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The Wonderful World of Ultracold

JILA scientists Eric Cornell, the late Deborah Jin, and Jun Ye have been world leaders in research on ultracold atoms and molecules.

In 1990, JILA physicists Eric Cornell and Carl Wieman began exploring the weird and wonderful world of ultracold matter. Their goal was to make a Bose-Einstein condensate (BEC) from a cloud of atoms cooled to near absolute zero. The key attraction that lured them to this challenge was the possibility that a BEC would display quantum behavior observable with ordinary laboratory equipment. If that proved true, they reasoned that a BEC might somehow be related to superconductivity, superfluidity, and laser light — three other systems exhibiting evidence of quantum behavior. From the beginning of their quest, the two researchers recognized that success would represent a major contribution to physics.

In June of 1995, Cornell and Wieman created the world’s first BEC — 70 years after Indian physicist Satyendra Bose and Albert Einstein had predicted it. To make the BEC, the researchers created a gas of rubidium atoms (87Rb) that, in theory, could occupy the same quantum state at ultracold temperatures. Using laser-based and magnetic cooling techniques, the researchers lowered the temperature of the gas of atoms to just a few billionths of a degree absolute zero (170 nK). Suddenly, a large fraction of the gas simultaneously went into the lowest possible energy state and formed a BEC, or "superatom." The superatom contained thousands of individual atoms that formed it. However, these atoms danced in unison and behaved as a single entity. The new BEC was an entirely new state of matter. It didn’t even exist in the coldest reaches of outer space.

Here on Earth, Cornell and Wieman’s journey of discovery opened up a rich new field of physics research. In 2001, it brought them the Nobel Prize in Physics, which they shared with MIT’s Wolfgang Ketterle, another talented scientist dedicated to research on ultracold matter. Today, nearly 20 years after the first JILA experiments on atomic collisions and the cooling of atoms to ultracold temperatures, research on ultracold matter continues to inform our understanding of the fundamental principles governing the physical world.

Early BEC experiments

After creating the first BEC, Cornell and Wieman performed many experiments to help them understand this new state of matter. They discovered that the behavior of a BEC was sensitive to even slight variations in temperature. Unexpectedly, they also observed the formation of two separate BECs in the same trap! Prior to this discovery, they had thought that since all the atoms in their experiments were the neighborly bosons, which happily occupy the same quantum state at ultracold temperatures, their atom clouds would predictably collapse into a single superatom. However, atoms have a property called spin, which can exist in two different states. Normally, atoms collide with each other in a gas cloud, knocking themselves into more or less in the same spin state. However, at ultralow temperatures, collisions between the atoms of 87Rb) are strongly suppressed. Identical atoms with different spins can coexist in a cold atom cloud and, once the temperature gets low enough, form two different BECs.

Cornell and Wieman studied how the two condensates interacted and pushed each other apart. They were also able to use atoms in one spin state as a cooling fluid to chill atoms in a second spin state. These efforts were part of an extensive research program on two-component condensates. Eventually, Cornell and Wieman were able to model the behavior of this two-BEC system as if it consisted of two distinct quantum fluids. One of their most dramatic experiments led to the creation of a vortex inside a BEC. In this experiment, the researchers placed atoms in one spin state near the center of a BEC, and then set atoms in a second spin state rotating around the inner ball of atoms. They created the vortex by blasting away the atoms in the center. Nearly a decade later, the Cornell group still performs sophisticated experiments to create, manipulate, and understand vortices inside a BEC.

Optical traps and magnetic resonances

Before the creation of the first BEC, Cornell had asked theorist Boudwijn Verhaar to explore how magnetic fields would affect atomic collisions at ultralow temperatures. Verhaar’s calculations revealed the existence of dramatic resonances in the magnetic field, which are now known as Feshbach resonances. The researchers immediately suspected that these resonances might provide a means for controlling ultracold atomic interactions. Consequently, they spent several years looking unsuccessfully for them. As soon as Cornell and Wieman adopted all-optical cooling techniques for their BEC experiments, it became much easier to not only find these magnetic resonances, but also to finely tune them. By 1999, Feshbach resonance physics had begun to play a major role in ultracold research at JILA.

In 2001, using Feshbach resonances and lasers, Cornell and Wieman cooled a cloud of atoms of a different isotope of rubidium (85Rb) to 3 nK, at which point the cloud formed a condensate. They found they could readily adjust the interactions in the cloud by varying the magnetic field in the vicinity of a Feshbach resonance. A world of possibility opened up. The researchers were able to use Feshbach resonance physics to study the instability of condensates when atomic interactions were attractive and explore the impact on a BEC’s quantum wave function when atomic interactions were large and repulsive. It was suddenly possible to think about probing BECs made of molecules and the quantum behavior of liquids!

In one Feshbach resonance experiment, the researchers suddenly changed the magnetic field around the condensate, creating a strong attraction between the atoms. The condensate first shrunk slightly and then exploded, throwing off a substantial fraction of the atoms. Most of the atoms surrounding the condensate also simply vanished. The process left behind a cold stable remnant. Paradoxically, the cold remnant was often larger than experimental conditions seemed to permit. All told, BEC collapse resembled a supernova (albeit on a vastly lower energy scale). Accordingly, Cornell and Wieman dubbed it the "Bosenova."

In other Feshbach resonance studies, JILA researchers found that they could create condensates that oscillated back and forth between condensates of atoms and molecules. This curious system was a quantum superposition of two chemically distinct species, the original atoms and molecules made of two of these atoms. Its discovery spurred a whole series of experiments, including those that led to condensates of fermionic atoms, which at first glance shouldn't even be able to form a superatom.

Taming the independent-minded fermions

Unlike bosons, fermions resist occupying the same quantum state as their neighbors. As they get colder, fermions flock to the lowest possible energy states. However, instead of collapsing into a single state, they stack up single file like people climbing a narrow staircase. When most of the lowest energy states are occupied by one fermion each, a degenerate Fermi gas forms.

In 1999, Deborah Jin and Brian DeMarco produced the world’s first degenerate Fermi gas of atoms (40K) in a magnetic trap at JILA. To do so, they adapted magnetic-trapping and evaporative-cooling techniques developed for creating BECs. Since then, Jin and her colleagues have observed the quantum behavior of this system. The quantum gas has more energy than a classical gas would because many atoms cannot fall into the lowest energy states, which are already occupied. Similarly, collisions that would normally knock an atom into its lowest energy state can’t happen if the lowest energy states are filled.

Early on, the Jin group set a goal of observing superfluidity in a dilute gas of fermionic atoms. Such a fermionic superfluid would be analogous to superconductivity in metals. Jin believed that it should be possible to adjust the interaction strength of ultracold atoms along a continuum between two limits: A BEC, which would form when pairs of atoms formed molecules, and a BCS-type superfluid. A BCS superfluid (named after Nobel Laureates John Bardeen, Leon Cooper, and Robert  Schrieffer) is made up of pairs of atoms that always move in synch with each other, just like the dancers shown here. Atom or electron pairs that move in sync are called Cooper pairs, after Leon Cooper. Cooper pairs of ultracold atoms are analogous to the Cooper pairs of electrons responsible for superconductivity.

Jin believed that she could help physicists improve their understanding of superconductivity and superfluidity by creating and observing condensates of (1) molecules made from fermionic atoms and (2) Cooper pairs of fermionic atoms. She was particularly interested in exploring the crossover point between the BEC and BCS limits because she suspected it might shed light on the workings of high-temperature superconductors.

In late 2003, Jin and her colleagues Markus Greiner and Cindy Regal used Feshbach physics to create and Bose condense diatomic molecules of 40K from an ultracold degenerate Fermi gas of atoms. The new ultracold molecules were bosons; unlike the fermionic atoms, they happily gathered in a single quantum state. In fact, if the temperature of the initial ultracold Fermi gas of atoms was sufficiently low, the molecules didn’t even have to be actively cooled to form a condensate. The molecular condensate was one extreme of the predicted BEC–BCS continuum.

Within a few months, the group again used Feshbach physics to create a "fermionic condensate" from (Cooper) pairs of ultracold atoms dancing in sync. The new condensate was a new phase of matter that formed in the crossover between BECs and superconductors. Not surprisingly, the strength of pairing in the ultracold condensate corresponded to a room temperature superconductor. The Jin group predicted that the new fermionic condensates would open the door to studies of superconductivity and superfluidity under extreme ultracold conditions. One day, this research may help physicists and engineers design more practical superconducting materials.

Since 2004, JILA scientists have continued their ground-breaking research on ultracold atoms, and, more recently, molecules. Links to highlights of more recent work on ultracold atoms appear below.

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