Confined gas rejects compromise.

Imagine pouring cold milk into a cup of hot coffee and finding that the milk stays cold while the coffee stays hot. Physicists have now achieved a similarly strange result by restricting atoms of an ultracold gas to motion in just one dimension.

In the new system, atoms of different energies collide but don't share energy in the usual way that, for example, equal amounts of hot and cold liquids make a lukewarm mix, notes David S. Weiss of Pennsylvania State University in University Park. He says that his team has devised an experimental means to investigate what causes and controls the energy-distribution process, called thermalization.

To do so the researchers first created a quantum state known as a Bose-Einstein condensate (SN: 9/18/04, p. 186) by trapping and cooling rubidium-87 atoms. Then, by crossing two laser beams to form thousands of light tubes capable of confining atoms, the team divvied up the atom cloud into a hundred-or-so atoms per tube. Finally, a third laser's pulse split each tube's contents into two portions and imparted unequal doses of momentum to each--the rough equivalent of creating hot and cold parts of each sample.

This set the portions in motion in one dimension within their tubes. The atoms repeatedly mixed, but their energy profiles didn't change, even after an estimated thousands of collisions, report Weiss and his Penn State colleagues Toshiya Kinoshita and Trevor Wenger in the April 13 Nature.

The system forestalls thermalization because the one-dimensional motion prevents the kind of momentum transfers that generate a broad distribution of energies, Weiss says.--P.W.