Quantum entanglement is a phenomenon that occurs when two or more particles generate and interact without a physical connection. Albert Einstein first described this phenomenon in 1935.
Almost 90 years later, scientists at the U.S. Department of Energy’s Oak Ridge National Laboratory demonstrated the possibility of spooky quantum entanglement witness capable of proving the presence of entanglement between magnetic particles, or spins, in a quantum material.
Entanglement witness is a technique that acts as a data analysis tool to determine which spins cross the threshold between the classical and quantum realms.
John Stewart Bell first introduced the technique in the 1960s. It confirms that the quantum theory questioned by other scientists had been correct. Although the technique relied on detecting one pair of particles at a time, hence is not useful for studying solid materials composed of trillions and trillions of particles.
Scientists in this study targeted and detected large collections of entangled spins using new entanglement witnesses. They extended this concept to characterize solid materials and study exotic behavior in superconductors and quantum magnets.
Using a combination of neutron scattering experiments and computational simulations, scientists tested three entanglement witnesses.
To ensure that the witnesses could be trusted, the team applied all three of them to a material they knew to be entangled. Two of the witnesses adequately indicated the presence of entanglement in this one-dimensional spin chain. These two witnesses were based on Bell’s approach.
On the other hand, the third witness fared exceptionally well at the same task. This witness was based on quantum information theory.
Allen Scheie, a postdoctoral research associate at ORNL, said, “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.”
The team confirmed the theoretical prediction that entanglement increases as temperature decreases and successfully differentiated between classical and quantum activity as part of the most comprehensive QFI demonstration since the technique was proposed in 2016.
ORNL neutron scattering scientist Alan Tennant, who leads a project focused on quantum magnets for the Quantum Science Center, or QSC, said, “The most interesting materials are full of quantum entanglement, but those are precisely the ones that are the most difficult to calculate.”
Previously, it was quite challenging to identify quantum materials. The method involves exploiting entanglement to develop novel devices and sensors while advancing quantum information science.
Streamlining this process with quantum Fisher information, or QFI, allows QSC scientists to focus on harnessing the power of substances like quantum spin liquids and superconductors for data storage and computing applications.
Allen Scheie, a postdoctoral research associate at ORNL, said, “The power of QFI comes from its connection to quantum metrology, in which scientists entangle multiple quasiparticles to shrink uncertainty and obtain exact measurements. The QFI witness reverses this approach by using the precision of an existing measurement to determine the minimum number of particles each spin is entangled with. This is a powerful way to reveal quantum interactions, which means that QFI is applicable to any quantum magnetic material.”
Having established that QFI could correctly categorize materials, the team tested a second one-dimensional spin chain, a more complex material featuring anisotropy. Thanks to this property, spins lie in a plane rather than rotating at random. The researchers applied a magnetic field to the spin chain and observed an entanglement transition, in which the amount of entanglement fell to zero before reappearing.
Using neutron scattering, scientists studied both spin chains. They then analyzed legacy data from experiments conducted decades ago at the ISIS Neutron Source in England and the Institut Laue-Langevin in France and new data from the Wide Angular-Range Chopper Spectrometer located at the Spallation Neutron Source, a DOE Office of Science user facility operated by ORNL. They also ran complementary simulations to validate the results against idealized theoretical data.
Tennant said, “By studying the distribution of neutrons that scatter off of 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.”
Scientists noted, “This combination of advanced computational and experimental resources provided answers about the nature of quantum entanglement originally asked by the founders of quantum mechanics.”
- A. Scheie, Pontus Laurell et al. Witnessing entanglement in quantum magnets using neutron scattering. DOI: 10.1103/PhysRevB.103.224434