Quantum entanglement occurs when two particles appear to communicate without a physical connection, a phenomenon Albert Einstein famously called “spooky action at a distance" almost ninety years ago. Knowing whether a material is exhibiting these quantum properties, or is behaving in a solely classical manner, is crucial to developing novel quantum devices.
Entanglement witnesses are techniques that act as data analysis tools to determine which spins cross the threshold between the classical and quantum realms. When first introduced, they confirmed quantum theory, a concept which had previously been under dispute.
Using a combination of neutron scattering experiments and computational simulations, a team including researchers from Oak Ridge National Laboratory, Helmholtz-Zentrum Berlin, the Technical University of Berlin, Institut Laue-Langevin, the University of Oxford and Adam Mickiewicz University have tested three quantum entanglement witnesses.
The conventional quantum witness technique relied on detecting one pair of particles at a time, a concept that is not feasible to apply to real-world materials. This study, published in Physical Review B, extended this concept by modelling large collections of entangled spins.
To test their witnesses, the researchers used experimental data on the compound KCuF3. The data used were a combination of recent experimental results and data collected in 2000 as part of one of the first commissioning experiments on the MAPS spectrometer.
“Prior to the MAPS measurement, Alan Tennant, Steve Nagler and I had been interested in KCuF3 for its potential to host exotic spinon excitations and a longitudinal mode which we successfully measured" explains Bella Lake from Helmholtz-Zentrum Berlin, who was part of the experimental team in 2000.
As well as supporting this current study, the data collected on MAPS has already appeared in several high-profile publications. First, they were used to test theories of universal scaling specifically as predicted for the Luttinger Liquid Quantum Critical point. Later, they were used to successfully test the Bethe Ansatz, which was solved in 2006 for the spin-1/2, Heisenberg antiferromagnetic chain.
“As a good experimental realisation of the spin-1/2 Heisenberg aniferromagnetic chain, KCuF3 is a great compound for testing theories," adds Bella, who is also an author on the current study testing entanglement witnesses.
Two of the three witnesses tested were based on the conventional method and adequately indicated the presence of entanglement in the one-dimensional spin chain present in KCuF3. The third witness was based on quantum information theory and did an even better job; “The quantum Fisher information, or QFI, witness showed a close overlap between theory and experiment, which makes it a robust and reliable way to quantify entanglement," said Allen Scheie, a postdoctoral research associate at ORNL.
Often thermal motions can cause fluctuations in a material that can be confused with quantum behaviour, and these cannot be eliminated above absolute zero. However, the team's QFI technique was able to successfully differentiate between the two, and confirm the theoretical prediction that entanglement increases as temperature decreases.
Quantum materials could be the key component of future devices and sensors, but it is challenging to quickly identify what materials exhibit quantum behaviour. The results of this study serve as a proof of principle that meaningful information about quantum entanglement can be extracted from experimentally measured spin-spin correlations.
“By studying the distribution of neutrons that scatter off a sample, which transfers energy, we were able to use neutrons as a gauge to measure quantum entanglement without relying on theories and without the need for massive quantum computers that don't exist yet," says ORNL neutron scattering scientist Alan Tennant.
The full paper can be found online at DOI: 10.1103/PhysRevB.103.224434