Supercooled Superfluid Could Lead to New Understanding of Matter

Researchers have demonstrated superfluidity in a cloud of atoms cooled to near absolute zero in work that could lead to high-temperature superconductors and a clearer understanding of neutron stars.

The group at the Massachusetts Institute of Technology (MIT; Cambridge, MA) was led by physics professor Wolfgang Ketterle, who shared the Nobel Prize in Physics in 2001 for his work on Bose-Einstein condensates (BECs), a supercooled form of matter in which atoms overlap into a blob-like state. The condensate consisted of bosons - atoms with an even number of electrons, protons, and neutrons. The new work focused on fermions, which have odd numbers of subatomic particles.

"Electrons are fermions. Neutrons are fermions. Quarks are fermions," Ketterle says. "I would even say the world of bosons is the smaller part of the world."

The team, which also included graduate students Martin Zweirlein, Andre Schirotzek, Christian Schunck, and Jamil Abo-Shaeer, cooled a gas of lithium-6 isotopes using laser-cooling techniques similar to those for creating a BEC. They then loaded the cloud of atoms into an optical dipole trap and applied an external magnetic field.

This so-called Fermi gas has been created before. While researchers suspected they had a superfluid, in which all the particles move together in lockstep, they had not proved it. Unlike an ordinary fluid, which moves smoothly at ever higher speeds when its container is rotated, a superfluid is a quantum wave that will only rotate in discrete amounts. The MIT group spun the fluid by moving a pair of 532-nm laser beams around the edges of the cloud. They then turned off the beam that was trapping the cloud and took images as it expanded. Instead of the smooth spinning of an ordinary liquid, they saw that the cloud was rotating around a series of vortices, proving it was a superfluid.

"In our field, you got it when you see it," Ketterle says.

"This experiment appears to nail the case for Fermi superfluidity," says John Thomas, a physics professor at Duke University (Durham, NC) whose group first created a Fermi gas in 2002.

"It proves superfluidity in ultracold Fermi gases," says Rudolf Grimm, a professor at the University of Innsbruck's Institute of Experimental Physics (Innsbruck, Austria). "Other experiments performed in the last two years have provided strong evidence for superfluidity, but this really is the 'smoking gun.'"

Clouds of supercooled lithium atoms that were held in magnetic fields of various strengths and then allowed to expand ballistically for 2 ms show vortices indicative of superfluidity. Photo courtesy Andre Schirotzek, MIT

The gas became a superfluid at 50 nK, which Ketterle called the highest transition temperature ever observed. That sounds extremely cold, but Ketterle points out, "If you take our low density and scale it up to the density of matter in a metal, we have a superconductor at room temperature."

Superconducting metals can transmit electrons with almost no loss. If room temperature versions could be created, they would have many applications. "Super-fluidity is for neutral particles exactly what super-conductivity is for electrons," Ketterle says. A major goal of future research will be to translate the mechanisms of superfluidity that scientists observe into the charged world of electrons.

He says that the Fermi gas, which has a density one billion times lower than a metal and in which the atoms are more easily controlled, acts as a sort of "model airplane" for studying the superconductors that interest scientists. "We want to use cold atoms and learn something about matter in general," he says. "Suddenly we have a test bed. We have experimental tools and theoretical tools where we can put atoms together in novel ways."

Thomas says the gases can also be a model for the behavior of atoms inside neutron stars and for the quark-gluon plasmas that dominated the early universe. As such, they could prove useful for astronomers and cosmologists trying to understand how the universe works.