Many quantum effects appear only in systems with just a few isolated particles. For decades scientists have asked whether much larger objects can also display clear signs of quantum behaviour. A study at TU Wien now answers this question for a crystal several centimetres across.
The crystal is a strange metal - a solid whose electrical properties do not fit the usual explanations used for ordinary metals - containing cerium, palladium and silicon. Scientists at Vienna University of Technology checked whether the particles inside it share quantum entanglement.
The scientists applied quantum Fisher information. This concept from quantum information theory measures how strongly a system reacts to a small disturbance. When particles are entangled the whole system can respond more than the sum of its separate parts, giving a clear signature of their connection.
Experiments with neutrons
Neutrons were directed at the crystal at a research reactor in Grenoble. In a normal solid one neutron would transfer energy to a single particle. Analysis of the scattering data showed a much stronger response. Using quantum Fisher information the scientists calculated that this could only occur if groups of at least nine particles were linked through multipartite entanglement and acted together. Multipartite entanglement describes a shared quantum state involving several particles at once.
The picture is closer to an anthill than to older thought experiments with cats. When an anthill is disturbed the colony reacts as a single unit rather than each insect on its own. The measurement therefore captures collective quantum behaviour inside a solid object that can be held in the hand.
The finding links the physics of complex solids with methods from quantum information science. Strange metals often appear near other unusual states such as high-temperature superconductivity. The coordinated response of entangled particles may explain why electric current in these materials flows with unusually low noise. The same approach can now be tried on other materials. Scientists also plan to examine whether strange metals could serve as platforms for quantum metrology, in which quantum effects enable extremely precise detection of small signals.
This research is published in Nature Physics. Physicists from Rice University participated in the project.