Neutron scattering reveals at least nine-partite entanglement in a strange metal

A team of physicists reports that neutron-scattering measurements on a centimeter-scale crystal of the strange metal Ce3Pd20Si6 revealed a signal strong enough to witness at least nine-partite entanglement among microscopic degrees of freedom inside the material.

The result, published in the peer-reviewed journal Nature Physics in a paper titled “Quantum Fisher information in a strange metal,” links measurable many-body entanglement to the unusual physics of strange metals. It does not mean researchers placed an entire crystal into a single Schrödinger-cat-like superposition. Instead, the experiment used a standard quantum-information quantity, quantum Fisher information, or QFI, as an entanglement witness — a way to prove a minimum level of collective quantum correlation from measured spin fluctuations.

The work brought together researchers from TU Wien, Universität Würzburg, the Institut Laue-Langevin in Grenoble and Rice University. Silke Paschen led the experimental effort, while Fakher F. Assaad led the theory work. Coauthor Federico Mazza was quoted in press materials describing the result in more intuitive terms.

The material, Ce3Pd20Si6, is a heavy-fermion compound that behaves as a strange metal under the relevant conditions. Strange metals are a longstanding puzzle in condensed-matter physics because they do not act like ordinary metals, whose electrons can usually be described as quasiparticles moving more or less independently. Understanding why strange metals break that picture could help clarify broader questions in quantum materials, including phenomena tied to unconventional superconductivity.

To probe the crystal, the team used inelastic neutron scattering — a technique that tracks how neutrons exchange energy and momentum with a material — on the ThALES cold-neutron triple-axis spectrometer at ILL. They measured the material’s dynamical spin response down to about 60 millikelvin and around a field-induced quantum critical point at about 1.73 tesla, where the system is tuned to the brink of a zero-temperature phase change.

From those neutron data, the researchers extracted the QFI. The paper reports a value of fQ = 8.2 ± 0.9 at 60 millikelvin. Using the QFI lower-bound criterion, the authors say that value witnesses a state with at least 9-partite entanglement. They combined the measurements with auxiliary-field quantum Monte Carlo simulations and interpret the result as evidence that strong multipartite entanglement is tied to strange-metal quantum criticality.

“In a normal material, one would expect a neutron to transfer its energy to an individual particle,” Mazza said in TU Wien and ILL press materials. “Instead, it indicates that groups of at least nine quantum-entangled entities act collectively.”

That distinction matters. The result is a conservative lower bound on entanglement depth, not a direct count of entangled atoms and not proof that the whole macroscopic crystal behaves as one entangled object. QFI, in this setting, is valuable because it can turn a measurable property of the material’s spin dynamics into a rigorous minimum statement about multipartite entanglement.

The authors argue that makes entanglement more than a buzzword in the strange-metal debate. “This provides evidence for a quantum state with high multipartite entanglement and offers a positive descriptor of strange metallicity that points towards its microscopic basis,” the paper says.

The paper also says that, “to the best of our knowledge,” the rise in QFI as temperature falls points to the largest entanglement depth yet reported in a quantum material. That is the authors’ qualified claim, rather than an independently established record.

Even with that caveat, the study gives researchers a new experimentally grounded way to connect quantum entanglement with one of condensed-matter physics’ most stubborn problems — and to do so in a crystal large enough for conventional neutron-scattering experiments, not just in finely controlled model systems.