The paper presents the first direct observation of superfluidity in a gas of fermionic atoms, which has been a major goal in ultracold atom research. Ultracold atomic gases, a million times thinner than air, offer the remarkable possibility to study the phenomenon of superfluidity in a very clean and highly controllable way. So far, this had been achieved only for gases of bosons, particles with integer spin.

Click to see full image. “Demonstrating fermionic superfluidity had been a long-standing goal in experiments with cold fermions.”

In this work we demonstrate superfluidity in a gas of fermions, particles with half-integer spin: after setting the gas in rotation, a regular array of vortices (mini-tornadoes) is observed.

To become superfluid, fermions have to team up and form pairs. These pairs are bosons and can thus "march in lockstep," forming one big quantum-mechanical matter wave.

Fermionic pairing and superfluidity are central to many diverse fields of physics, as fermions (such as protons, neutrons, and electrons) are the building blocks of matter. For example, the phenomenon is closely connected to superconductivity of electrons in a metal, where electron pairs flow without any resistance or loss. As such, our work is of immediate interest not only to atomic physicists, but also to researchers in the fields of condensed matter physics, nuclear physics, and astronomy.

It describes the discovery of high-temperature superfluidity in a strongly interacting Fermi gas. This also presents a synthesis of knowledge, as several experiments over the past years had already uncovered pieces of the puzzle (in six laboratories around the world: JILA (Boulder), Duke, Paris, Rice, Innsbruck, and MIT).

It was known before that the gas was strongly interacting, that it consisted of pairs of fermions, and that these pairs could condense into a very low-energy state. Our observation of vortices in the rotating gas provided the "smoking gun" that this gas was actually a superfluid.

A superfluid gas can flow without resistance. It can be clearly distinguished from a normal gas when it is rotated. A normal gas rotates like an ordinary object, but a superfluid can only rotate when it forms vortices similar to mini-tornadoes. This gives a rotating superfluid the appearance of Swiss cheese, where the holes are the cores of the mini-tornadoes.

Demonstrating fermionic superfluidity had been a long-standing goal in experiments with cold fermions. Our fermion of choice was the lithium-6 isotope comprising three protons, three neutrons, and three electrons. Since the total number of constituents is odd, lithium-6 is a fermion.

Using laser and evaporative cooling techniques, we cooled the gas close to absolute zero. Next, the gas was trapped in the focus of an infrared laser beam; the electric and magnetic fields of the infrared light held the atoms in place.

The last step was to spin a green laser beam around the gas to set it into rotation. This was just like using a spoon to stir up the coffee in your mug and making the liquid spin around. A shadow picture of the cloud showed its superfluid behavior: the cloud was pierced by a regular array of vortices, each about the same size.

We were able to view these superfluid vortices at extremely cold temperatures, when the fermionic gas was cooled to about 50 billionths of a degree Kelvin, very close to absolute zero (-273 degrees C or -459 degrees F). Although "ultracold" in absolute terms, this temperature is actually quite "high" when compared with the energy content of the gas (which is very small due to its ultra-low density). In this sense, the temperature of the ultra-dilute superfluid observed by our group exceeds by far the transition temperature for high-temperature superconductors. Scaled to the density of electrons in a metal, the superfluid transition would occur far above room temperature.

This is fascinating, as even the remote possibility of making a room-temperature superconductor spurs hopes that we will one day transport electricity without any loss.