Atomic Quantum Fluids Spin and Spin

Six years ago, researchers hit the physics jackpot when they created a new state of matter: supercooled rubidium atoms locked together in one quantum mechanical state. These so-called Bose-Einstein condensates (BEC) of atomic vapor resemble much denser substances known as quantum fluids. Lately physicists have put atomic BECs through some bizarre, quantum-fluid-like contortions, culminating now in arrays of long-lasting vortices.

The first and classic quantum fluid is made of liquid helium. When cooled below its 4.2-kelvin boiling point, the liquid becomes calm, and then at 2.17 kelvin loses its viscosity. With all the jiggling from thermal energy removed, the wave functions of all the helium atoms lock together in a common quantum state.

Such superfluids break up into tiny whirlpools when they are rotated. The vortices are strikingly long-lived. In theory, without viscosity to stop them, they could persist forever. For many physicists, this vortex phenomenon is the acid test for superfluidity. "Superfluidity has many manifestations," says Wolfgang Ketterle of the Massachusetts Institute of Technology, "but for many people vortices are the most direct evidence."

When the much more rarefied atomic BECs came along, physicists wondered whether they would pass that test. Over the past year or so, most have concluded that they do. Researchers create the vortices by using a laser beam, in effect as an optical spoon to stir up the condensate.

Now a team has created the most elaborate vortex phenomenon yet in an atomic BEC. As researchers stir more vigorously, more and more vortices appear, packing themselves into lattices. Ketterle and colleagues spawned highly ordered arrays of more than 100 vortex lines, lasting as long as 40 seconds, they report on 23 March in Science Online. "If you want to see superfluid motion, you are looking for something persistent," Ketterle explains. "We can watch the vortices make 50,000 rotations. In a classical gas, these would be completely damped out."

Such experiments have left most physicists convinced that cold atomic vapors behave like superfluids. "Everybody believed that superfluidity was going on in these things," says Bill Phillips of the National Institute of Standards and Technology in Gaithersburg, Maryland, "but seeing it unequivocally was something people were looking for."

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