In the world of fundamental particles, you are either a fermion or a boson
but a new study from the University of Cambridge shows, for the first time,
that one can behave as the other as they move from one place to another.
Researchers from the Cavendish Laboratory have modeled a quantum walk of
identical particles that can change their fundamental character by simply
hopping across a domain wall in a one-dimensional lattice.
Their findings, published as a Letter in Physical Review Research, open up a
window to engineer and control new kinds of collective motion in the quantum
world.
All known fundamental particles fall in two groups: either a fermion
("matter particle") or a boson ("force carrier"), depending on how their
state is affected when two particles are exchanged. This "exchange
statistics" strongly affects their behavior, with fermions (electrons)
giving rise to the periodic table of elements and bosons (photons) leading
to electromagnetic radiation, energy and light.
In this new study, the theoretical physicists show that, by applying an
effective magnetic field that varies in space and with the particle density,
it is possible to coax the same particles to behave as bosons in one region
and (pseudo)fermions in another. The boundaries separating these regions are
invisible to every single particle and, yet, dramatically alters their
collective motion, leading to striking phenomena such as particles getting
trapped or fragmenting into many wave packets.
"Everything that we see around us is made up of either bosons or fermions.
These two groups behave and move completely differently: bosons try to bunch
together whereas fermions try to stay separate," explained first author Liam
L.H. Lau, who carried out this research during his undergraduate studies at
the Cavendish Laboratory and is now a graduate student at Rutgers
University.
"The question we asked was what if the particles could change their
character as they moved around on a one-dimensional lattice, our notion of
space."
This research is partly motivated by the remarkable prospects of being able
to control the nature of particles in the laboratory. In particular, certain
two-dimensional materials have been found to host particle-like excitations
that are in between bosons and fermions—called "anyons"—which could be used
to build robust quantum computers. However, in all setups so far, the nature
of the particles is fixed and cannot be changed in space or time.
By analyzing mathematical models, the present study shows how one can
juxtapose bosonic, fermionic, and even "anyonic" spatial domains in the same
physical system, and explores how two particles can move in surprising ways
through these different regions.
"The boundaries separating these regions are very special, because they are
transparent to single particles and, yet, control the final distribution by
how they reflect or transmit two particles arriving together!" said Lau. The
researchers illustrate this "many-body" effect by studying different
arrangements of the spatial domains, which give rise to strikingly different
collective motion of the two particles.
"These variable two-particle interferences are fascinating in their own
rights, but the new questions they open up for many particles are even more
exciting," said Dr. Shovan Dutta, the study's co-author who conceived and
supervised the research in the Cavendish and has recently moved to the Max
Planck Institute for the Physics of Complex Systems.
"Our work builds on recent progress in engineering artificial magnetic
fields for neutral atoms, and the predictions can be tested experimentally
in existing optical-lattice setups," added Dutta. "This will open access to
a rich class of controllable many-particle dynamics and, potentially,
technological applications, including in quantum sensing."
Reference:
Liam L.H. Lau et al, Quantum walk of two anyons across a statistical
boundary, Physical Review Research (2022).
DOI: 10.1103/PhysRevResearch.4.L012007
Tags:
Physics