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Mismatched Cold Atoms Hint at a Stellar New Superfluid

Science  23 Dec 2005:
Vol. 310, Issue 5756, pp. 1892a
DOI: 10.1126/science.310.5756.1892a

A puff of ultracold atoms may help physicists decipher the weird nuclear matter in the hearts of neutron stars. Two groups report online in Science this week that when they tweaked such frigid atoms to mimic superdense nuclear matter, the atoms continued to pair up and flow without resistance, just as electrons do in a superconductor. One group even claims evidence of a new type of resistance-free flow, or superfluidity.

Theorists have predicted that new forms of superfluidity might exist in neutron stars, and an atomic analog may enable them to test those ideas directly. “To have that become something you can study in the laboratory is like a gift from heaven,” says theorist Frank Wilczek of the Massachusetts Institute of Technology (MIT) in Cambridge.

Find a partner.

When atoms spinning one way outnumber those spinning the other, they still can pair and flow freely—perhaps like matter in a neutron star.

In each experiment, a gas of the isotope lithium-6 is trapped in a laser beam and chilled to less than a millionth of a kelvin. Quantum mechanics dictates that no two identical lithium-6 atoms can fill the same energy “state,” so the trapped atoms stack into the energy ladder of quantum states two at a time—one spinning one way and the other spinning the opposite way. Researchers then apply a magnetic field to make the atoms attract or repel one another.

When equal numbers spin each way while the atoms repel, opposite-spinning atoms form loose “Cooper pairs” whose connection depends on the motion of the other atoms (Science, 6 February 2004, p. 741). These pairs flow through one another without resistance, as Martin Zwierlein, Wolfgang Ketterle, and colleagues at MIT proved in June, when they tried to rotate the cloud of atoms (Science, 24 June, p. 1848). Instead of turning as a whole, it sprouted tiny whirlpools called vortices—hallmarks of superfluidity.

Now, the MIT experimenters report online (www.sciencemag.org/cgi/content/abstract/1122318) that superfluidity persists when atoms spinning one way outnumber potential partners by as much as 70%. The imbalanced gas mimics the dense soup of subatomic “quarks” at the center of a neutron star, as there some types of quarks outnumber others.

Whether the particles are atoms or quarks, standard theory forbids superfluidity when one type of them stacks to higher energy than the other, Ketterle says. But, he says, the results jibe with the notion that extra members of the majority are squeezed to sides of the laser trap, leveling the energy stacks in the middle.

More speculatively, Guthrie Partridge, Randall Hulet, and colleagues at Rice University in Houston, Texas, claim online (www.sciencemag.org/cgi/content/abstract/1122876) that the lithium superfluid remains mixed at small imbalances. Atoms spinning in opposite directions absorb light of different colors. By measuring the absorption of the colors in various parts of the cloud, the researchers showed that the extra atoms migrated to the edges only when the imbalance exceeded 9%.

No one has ever detected an imbalanced superfluid before, although Wilczek and others have devised scenarios in which one could exist. “On the face of it, [Hulet's result] is consistent with the kind of superfluid we've been predicting,” Wilczek says, “but it's by no means proof.”

The Rice researchers haven't shown that their gas is ever superfluid, Ketterle says. Hulet agrees, but he says that previous experiments show the gas is superfluid when the imbalance is zero, and the easiest explanation is that his team is seeing a transition from one superfluid to another. “Anything else, while not ruled out, would have to be even more exotic,” Hulet says.

Future experiments will put the purported superfluid to the test. Regardless of the outcome, however, ultracold atoms have begun to live up to their potential as a portal into new and exotic physics.

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