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Proof of quantum effects in a strange metal
Image Credit: © TU Wien / Harald Ritsch
Scientific Frontline: Extended "At a Glance" Summary: Macroscopic Quantum Entanglement (Schrödinger's Anthill)
The Core Concept: For the first time, physicists have detected a high degree of multipartite quantum entanglement within a macroscopic, centimeter-sized crystal of a "strange metal." This demonstrates that massive objects made of countless particles can collectively exhibit fundamental quantum effects.
Key Distinction/Mechanism: Rather than attempting to force an entire object into a superposition state (akin to the theoretical Schrödinger's cat), researchers measured the material's sensitivity to neutron bombardment. Using a metric called quantum Fisher information, they found that the material responds to disturbances collectively—much like a disturbed anthill—with groups of at least nine particles acting as single, quantum-entangled entities rather than independent atoms.
Major Frameworks/Components:
- Quantum Fisher Information: A theoretical tool from quantum information science used to quantify the sensitivity of a many-body system to external changes, directly indicating its degree of entanglement.
- Strange Metals: A complex class of materials (in this experiment, a crystal of cerium, palladium, and silicon) known for highly unusual quantum properties, such as suppressing electrical current fluctuations.
- Neutron Scattering: An experimental technique where neutrons are fired at the crystal to observe the transfer of energy and measure the resulting collective particle response.
Branch of Science: Solid-State Physics, Quantum Physics, and Quantum Metrology.
Future Application: The enhanced, highly sensitive nature of these entangled strange metals could be leveraged in advanced quantum technologies, particularly for developing high-precision sensors and measurement tools in quantum metrology.
Why It Matters: This discovery establishes a direct bridge between solid-state physics and quantum information theory, proving that large-scale materials can maintain strong, multipartite quantum entanglement. It also provides a critical new explanation for the poorly understood behaviors of strange metals and high-temperature superconductors.
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| Federico Mazza at ILL Photo Credit: © ILL |
At TU Wien, a high degree of quantum entanglement has been detected for the first time in a centimeter-sized crystal of a strange metal.
Many quantum effects can be observed only when a small number of particles is studied—individual atoms, molecules, or photons, for example, carefully shielded from the rest of the world. But what about macroscopic objects consisting of an unimaginably large number of particles? Can they, too, display effects that provide a direct glimpse into the quantum world?
Experimentalists at TU Wien have now shown that this is possible: a centimeter-sized crystal of a so-called strange metal was investigated, and a high degree of quantum entanglement was evidenced. This was made possible by a clearly defined method from quantum information theory: quantum Fisher information.
It establishes a new bridge between solid-state physics and quantum physics: quantum entanglement can be directly quantified in a macroscopic strange-metal material.
Cats or Ants?
The question of whether the strange statements of quantum theory can also be applied to large, macroscopic objects is almost as old as quantum theory itself. Erwin Schrödinger famously asked whether a cat could be both dead and alive at the same time. Since then, many experiments have attempted to deliberately generate quantum effects in ever-larger systems.
"Our approach is different," says Prof. Silke Bühler-Paschen of the Institute of Solid State Physics at TU Wien. "We do not try to bring the crystal as a whole into a superposition of two states. Instead, we ask whether its constituents are—collectively—in such a state of entanglement." The experiment is therefore less reminiscent of Schrödinger's cat than of an anthill: when it is disturbed, it is not a single ant that reacts, but the entire colony as a collective.
Quantum Fisher Information: Entanglement Enhances Sensitivity
The theoretical basis for this approach was developed by Innsbruck quantum physicist Peter Zoller and his team. They showed that the concept of quantum Fisher information can be used to detect quantum entanglement even in large many-body systems.
"Quantum Fisher information quantifies how sensitively a quantum system responds to a change," explains Bühler-Paschen. "For a collection of independent particles, the response is limited because each particle contributes on its own. However, if the particles are entangled, the entire system can respond more strongly than the sum of its individual parts. This enhanced sensitivity is precisely what makes entanglement such a valuable resource for quantum metrology, where one aims to detect extremely small signals with the highest possible precision. By measuring how strongly a system responds to a perturbation, one can therefore infer the degree of entanglement present in the material."
The TU Wien team produced a crystal made of cerium, palladium, and silicon—a strange metal already known to display highly intriguing quantum properties, many of which are still not fully understood. At the ILL in Grenoble, Ph.D. student Federico Mazza bombarded the crystal with neutrons and measured how the material responded.
One Neutron Asks a Question—At Least Nine Particles Answer
"In a normal material, one would expect a neutron to transfer its energy to an individual particle," Mazza says. "But by analyzing the data using 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." This provides direct evidence of high multipartite quantum entanglement in a solid—a macroscopic object large enough to be comfortably held in one's hand.
The Background: Research on Strange Metals
The scientific motivation for the study was to better understand the strange-metal behavior of the crystal—behavior that is also observed in other classes of materials, such as high-temperature superconductors. In recent years, research on this topic has intensified, and increasingly unusual properties have come to light. In a collaboration between TU Wien and Rice University in the United States, it was found in 2025 that electric current flows through such materials in an astonishingly "quiet," low-noise manner. The discovery of entanglement now provides a new possible explanation for this phenomenon: the particles have not disappeared but instead coordinate to suppress current fluctuations.
"What we see here is not a detail of one particular material, but a general physical principle," says Fakher Assaad of the University of Würzburg, lead theorist of the work. "Strong entanglement appears to be directly linked to the unusual behavior of strange metals."
"The results are a great success for us," says Silke Bühler-Paschen. "They confirm that our unusual approach of using methods from quantum information science for solid-state physics studies of novel materials can reveal fundamentally new insights." The next goal is already set: "We want the transfer of knowledge between the two fields to also work in the other direction. 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."
Published in journal: Nature Physics
Title: Quantum Fisher information in a strange metal
Authors: Federico Mazza, Sounak Biswas, Xinlin Yan, Andrey Prokofiev, Paul Steffens, Qimiao Si, Fakher F. Assaad, and Silke Paschen
Source/Credit: Technische Universität Wien
Edited by: Scientific Frontline
Reference Number: phy061626_01