A Strange Metal Just Made Quantum Entanglement Visible at the Macro Scale

For decades, quantum entanglement—what Einstein famously called "spooky action at a distance"—was a delicate phenomenon confined to the microscopic world of photons and atoms. A landmark experiment published on June 16, 2026, in Nature Physics has changed that. Physicists at TU Wien have detected a high degree of genuine multipartite quantum entanglement inside a centimeter-sized crystal of a strange metal, an object large enough to hold in your hand and see with the naked eye. This discovery not only pushes the boundaries of macroscopic quantum phenomena but also provides a powerful new explanation for the mysterious behavior of strange metals , .

The Material: A Quantum Secret Inside a Strange Metal

The crystal at the center of this breakthrough is Ce₃Pd₂₀Si₆, a heavy-fermion compound made of cerium, palladium, and silicon. This material is already a celebrity in condensed matter physics for exhibiting "Kondo destruction quantum criticality." At a quantum phase transition—a change in the material's fundamental state at absolute zero—the usual behavior of electrons breaks down. In normal metals, electrons can be treated like independent quasiparticles. But in strange metals like Ce₃Pd₂₀Si₆, that picture collapses, leading to a resistivity that increases linearly with temperature in a way that has defied explanation for decades , .

From Quantum Information to Solid-State Physics: The Power of QFI

The key to this discovery wasn't a new microscope but a concept borrowed from quantum information theory: quantum Fisher information (QFI). Originally developed by quantum physicist Peter Zoller and his group at the University of Innsbruck, QFI quantifies how sensitively a quantum system responds to a tiny perturbation. If a system's sensitivity exceeds a well-defined classical limit—meaning it reacts more strongly than the sum of its independent parts—that enhanced sensitivity can only come from quantum entanglement , . Think of it like a choir: if you poke one singer and the entire choir's harmony shifts in a way impossible to explain by individual reactions, you know they are connected in a deep, collective state.

This framework allowed the team to extract a direct entanglement measure from a messy, bulk solid for the first time, a radical departure from traditional experiments that rely on isolating and preparing pristine quantum states.

Neutrons Reveal Entangled Collectives

The researchers, led by Prof. Silke Bühler-Paschen at TU Wien, didn't try to put the whole crystal into a quantum superposition—a practical impossibility for such a large object. Instead, they used inelastic neutron scattering at the Institut Laue–Langevin (ILL) in Grenoble, France. By bombarding the crystal with neutrons and measuring how the spins of its particles moved and fluctuated together as a function of energy, momentum, and temperature, they obtained the material's dynamical spin correlation function, S(q, ω, T) .

When they applied the QFI formalism to the scattering data, the results were stunning. The crystal's collective response was far too strong to be explained by independent particles. The QFI density reached f_Q = 8.2 ± 0.9 at the lowest measured temperature, a value that mathematically requires a group of at least nine quantum-entangled entities acting together. The entanglement peaked at the experiment's coldest temperature of 60 mK (millikelvin) and near a magnetic field of roughly 1.73 Tesla, precisely at the quantum critical point where Kondo destruction occurs. When the crystal was cooled from 10 K down to 60 mK, the QFI density surged nearly 40-fold with no sign of leveling off, suggesting that even stronger entanglement could exist at even lower temperatures , .

Why Strange Metals Have Been So Puzzling

These results offer a compelling new story for one of condensed matter physics' biggest puzzles. Strange metals don't conduct electricity like ordinary metals. Their electrons seem to lose their individual identity, forming a collective quantum soup. This experiment directly links the breakdown of quasiparticles at a Kondo destruction quantum critical point to a surge in multipartite entanglement , . The signature linear-in-temperature resistivity may not be the fingerprint of disorder or simple scattering, but rather the signature of a highly entangled, collective quantum state.

From Fundamental Physics to Quantum Sensors

Beyond explaining bizarre metals, this work carves a practical path forward for quantum technology. Quantum Fisher information is not just an entanglement detector; it is also the central quantity in quantum metrology, the science of ultra-precise measurement. A material that naturally harbors strong, stable internal entanglement is a ready-made platform for a highly sensitive sensor. If such entanglement can be maintained at higher, more practical temperatures, these materials could be used for everything from magnetic field sensing to detecting gravitational waves. The study opens the door to using QFI-based techniques to systematically screen other quantum materials, potentially including high-temperature superconductors, for macroscopic entanglement , .

A Global Effort Bridging Two Worlds

The study was a major international collaboration. The experimental work was led by Silke Bühler-Paschen's group at TU Wien, with PhD student Federico Mazza conducting the neutron scattering at ILL Grenoble. The theoretical foundation was built by Peter Zoller's team at the University of Innsbruck, with additional contributions from Qimiao Si at Rice University, whose group has long studied the Kondo destruction mechanism in Ce₃Pd₂₀Si₆.

By uniting the abstract tools of quantum information with the messy reality of a bulk solid, the team has built a bridge between two previously distant fields. They have given physicists a new way to witness entanglement not in a controlled vacuum chamber, but inside a small, glittering crystal sitting in plain sight.