By Davide Castelvecchi

In his article "Demons, Entropy and the Quest for Absolute Zero," physicist Mark G. Raizen describes how to cool a rarefied gas down to temperatures of just millionths of a degree above absolute zero. The starting point is to take a gas that has already been cooled to one one-hundredth of a kelvin (using a device called an atomic coilgun, also described in the text) and place it in a magnetic trap. Then the serious freeze can begin, using the new technique of single-photon cooling.

Single-photon cooling exploits a one-way gate inspired by a 19th-century thought experiment by James Clerk Maxwell. The great Scottish physicist theorized the existence of a "demon" that seemed able to concentrate the atoms of a gas into a smaller volume without raising their temperature, thus reducing their entropy. That feat seemed to violate the second law of thermodynamics, according to which entropy can never decrease. (What's the catch? Read the article to find out.)

Raizen's idea is that a one-way gate can help cool a gas in two steps: First let the gate concentrate atoms into a smaller volume (but without raising their temperature), then allow them to expand to the original volume (which brings their temperature down).

Click on "START" to see how it works.