Physicists Hot for Ultra Cold: A Laser's Light Tickle Tricks Molecules into Sitting Still

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Molecules are hot. They zip, spin and vibrate with frenetic motion. They jiggle and twist on the inside and bounce on the outside, imparting structure and physical properties to nearly everything that exists. But by achieving temperatures colder than any in the natural world, physicists can almost stop these speed demons cold.

Like surgeons who slow a beating heart by packing ice around a patient's chest, physicists have recently coaxed molecules into ultracold states in which motion is nearly gone. Researchers are left with intriguing, exquisitely controllable new specimens to poke and prod, enabling experiments that would be impossible with everyday hot molecules that rotate and vibrate at their usual frenzied pace.

To still these jittery molecules, temperatures must descend to about 350 nanokelvins, only a sliver above absolute zero and far colder even than the depths of outer space (about 3 kelvins, roughly 300 degrees Celsius colder than room temperature).

Researchers have now figured out ways to reach such lows, using precise laser pulses to trick molecules into giving up energy in the form of light. As the temperature drops, energy is siphoned away from the molecules. These new experiments have created a large, stable supply of ultracold molecules stationary enough to operate on.

"This is the breakthrough," says Matthias Weidemuller, a physicist who was formerly at the University of Freiburg in Germany and whose group recently succeeded in making an ultracold lithium-cesium molecule. "The thing that drives the whole field is to create ultracold systems that you can manipulate and observe."

What motivates the scientists is the potential of ultracold molecules as new tools for research. Such tools could help answer questions about the relationship between individual molecules as they collide: Cold chemical reactions are so slow that physicists have enough time to catch nuances of the interactions between molecules. Researchers could harness lattices of trapped, frozen molecules to explore the application of quantum mechanics to data storage and transmission. And more fundamentally, ultracold molecules may enable physicists to discover new, exotic phases of matter, such as a kind of superfluid in which molecules act across long ranges to influence one another in a frictionless system.

A new abundant supply of ultracold molecules has bumped physicists into their own excited state of high energy. "There is an awful lot of detail and rich physics to explore," says Paul Julienne, a theoretical physicist at the National Institute of Standards and Technology in Gaithersburg, Md.

The jittery molecule

Molecules, unlike the spherical atoms that compose them, are lumpy. Physicist Jun Ye says atoms are basketballs, and molecules American footballs. Two basketballs bounce off each other in a predictable way, says Ye, of the University of Colorado at Boulder. But molecules have awkward angles and unwieldy curves, making interactions less predictable.

"Atoms are easier to control," says Ye. "Molecules are more complex, and more exciting to study than atoms."

Physicists have been able to freeze atoms using laser light for years. But molecules--which come in a wide variety of shapes, sizes and charges--have proved to be a greater challenge. "The techniques to ultracool atoms can't be used for molecules," says Weidemuller, who is now at Heidelberg University. "That's why one had to come up with a good trick to make them slow."

Physicists had identified two approaches for creating ultracold molecules: Hot molecules could be cooled, or already cold atoms could be cajoled into joining.

Attempts by several groups started with hot molecules, but that approach proved extremely difficult. While several experiments resulted in a few weakly bound cold molecules, the molecules weren't cold enough. …