TU Wien Physicists Detect Record-Deep Quantum Entanglement in a Centimeter-Sized Strange-Metal Crystal

Overview

Physicists led by a team at TU Wien have found evidence of what may be the deepest quantum entanglement yet recorded in any material: a signal indicating that groups of at least nine particles act as a single entangled unit inside a strange-metal crystal large enough to hold in one hand. The measurements, taken on the ThALES cold-neutron spectrometer at the Institut Laue-Langevin (ILL) in Grenoble, point to “the largest entanglement depth reported so far in any quantum material,” according to ILL.

What We Know

The crystal is a heavy-fermion compound made of cerium, palladium and silicon, with the formula Ce3Pd20Si6, according to Physics World and the study posted on arXiv. It belongs to a class of materials called strange metals, whose electrical resistivity rises in direct proportion to temperature at low temperatures rather than following the square-in-temperature dependence expected of ordinary metals, per ILL. Physicists consider the strange-metal state a “parent” state of high-temperature superconductivity, according to Physics World, which is part of why pinning down its underlying mechanism matters beyond this one material.

PhD student Federico Mazza of TU Wien carried out the measurements at the ILL, exposing the crystal to neutrons on the ThALES cold-neutron triple-axis spectrometer, which the study describes as offering energy resolution of about 0.07 meV, according to phys.org and the arXiv paper. The team cooled the crystal to 60 millikelvin under a magnetic field of 1.73 tesla, near a magnetic-field-tuned Kondo destruction quantum critical point described in the study as a state where “strange metallicity is associated with fluctuations beyond a Landau order parameter,” according to ILL and the arXiv paper.

To probe for entanglement in the neutron data, the researchers applied quantum Fisher information (QFI), a tool borrowed from quantum metrology that “quantifies how sensitively a quantum system responds to a change,” according to Physics World. Mazza explained why that matters for interpreting the result: “In a normal material, one would expect a neutron to transfer its energy to an individual particle,” he said, according to ILL. Instead, the team found that groups of at least nine quantum-entangled entities act collectively inside the crystal, according to ILL and Physics World. The underlying paper puts a precise figure on that finding: at the lowest temperature tested, the QFI witness reached a value of 8.2, which the authors call “the very most conservative estimate” of entanglement depth, according to the arXiv paper.

Study lead Silke Bühler-Paschen, a solid-state physicist at TU Wien’s Institute of Solid State Physics, said the finding confirmed a long-standing hunch: “We had suspected that some of the intriguing properties of this state might be related to entanglement but were not able to pin it down until now,” she said, according to Physics World. The theoretical framework for using QFI to detect entanglement in large many-body systems was developed by Innsbruck quantum physicist Peter Zoller, while University of Würzburg physicist Fakher Assaad served as lead theorist on the study, according to phys.org and ScienceDaily. The crystal itself is large enough to fit in the palm of a hand, according to ScienceDaily, a striking scale for a phenomenon usually confined to isolated particles or engineered qubits. The work, titled “Quantum Fisher information in a strange metal,” was carried out by Mazza, Sounak Biswas, Xinlin Yan, Andrey Prokofiev, Paul Steffens, Qimiao Si, Assaad and Bühler-Paschen, and was recently published in Nature Physics, according to The Quantum Insider and the arXiv listing.

What We Don’t Know

The measurements were made in one specific compound, Ce3Pd20Si6, tuned close to its own quantum critical point. Whether the same deep entanglement signature shows up in other strange-metal families — including the cuprate and iron-based compounds most closely tied to high-temperature superconductivity — is untested territory; the study’s authors frame their result only as work that “opens a new direction for studies across strange metal platforms,” according to the arXiv paper. It also remains unclear how directly this entanglement depth explains specific strange-metal behaviors, such as the material’s unusual low-noise electrical current, beyond the correlational link the team has now measured.