Molecules could be super-cooled to within a whisker of absolute zero, suggest physicists who have for the first time chilled atoms in a fridge made of mirrors. A rash of breakthroughs in physics stemmed from the invention of a technique to reduce the temperature of atoms. Called "laser cooling", this led to more accurate atomic clocks and even created new forms of matter. But laser cooling does not work for molecules. The ability to super-cool molecules could open up new realms of research says Vladan Vuletic, from the Centre for Ultracold Atoms at MIT in Cambridge, Massachusetts. For example, experiments on molecules can probe the fundamental forces and the properties of the electron. "Just as cold atoms have improved the precision of atomic clocks, colder molecules could improve the precision of these measurements," he says. "We have realised a new mechanism that will allow us to cool particles that cannot be cooled by standard techniques," says Gerhard Rempe, who led the team at Max Planck Institute for Quantum Optics in Garching, Germany. So far, however, Rempe's team have only tested the technique on single atoms. The experiment they carried out involved just one rubidium atom. The researchers injected the atom into the space between two mirrors, held 100 microns apart. Everything else had been pumped out of the cavity, so the atom went into a perfect vacuum. An infrared laser beamed light into the gap, and this bounced backwards and forwards between the two mirrors. The atom interacted with the reflected light in such a way that its frequency increased. This meant that the photons escaping the cavity had a higher energy than those that were pumped in. Because energy is always conserved, the rubidium atom lost a corresponding amount of energy and so cooled down. "We haven't measured the temperature, but we start with millikelvin and we think we are getting to something like a hundred microkelvin," Rempe told New Scientist. He believes they could achieve even lower temperatures with better lasers. New Scientist