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."
For more details, see the JQI news announcement, "The Impact of Quantum Matter" at http://jqi.
View an animation of the atom interactions on the NIST YouTube channel at http://www.
* R.A. Williams, L.J. LeBlanc, K. Jiménez-García, M.C. Beeler, A.R. Perry, W.D. Phillips, I.B. Spielman. Synthetic Partial Waves in Ultracold Atomic Collisions . Science Express, 8 December 2011.