Quantum phenomena are usually the domain of the impossibly small - individual atoms, molecules, or photons that need to be coddled in isolation. But what if those same weird effects could exist in something you could actually hold in your hand? Researchers at TU Wien have now provided compelling evidence that they can, using a centimeter-sized crystal made from a material known as a strange metal. They detected a high degree of quantum entanglement, one of quantum physics’ greatest hits, using a technique from quantum information science called quantum Fisher information. The results create a new connection between quantum information and solid-state physics, showing that entanglement can be measured directly in a macroscopic strange metal.
The question of whether quantum mechanics applies only to tiny particles or also to larger objects has been debated since the early days of the field. Erwin Schrödinger famously illustrated the mystery with his thought experiment involving a cat that is simultaneously alive and dead until observed - a scenario that, mercifully, remains theoretical. Since then, scientists have repeatedly pushed the limits of how large a system can display quantum behavior. The TU Wien team approached the question from a different angle. “Our approach is different,” says Prof. Silke Bühler-Paschen from 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.” Rather than Schrödinger’s cat, Bühler-Paschen says the experiment is more like an anthill: when disturbed, the response comes from the colony acting together, not from any individual ant. The researchers wanted to determine whether the particles inside the crystal behave in a similarly coordinated way.
The theoretical framework behind the experiment was developed by Innsbruck quantum physicist Peter Zoller and his colleagues. Their work showed that quantum Fisher information can identify quantum entanglement even in complex systems made up of enormous numbers of interacting particles. “The 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.” In simple terms, a strongly entangled system reacts more dramatically to disturbances than a collection of independent particles, allowing researchers to estimate how much entanglement is present.
To test the idea, the researchers created a crystal composed of cerium, palladium, and silicon. This material belongs to the class of strange metals, which have long fascinated physicists because they display unusual quantum properties that remain only partly understood. At the Institut Laue-Langevin (ILL) in Grenoble, PhD student Federico Mazza fired neutrons at the crystal and measured its response. “In a normal material, one would expect a neutron to transfer its energy to an individual particle,” says Mazza. “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 measurements provide direct evidence of strong multipartite quantum entanglement inside a solid crystal that is large enough to fit comfortably in the palm of your hand.
The researchers originally set out to better understand why strange metals behave so differently from conventional materials. Similar behavior is also found in other systems, including high-temperature superconductors. Interest in strange metals has grown rapidly in recent years as scientists continue uncovering unexpected properties. In 2025, researchers from TU Wien and Rice University reported that electrical current moves through these materials with unusually low electrical noise. The newly observed quantum entanglement may help explain why: rather than acting independently, the particles appear to coordinate their behavior in a way that suppresses current fluctuations. “What we see here is not a detail of one particular material, but a general physical principle,” says Fakher Assaad from the University of Würzburg, lead theorist of the work. “Strong entanglement appears to be directly linked to the unusual behavior of strange metals.” The researchers believe the work demonstrates the value of bringing together ideas from quantum information science and condensed matter physics. “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 insight.” The team is now looking ahead to the reverse exchange of ideas. They hope to determine whether strange metals could eventually become useful for quantum technologies, including highly sensitive quantum metrology systems capable of detecting extremely small signals with exceptional precision.