Cold Atoms Act as Distant Messengers

Particles can interact directly by repelling or attracting each other, but how do particles interact when far apart? Scientists at the University of Chicago have discovered that atoms can actually exchange information using intermediary particles. Such research represents the first time this particular phenomenon has been observed in a cold atom system, where atoms are maintained at temperatures close to absolute zero to reveal associated quantum mechanical properties.

The team of physicists embedded cesium atoms in a gas of lithium atoms. The lithium atoms are a particular type of particle called a fermion, which has the special property that no two can occupy the same quantum state in a system—so they collectively form what is called a Fermi sea at low temperatures. In the researchers’ experiment, a cesium atom first interacts with a lithium atom, causing a ripple that propagates on the surface of the Fermi sea. When the ripple reaches a second cesium atom, it receives the information from the first cesium atom.

“You can imagine the Fermi sea as a messenger,” says Cheng Chin, a professor of physics who leads the lab where the findings were made. “One particle transfers a message through the sea to the other particle.”

Researchers studied these interactions in cold-atom systems because the systems are more easily controlled and can serve as analogs for more complex systems. For example, the long-range nature of this mediated interaction creates new possibilities in quantum simulations.

“There are all sorts of predictions of new quantum phases that you might be able to realize by adding these long-range interactions into your system,” says Brian DeSalvo, postdoctoral researcher and co-lead author.

Though this is the first time mediated interactions have been observed in cold-atom systems, these interactions exist in other branches of physics. “This might allow us to explore other quantum phenomena related to high-energy and nuclear physics, where mediated interactions play an essential role,” Chin says.

Chin’s team is now doing system upgrades so that they can learn more about the mediated interactions.

“The experiments we’ve done prove that the interactions exist and tell us something about their strength, but they don’t tell us a lot of details about their range or behavior,” say Krutik Patel, graduate student and co-lead author. “Now, we want to see their strength as a function of distance.”