Physicists Get Hot New Results With Cold Atoms

Using some of the coldest atoms in the universe, physicists at the National Institute of Standards and Technology have been able to observe exotic phenomena that do not exist outside of specialized laboratories.

Using unusual giant molecules or new forms of ultracold matter, physicists are gaining new understandings of how nature works.

NIST physicists recently have made significant advances using cold atoms in experiments with optical lattices, in cold atom collisions and in achieving the first Bose-Einstein condensation. The atomic temperatures necessary to conduct these experiments are far colder than the deep reaches of interstellar space, just a few millionths to a few billionths of a degree above absolute zero.

Such temperatures are possible using lasers and magnetic fields to slow, cool and trap atoms. A laser beam aimed at a stream of atoms can exert pressure against the atoms, slowing their speed and cooling their temperature simultaneously, explains William D. Phillips, leader of NIST's Laser Cooled and Trapped Atoms Group. Laser-cooling atoms is analogous to spraying a stream of water at a succession of rapidly volleyed tennis balls.

Physicists can trap atoms in an "optical lattice" by shining lasers at them from several different directions. The intersecting laser beams create a periodic interference pattern, a lattice of bright and dark regions. The light-shifted energy of the atoms is lowest in the bright regions, so atoms caught in such an array of lasers will slip into the bright regions as they slow down, much like eggs fit in an egg carton.

One of the most recent accomplishments of the Phillips group is the demonstration that an optical lattice can diffract laser light in the same way that crystals diffract X-rays (scientists use X-rays to study the structure of crystals). The effect is called Bragg scattering, and it allows scientists to observe changes in the motion of trapped atoms as they grow colder in the optical lattice.

"We can tell how long it takes atoms in the trap to come to an equilibrium," Phillips says. "It tells us things we couldn't measure before about the cooling process."

Another type of experiment, which also is allowing scientists to measure previously unmeasurable phenomena, uses cold atoms in collisions. For years, scientists have known that two neutral atoms are attracted to each other due to transient shifts in their electron orbitals. However, the force of this attraction is also influenced by the distance between the two atoms and the time it takes the force-field to move between the atoms, Phillips explains.

For the first time ever, the NIST team has measured this effect. "We can see the effect of the finite speed of light on the energy fields of two atoms," he says. "The cold atom collisions have allowed us to see an effect that up until now has eluded measurement."

The physicists collided cold sodium atoms with each other while shining a laser at them. The result is a weakly bound molecular state that does not exist in nature, is many times larger than an ordinary molecule and only exists in the lab for roughly 10 nanoseconds.

Measuring the energy levels of this unusual sodium molecule has given scientists a more accurate way to measure the lifetime of sodium atoms. Data from experiments like this should help guide scientists doing large-scale modeling calculations by providing benchmark data for applications ranging from astrophysical phenomena to advances in the lighting industry.

Improvements in laser cooling and trapping techniques allowed another group of researchers including Eric Cornell and Carl Wieman of JILA, a joint program of NIST and the University of Colorado at Boulder, on June 5, 1995, to achieve the coldest temperature ever recorded and produce an exotic new form of matter known as a Bose-Einstein condensate. Since then, other laboratories have reported similar results.

The new state of matter had been predicted decades ago by Albert Einstein and Indian physicist Satyendra Nath Bose. In the condensate, a gas of rubidium atoms coalesced into a single quantum state, indistinguishable from each other, even in principle. The condensate formed at a temperature of about 20 billionths of a degree above absolute zero, a temperature lower than had ever been achieved previously. Phillips expects Bose-Einstein condensates to usher in a new era of exploration and precision measurements using cold atoms in new ways.

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