TU Wien has detected strong quantum entanglement for the first time in a centimeter-sized crystal of a strange metal. Many quantum effects are easiest to detect in very small systems, such as individual atoms, molecules or photons, that are carefully isolated from their surroundings. But physicists have long wondered whether much larger objects, made of enormous numbers of particles, can also reveal unmistakable signs of quantum behavior. Experimentalists at TU Wien have now shown that they can. The group studied a centimeter-sized crystal of a so-called strange metal and found evidence of a high level of quantum entanglement. The measurement was made possible by a precise tool from quantum information theory called quantum Fisher information. The result creates a new link between solid state physics and quantum physics. It shows that quantum entanglement can be directly measured in a large strange metal material. Cats or ants? The question of whether the strange predictions of quantum theory can apply to large, everyday scale objects goes back almost to the beginning of quantum mechanics. Erwin Schrödinger famously asked whether a cat could be dead and alive at the same time. Since then, many experiments have tried to deliberately produce quantum effects in increasingly large systems. “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.” The experiment is therefore closer to the behavior of an anthill than to Schrödinger’s cat. When an anthill is disturbed, the response does not come from one ant alone, but from the colony acting collectively. Quantum Fisher information: entanglement enhances sensitivity The theoretical foundation for