Quantum entanglement occurs when two particles appear to communicate without
a physical connection, a phenomenon Albert Einstein famously called "spooky
action at a distance." Nearly 90 years later, a team led by the U.S.
Department of Energy's Oak Ridge National Laboratory demonstrated the
viability of a "quantum entanglement witness" capable of proving the
presence of entanglement between magnetic particles, or spins, in a quantum
material.
The team—including researchers from ORNL, Helmholtz-Zentrum Berlin, the
Technical University of Berlin, Institut Laue-Langevin, Oxford University
and Adam Mickiewicz University—tested three entanglement witnesses using a
combination of neutron scattering experiments and computational simulations.
Entanglement witnesses are techniques that act as data analysis tools to
determine which spins cross the threshold between the classical and quantum
realms.
First introduced by John Stewart Bell in the 1960s, entanglement witnesses
confirmed that the quantum theory questioned by other scientists had been
correct. Bell's technique relied on detecting one pair of particles at a
time, but this approach is not useful for studying solid materials composed
of trillions and trillions of particles. By targeting and detecting large
collections of entangled spins using new entanglement witnesses, the team
extended this concept to characterize solid materials and study exotic
behavior in superconductors and quantum magnets.
To ensure that the witnesses could be trusted, the team applied all three of
them to a material they knew to be entangled because of a previous spin
dynamics study. Two of the witnesses, which are based on Bell's approach,
adequately indicated the presence of entanglement in this one-dimensional
spin chain—a straight line of adjacent spins that communicate with their
neighbors while disregarding other particles—but the third, which is based
on quantum information theory, fared exceptionally well at the same task.
"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 and a lead author of the team's proof-of-concept paper published in
Physical Review B.
Because fluctuations in a material that appear to be quantum in nature can
be caused by random thermal motion, which only vanishes at absolute zero on
the temperature scale, most modern methods cannot distinguish between these
false alarms and actual quantum activity. The team not only confirmed the
theoretical prediction that entanglement increases as temperature decreases
but also successfully differentiated between classical and quantum activity
as part of the most comprehensive QFI demonstration since the technique was
proposed in 2016.
"The most interesting materials are full of quantum entanglement, but those
are precisely the ones that are the most difficult to calculate," said ORNL
neutron scattering scientist Alan Tennant, who leads a project focused on
quantum magnets for the Quantum Science Center, or QSC, a DOE National
Quantum Information Science Research Center headquartered at ORNL.
Previously, the challenge of quickly identifying quantum materials presented
a significant roadblock to the center's mission, which involves exploiting
entanglement to develop novel devices and sensors while advancing the field
of quantum information science. Streamlining this process with QFI allows
QSC researchers to focus on harnessing the power of substances such as rare
phases of matter called quantum spin liquids and materials that do not
resist electricity called superconductors for data storage and computing
applications.
"The power of QFI comes from its connection to quantum metrology, in which
scientists entangle multiple quasiparticles to shrink uncertainty and obtain
extremely precise measurements," Scheie said. "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 really
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, which is a property that causes spins to 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. They published
this finding in Physical Review Letters.
To achieve these results, the researchers studied both spin chains using
neutron scattering and then analyzed legacy data from experiments conducted
decades ago at the ISIS Neutron Source in England and the Institut
Laue-Langevin in France along with 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.
Neutrons, which Tennant describes as "beautifully simple," are an ideal tool
for probing the properties of a material because of their neutral charge and
nondestructive nature.
"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," Tennant said.
According to the team, this combination of advanced computational and
experimental resources provided answers about the nature of quantum
entanglement originally asked by the founders of quantum mechanics. Scheie
expects that QFI calculations are likely to become part of the standard
procedure for neutron scattering experiments that could eventually
characterize even the most mysterious quantum materials.
References:
A. Scheie et al, Witnessing entanglement in quantum magnets using neutron
scattering, Physical Review B (2021).
DOI: 10.1103/PhysRevB.103.224434
Pontus Laurell et al, Quantifying and Controlling Entanglement in the
Quantum Magnet Cs2CoCl4, Physical Review Letters (2021).
DOI: 10.1103/PhysRevLett.127.037201
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