Research team further explores exotic Fermion superfluid

A research team that created an exotic new form of matter in 2003 has now shown for the first time how to arrange that matter into complex molecules.

Cheng Chin, now a Chicago faculty member, and his colleagues conducted the experiments under the leadership of Rudolf Grimm at Innsbruck University in Austria. Their experiments may lead to a better scientific understanding of superconductivity and advance a growing new field called superchemistry. In the long term, they may also provide a strategy that could aid the development of quantum computers.

“In this field, it’s hard to predict what’s going to happen because none of this was possible before 2003,” said Chin, an Assistant Professor in Physics and the Physical Sciences Collegiate Division. Chin, Grimm and five colleagues reported their findings online in the Friday, April 1 issue of the journal Physical Review Letters.

The new form of matter the team produced in 2003 is called a Fermion superfluid, which exists only at temperatures hundreds of degrees below zero. Superfluids exhibit characteristics distinctively different from the solids, liquids and gases that dominate everyday life. Most notably, superfluids can flow ceaselessly without any energy loss whatsoever. Science magazine named this research one of the top 10 breakthroughs of 2004.

In creating the Fermion superfluid, the team extended the work that earned the Nobel Prize in Physics for Eric Cornell, Wolfgang Ketterle and Carl Wieman in 2001. Those scientists had succeeded in creating the first Bose-Einstein condensate. Building on the work of Satyendra Nath Bose, Albert Einstein predicted in the 1920s that a special state of matter would form when a group of atoms collapsed into their lowest energy state. In this state, now named for them, all of the atoms behave as if they are all one giant atom.

Cornell, Ketterle and Wieman created their Bose-Einstein condensate out of bosons, one of the two major categories of particles. Bosons carry force, while fermions, the other category of particles, comprise matter. Chin and the Innsbruck team showed in 2003 that, with some difficulty, fermions—in this case, lithium atoms—also can be coaxed into a Bose-Einstein condensate.

“Lithium atoms themselves cannot become condensed. They are not bosons,” Chin said. “But once they are paired, they become bosons, and you can go to this superfluid state.”

The laws of quantum mechanics forbid fermions from condensing. Chin and his colleagues used a technique called Feshbach resonance to bind two atoms into a simple molecule that behaves like a boson. The process is carried out in a magnetic field and resembles the type of electron pairing that causes superconductivity—the unimpeded flow of electricity at temperatures near absolute zero (minus 459.6 degrees Fahrenheit)—in solids.

This type of electron pairing is called Cooper pairing. Cooper pairings are the long-distance marriages of the subatomic world, where electrons are bonded at distances far greater than usual. “We have now discovered a handle to adjust the atomic interactions, which allows us to synthesize complex objects similar to Cooper pairs of atoms,” Chin said.

Approximately two years ago, the Innsbruck scientists found a deep connection between Bose-Einstein condensates and the bonding of Cooper pairs. They learned that they could use a pair of atoms to simulate a Cooper pair of electrons. And more importantly, they showed that Cooper pairs of atoms can be converted into molecules.

In their latest achievement, Chin and his colleagues have learned how to go one step further and bind the simple molecules made of cesium atoms into even larger clusters at temperatures near absolute zero.

“Since 2003, the controlled synthesis of simple molecules made of two atoms has opened up new frontiers in the field of ultracold quantum gases,” said Rudolf Grimm, a professor of experimental physics at Innsbruck University and a co-author of the Letters paper. Their present work now shows that ultracold simple molecules can be merged to form more complex objects consisting of four atoms, he said.

An important feature of this synthesis process is its tenability, Chin said. “In a magnetic field you can experimentally adjust it to any value, so we can control the formation process.”

The synthesis of ultracold molecules is so new, it is difficult to predict potential applications, Chin said. But it puts a new field called superchemistry on firm experimental footing. In superchemistry, scientists are able to precisely control the pairings and interactions of the atoms and molecules in Bose-Einstein condensates.

As ultracold molecules are synthesized into complex quantum objects, phenomena hidden at the subatomic scale will now become visible almost to the naked eye. Control of quantum objects may ultimately lead to the realization of a quantum computer, Chin said. Although possibly still decades from fruition, a quantum computer based on atoms would work much faster than conventional computers. The idea would be to use atoms in ultracold gas as bits, the basic units of information storage on a computer, with Feshbach resonance controlling their interactions to perform computations.

As Chin sets up his laboratory at Chicago, he plans to continue studying quantum manipulation and computation based on cold atoms and molecules in collaboration with Grimm’s Innsbruck team. “Based on the speed of progress in this field, I think there probably will be more surprises soon,” Chin said.