Schematic drawing of collision between two BECs (the gray blobs) that have been “dressed” by laser light (brown arrows) and an additional magnetic field (green arrow). The fuzzy halo shows where atoms have been scattered. The non-uniform projection of the scattering halo on the graph beneath shows that some of the scattering has been d-wave and g-wave. |
Physicists
at the National Institute of Standards and Technology (NIST) have found
a way to manipulate atoms’ internal states with lasers that
dramatically influences their interactions in specific ways. Such
light-tweaked atoms can be used as proxies to study important phenomena
that would be difficult or impossible to study in other contexts. Their
most recent work, appearing in Science,
demonstrates a new class of interactions thought to be important to the
physics of superconductors that could be used for quantum computation.
Particle
interactions are fundamental to physics, determining, for example, how
magnetic materials and high temperature superconductors work. Learning
more about these interactions or creating new “effective” interactions
will help scientists design materials with specific magnetic or
superconducting properties.
Because
most materials are complicated systems, it is difficult to study or
engineer the interactions between the constituent electrons. Researchers
at NIST build physically analogous systems using supercooled atoms to
learn more about how materials with these properties work.
“Basically,
we’re able to simulate these complicated systems and observe how they
work in slow motion,” says Ian Spielman, a physicist at NIST and fellow
of the Joint Quantum Institute (JQI), a collaborative enterprise of NIST
and the University of Maryland.
According
to Ross Williams, a postdoctoral researcher at NIST, cold atom
experiments are good for studying many body systems because they offer a
high degree of control over position and behavior of the atoms.
“First,
we trap rubidium-87 atoms using magnetic fields and cool them down to
100 nanokelvins,” says Williams. “At these temperatures, they become
what’s known as a Bose-Einstein condensate. Cooling the atoms this much
makes them really sluggish, and once we see that they are moving slowly
enough, we use lasers to ‘dress’ the atoms, or mix together different
energy states within them. Once we have dressed the atoms, we split the
condensate, collide the two parts, and then see how they interact.”
According
to Williams, without being laser-dressed, simple, low-energy
interactions dominate how the atoms scatter as they come together. While
in this state, the atoms bang into each other and scatter to form a
uniform sphere that looks the same from every direction, which doesn’t
reveal much about how the atoms interacted.
When
dressed, however, the atoms tended to scatter in certain directions and
form interesting shapes indicative of the influence of new, more
complicated interactions, which aren’t normally seen in ultracold atom
systems. The ability to induce them allows researchers to explore a
whole new range of exciting quantum phenomena in these systems.
While
the researchers used rubidium atoms, which are bosons, for this
experiment, they are modifying the scheme to study ultracold fermions, a
different species of particle. The group hopes to find evidence of the
Majorana fermion, an enigmatic, still theoretical kind of particle that
is involved in superconducting systems important to quantum computation.
“A
lot of people are looking for the Majorana fermion,” says Williams. “It
would be great if our approach helped us to be the first.”
Synthetic Partial Waves in Ultracold Atomic Collisions
JQI news announcement, “The Impact of Quantum Matter”
