU Wien.
Quantum states woven through a strange metal crystal. The illustration represents the idea behind the TU Wien result: particles inside a solid that respond to a disturbance as one entangled collective, not as independent individuals. Illustration: Harald Ritsch / TU Wien.

In a laboratory at the Institut Laue-Langevin in Grenoble, France, a crystal smaller than a matchbox sat in a beam of neutrons, cooled to within a tiny fraction of a degree of absolute zero. The crystal is a centimeter-sized chunk of a metallic compound made of cerium, palladium, and silicon, a material physicists call a strange metal. When a single neutron struck it, the response did not come from one particle. Groups of at least nine particles answered together, locked in the coordinated quantum state physicists call multipartite entanglement.

The result, published on June 15, 2026, in the journal Nature Physics, is entanglement in something you could hold in one hand. The team behind it, led by Silke Bühler-Paschen at TU Wien (Vienna University of Technology), reports it as the largest entanglement depth measured so far in any quantum material.

Entanglement usually lives somewhere else

Quantum entanglement is the connection between particles whose states are linked: measure one, and you instantly know something about the other, no matter how far apart they are. It is normally demonstrated with the smallest, most isolated systems physicists can build: pairs of photons, trapped atoms, or single ions held in place by lasers and cooled to near stillness.

For almost a century, since Erwin Schrödinger posed his famous cat thought experiment, physicists have asked how far quantum behavior can climb the size ladder. Can a big, ordinary-looking object made of enormous numbers of particles show unmistakable signs of quantum mechanics? Most attempts have tried to place a whole object into a superposition of two states at once, a strategy that gets harder as objects get bigger.

The TU Wien group took a different route. "We do not try to bring the crystal as a whole into a superposition of two states," Bühler-Paschen said. "Instead, we ask whether its constituents are, collectively, in such a state of entanglement." She compares the material to an anthill: when an anthill is disturbed, the response does not come from one ant alone, but from the whole colony acting together.

One neutron asks a question, at least nine particles answer

The crystal, known to chemists as Ce3Pd20Si6, is a heavy-fermion compound, a class of materials in which electrons behave as if they were much heavier than normal. It was already famous in physics circles for hosting a strange-metal quantum critical point: a boundary where the material's behavior changes abruptly at absolute zero, tied to the breakdown of Kondo screening, the process by which conduction electrons normally bind to and mask the magnetic moments of atoms.

PhD student Federico Mazza carried out the measurements on the ThALES spectrometer at the ILL, the world's leading neutron research facility. He cooled the crystal to 60 millikelvin, about sixty thousandths of a degree above absolute zero, and held it in a magnetic field of 1.73 tesla, tuned precisely to the material's quantum critical point. Then he fired neutrons at it and measured how the scattered neutrons carried information about the crystal's response.

Federico Mazza, a PhD student at TU Wien, standing in front of the ThALES neutron spectrometer at the Institut Laue-Langevin in Grenoble, France, surrounded by its cylindrical tanks, cables, and cooling lines.
Federico Mazza, the TU Wien PhD student who ran the measurements, in front of the ThALES neutron spectrometer at the Institut Laue-Langevin in Grenoble, France. Photo: ILL Grenoble.

"In a normal material, one would expect a neutron to transfer its energy to an individual particle," Mazza said. "But by analyzing the data using the quantum Fisher information, we found a response that cannot be explained in terms of independent particles. Instead, it indicates that groups of at least nine quantum-entangled entities act collectively."

The tool: quantum Fisher information

The analysis relied on a quantity from quantum information theory called quantum Fisher information, or QFI. It measures how strongly a quantum system responds to a small change. For a collection of independent particles, the response is limited: each particle contributes on its own. But if the particles are entangled, the whole system can respond more strongly than the sum of its parts, and that extra sensitivity is the signature of entanglement.

Diagram comparing two situations: on the left, separate particles each produce a small individual response shown as small arrows. On the right, connected particles form a network that responds as one unit with a single large arrow, illustrating how entanglement amplifies a system's response.
Why quantum Fisher information works: independent particles each respond a little on their own (left), while an entangled group responds as one, more strongly than the sum of its parts (right). Illustration: Impossible Universe Editorial Team.

The theoretical framework came from Peter Zoller's group at the University of Innsbruck, which showed that QFI can reveal entanglement even in large many-body systems, the kind found inside real solids. Extracting it from neutron scattering data is what makes the method work on a bulk crystal rather than a handful of trapped particles.

The team's analysis gave an entanglement depth of at least nine entities, and the signal grew in a scale-free way as the temperature dropped. "To the best of our knowledge, the pronounced scale-free increase in the QFI with decreasing temperature points to the largest entanglement depth reported so far in any quantum material," the authors write in the paper.

Why strange metals are strange

Strange metals get their name from a simple anomaly. In an ordinary metal, electrical resistance falls with temperature in a smooth, predictable way. In a strange metal, resistance falls in a straight line: linear in temperature, down to the lowest measurable values. The behavior was first seen in high-temperature superconductors, the cuprates, and has since turned up in several other exotic material classes. Nobody has a complete explanation for it.

The new result adds a clue. In 2025, a collaboration between TU Wien and Rice University found that electric current flows through strange metals in a surprisingly quiet way, with unusually low electrical noise. The entanglement measured now offers a possible explanation: the charge carriers have not disappeared or gone quiet on their own; they coordinate, and their collective response suppresses the fluctuations.

"What we see here is not a detail of one particular material, but a general physical principle," said Fakher Assaad of the University of Würzburg, the lead theorist on the study. "Strong entanglement appears to be directly linked to the unusual behavior of strange metals."

What this is not

The result is entanglement, but it is worth being precise about what that means. The crystal is not in a superposition of two macroscopic states, the way Schrödinger's hypothetical cat was. Its constituent particles are entangled with each other, and the evidence comes from an indirect analysis of scattering data, not from a direct Bell-type test on individual particles.

The measurement also requires extreme conditions: 60 millikelvin and a carefully tuned magnetic field. This is not a room-temperature quantum device, and the crystal does not display strange-metal behavior under everyday conditions.

The researchers' stated next goal is to make the exchange of ideas between quantum information science and solid-state physics work in both directions. "Our aim is to explore whether strange metals may one day find applications in quantum technologies, for example in high-precision measurements for quantum metrology," Bühler-Paschen said. Entanglement is the resource that makes such measurements more sensitive, and a strange metal is a place where that resource occurs naturally, in bulk, inside a crystal you could hold.

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Hero image: Illustration of quantum states in a strange metal, Harald Ritsch / TU Wien. Inline images: Federico Mazza at the ThALES spectrometer, ILL Grenoble; quantum Fisher information concept diagram, Impossible Universe Editorial Team (generated).