Particles with different energies make up a liquid or gas. The hotter the gas, the higher the number of particles there are with high energies. In order to cool such a system, the scientists repeatedly and intentionally removed the quickest particles with the highest energy. The remaining ones interact, redistributing the energy, and the gas then adjusts at a lower energy level and thus lower temperature. This inner temperature compensation is named “thermalization.”
"This is the standard approach used in experiments with cold atoms”, explained Bernhard Rauer of the Institute for Atomic and Subatomic Physics at the Vienna University of Technology. However, the researchers led by Rauer and Jörg Schmiedmayer conduct experiments with one-dimensional gases which behave differently due to their special spatial structure.
The particles in this experimental setup are trapped in such a narrow electromagnetic field that they can only move in one direction and only exchange energies with each other. Nevertheless, the temperature fell to much lower energies than one would expect according to the simplified picture of slower and faster moving particles.
In their experiments, the Viennese physicists “discovered a new mechanism which is not based on thermalization,” as Rauer explained. “In such an extremely cold state in which the atoms exist, one can better understand their behavior if one does not focus on the movement of the individual particles but collective waves, similar to waves of water, distributed over several particles.” The energy of the system is stored in these quantum waves, which become smaller and smaller the more particles are removed from the gas. The system cools in a quantum physical manner when throwing out these particles. “It is a good tool for us to increase the temperature. The colder one gets these systems, the more their quantum physical properties are demonstrated,” Rauer added.
Service: Link to the publication: http://dx.doi.org/10.1103/PhysRevLett.116.030402