Quantum phenomena are usually associated with extremely small objects — individual atoms, molecules, or photons carefully isolated from their surroundings. But can those same strange quantum effects also exist in objects large enough to see and hold?
Researchers at TU Wien have now provided compelling evidence that they can. By studying a centimeter-sized crystal made from a type of material known as a strange metal, the team detected a high degree of quantum entanglement — one of the most remarkable features of quantum physics.
From Schrödinger's Cat to an Anthill
Whether quantum mechanics applies only to tiny particles or also to larger objects has been debated since the early days of the field. Physicist Erwin Schrödinger famously illustrated the mystery with his thought experiment involving a cat that is simultaneously alive and dead until observed.
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 thinking of Schrödinger's cat, Bühler-Paschen says the experiment is more like an anthill. When an anthill is disturbed, the response comes from the colony acting together rather than from any individual ant.
Quantum Fisher Information Reveals Hidden Entanglement
The theoretical framework 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 of enormous numbers of interacting particles.
"The quantum Fisher information quantifies how sensitively a quantum system responds to a change," explains Bühler-Paschen. "If particles are entangled, the entire system can respond more strongly than the sum of its individual parts."
To test the idea, the researchers created a crystal composed of cerium, palladium, and silicon. At the Institut Laue-Langevin 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 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 large enough to fit comfortably in the palm of your hand.
Solving the Mystery of Strange Metals
The researchers originally set out to better understand why strange metals behave so differently from conventional materials. 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. "Strong entanglement appears to be directly linked to the unusual behavior of strange metals."
The team is now looking ahead to determine whether strange metals could become useful for quantum technologies, including highly sensitive quantum metrology systems capable of detecting extremely small signals with exceptional precision.
