Quantum electrodynamics (QED) is the fundamental quantum theory governing
the behavior of charged particles and light in vacuum. The strength of the
interactions in QED is quantified by the fine structure constant α, which in
our universe is both immutable and eternal (α ~ 1/137). The smallness of the
fine structure constant has far-reaching consequences in the physical
world—it determines the number of stable chemical elements, enables
long-distance, light-based communication, etc.
One of the great recent insights of condensed matter physics is that
QED-like theories describe the behavior of quantum spin ice, a class of
fractionalized magnets. Rather than being ordered in a simple pattern, the
atomic spins in these systems fluctuate in intricate patterns down to the
lowest measurable temperatures. The resulting phase is characterized by the
presence of magnetic charges which interact with light-like waves in the
spin background.
Researchers at Boston University, Massachusetts Institute of Technology
(MIT) and Max-Planck-Institut für Physik komplexer Systeme have recently
carried out a study investigating the fine structure constant that emerges
in the QED of quantum spin ice. Their paper, published in Physical Review
Letters, shows that in quantum spin ice, this fundamental constant is large,
which means that these magnetic systems could be ideal for studying physical
phenomena arising from strong particle interactions.
"We were thinking about possible signatures of the emergent QED in quantum
spin ice and found that the most distinctive signatures involved effects of
interactions between the emergent charges and photons," Christopher R.
Laumann and Siddhardh C. Morampudi, two of the researchers who carried out
the study, told Phys.org via email. "We then realized that the basic
dimensionless number (the emergent fine-structure constant) characterizing
the strength of this interaction was not yet determined in any previous
work, and previous works had only focused on characterizing the emergent
speed of light."
Laumann, Morampudi and their colleagues set out to investigate the fine
structure constant of quantum spin ice, as they believed this would offer a
more complete characterisation of their QED. The observation of a relatively
large α value was a pleasant surprise for them, as such a value would
enhance the interaction-mediated signatures of the emergent QED.
"Using large-scale exact diagonalization to obtain the energy cost of an
electric flux tube, we were able to extract the electric charge," Laumann
and Morampudi said. "This then allowed us to get from the lattice model to
the long-wavelength emergent QED in computationally accessible finite-size
systems."
The numerical simulations carried out by Laumann, Morampudi and their
colleagues are the first to calculate the fine structure constant in an
emergent QED, specifically one realized in quantum spin ice. The team showed
that in the system they simulated, the α constant is typically one order of
magnitude larger than the fine-structure constant of usual QED. In addition,
they demonstrated that in quantum spin ice the constant can be tuned all the
way from zero to the strongest coupling with which QED confines.
"The fine-structure constant of usual QED is small and fixed as provided by
nature," Laumann and Morampudi said. "Having an emergent QED with a large
and also tunable fine-structure constant provides a nice playground for
understanding processes in QED which are heavily suppressed due to the small
coupling."
One of the primary theoretical tools for studying quantum field theories is
perturbation theory. Over the past few decades, however, many researchers
have begun to explore what happens to field theories at strong coupling, in
instances where perturbation theory is not a particularly useful construct.
"This has led to a wide variety of non-perturbative tools whose
effectiveness can be tested if we have an experimental playground for
strong-coupling QED in quantum spin ice," Laumann and Morampudi said. "Our
work also identifies quantum spin ice as a great target for fast-evolving
quantum simulators, with the promise of uncovering interesting physics of
strong-coupling QED as a reward."
In recent years, a growing number of physicists have started conducting
studies investigating quantum spin ice candidates, particularly rare-earth
pyrochlores. Some of the candidates identified in these studies could
exhibit additional interactions that causes the systems to become ordered,
rather than remaining in a quantum spin liquid phase. The large fine
structure constant calculated by Laumann, Morampudi and their colleagues
implies the presence of significant interaction-mediated effects, such as a
large enhancement of the inelastic neutron scattering cross-section near
threshold.
"There have been tantalizing hints of the right physics in some of the
materials, but disorder and the small energy scales (limiting experimental
resolution in neutron scattering for example) have been limiting factors so
far," Laumann and Morampudi said. "In our next studies, we plan to explore
more implications of the large fine structure constant in potential
realizations of quantum spin ice, and push towards simulations of them in
near-term quantum computers. Our hope is to better understand how open
questions in strong-coupling QED could potentially be answered in such
settings."
Reference:
Salvatore D. Pace et al, Emergent fine structure constant of quantum spin
ice is large. Physical Review Letters(2021).
DOI: 10.1103/PhysRevLett.127.117205
Siddhardh C. Morampudi et al, Spectroscopy of Spinons in Coulomb Quantum
Spin Liquids, Physical Review Letters (2020).
DOI: 10.1103/PhysRevLett.124.097204
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