Quantum Fisher Information: Spilling the Secrets of Quantum Entanglement

A material’s spins, depicted as red spheres, are probed by scattered neutrons. Applying an entanglement witness causes the neutrons to form a kind of quantum gauge capable of distinguishing between classical and quantum spin fluctuations. Credit: Image courtesy of Nathan Armistead, Oak Ridge National Laboratory

Because of quantum physics, quantum materials behave in surprising ways. For instance, they can be superconductors, which can allow electricity to flow with no resistance. These exceptional materials could lead to completely new technologies.

In an advancement for quantum materials, scientists assessed the ability of techniques called entanglement witnesses to accurately identify pairs of entangled magnetic particles. Entanglement is when one of these particles, or “spins,” mirrors another’s properties and behavior no matter how great the distance between them is. Three entanglement witnesses were evaluated in this research. Of the three, quantum Fisher information (QFI) performed the best, routinely locating entanglement in complex materials. QFI also differentiated between true quantum activity and non-quantum activity that can appear quantum due to random thermal motion. Additionally, the experiments confirmed that entanglement increases as temperature decreases.

The Impact

This work is the most in-depth examination of QFI’s capabilities to date. It is also the first to apply the technique to massive solid materials by examining many pairs of entangled spins simultaneously. With QFI, researchers can more quickly identify entangled quantum materials such as quantum spin liquids, quantum magnets, and superconductors. These materials are excellent for applications such as data storage and computing. Incorporating QFI calculations into future neutron scattering experiments could help scientists characterize even more complex quantum materials.

Summary

Proving the presence of entanglement in one-dimensional spin chains—linear lines of connected spins within quantum materials—has historically been a significant challenge in quantum information science. The research team observed QFI tackling this challenge by applying the witness to neutron scattering experiments at the Spallation Neutron Source, a Department of Energy user facility. Because of their neutral charge and non-destructive nature, the neutrons provided valuable insights into the properties of two different spin chains. To validate their results, the scientists also ran computational simulations and analyzed data from older experiments conducted at the ISIS Neutron Source and the Institut Laue-Langevin.

References:

“Witnessing entanglement in quantum magnets using neutron scattering” by A. Scheie, Pontus Laurell, A. M. Samarakoon, B. Lake, S. E. Nagler, G. E. Granroth, S. Okamoto, G. Alvarez and D. A. Tennant, 28 June 2021, Physical Review B.

DOI: 10.1103/PhysRevB.103.224434

“Quantifying and Controlling Entanglement in the Quantum Magnet Cs₂CoCl₄” by Pontus Laurell, Allen Scheie, Chiron J. Mukherjee, Michael M. Koza, Mechtild Enderle, Zbigniew Tylczynski, Satoshi Okamoto, Radu Coldea, D. Alan Tennant and Gonzalo Alvarez, 13 July 2021, Physical Review Letters.

DOI: 10.1103/PhysRevLett.127.037201

This work was funded by the Department of Energy Office of Science, DOE’s Scientific Discovery through Advanced Computing program, Oak Ridge National Laboratory’s Laboratory Directed Research and Development program, the Quantum Science Center, the Center for Nanophase Materials Sciences, and the European Research Council under the European Union Horizon 2020 Research and Innovation Programme.