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.

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.
Image courtesy of Nathan Armistead, Oak Ridge National Laboratory

The Science

Quantum materials behave in surprising ways because of quantum physics. For example, they can be superconductors, which can allow electricity to flow with no resistance. These materials could lead to completely new technologies. In an advance for quantum materials, scientists tested 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 regardless of the distance between them. This research evaluated three entanglement witnesses. 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. In addition, the experiments confirmed that entanglement increases as temperature decreases.

The Impact

This work is the most thorough 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, scientists can more quickly identify entangled quantum materials such as quantum spin liquids, quantum magnets, and superconductors. These materials are ideal for applications such as data storage and computing. Incorporating QFI calculations into future neutron scattering experiments could help research teams 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 major challenge in quantum information science. The 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 nondestructive nature, the neutrons provided valuable insights into the properties of two different spin chains. To validate their results, the researchers also ran computational simulations and analyzed data from older experiments conducted at the ISIS Neutron Source and the Institut Laue-Langevin.

Contact

Alan Tennant
University of Tennessee Knoxville
[email protected]

Funding

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.

Publications

Scheie, A. et al., Witnessing entanglement in quantum magnets using neutron scattering. Physical Review B 103, 224434 (2021). [DOI: 10.1103/PhysRevB.103.224434]

Laurell, P. et al., Quantifying and Controlling Entanglement in the Quantum Magnet Cs2CoCl4. Physical Review Letters 127, 037201 (2021). [DOI: 10.1103/PhysRevLett.127.037201]

Related Links

ORNL News: Key witness helps scientists detect ‘spooky’ quantum entanglement in solid materials

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