It's almost axiomatic: Shake something and its temperature will rise. But that rule of thumb doesn't necessarily hold if that something is a cloud of ultracold atoms. A gentle jostle can further chill the atoms, physicists report.
Vibrations carry energy, so shaking an object generally increases its energy and drives its temperature up. That's why physicists often go to great lengths to shield low-temperature experiments from any kind of rumble. When trying to reach temperatures close to absolute zero, researchers routinely mount their rigs on massive tables or sand-filled pillars that damp vibrations. Now, however, a team of physicists has shown that a subtle shake can further cool a cloud of atoms already refrigerated to less than a thousandth of a degree.
The trick is to shake the cloud at just the right frequency, report Tsutomu Yabuzaki, Mitsutaka Kumakura, and colleagues at Kyoto University in Japan. The researchers trapped roughly 200 million atoms of rubidium-87 in a carefully crafted magnetic field deep within a vacuum chamber. The atoms formed a cigar-shaped cloud roughly 5 millimeters long and 2 millimeters wide, and the researchers then shook the cloud side to side at frequencies ranging from 10 to 500 cycles per second. At very low frequencies the shaking had little effect, while at high frequencies it heated the atoms. But in a narrow range around 40 cycles per second, the jiggling cooled the cloud, the researchers report in a paper to be published in the journal Physical Review A. In that range, the shaking frequency roughly matched the frequency at which the most energetic atoms rattled back and forth from one edge of the cloud to the other. The frequency match caused those atoms to absorb even more energy in a process known as resonance. They then flew out of the trap, taking a disproportionate amount of energy with them and cooling the remaining atoms.
"It's cute," says Eric Cornell, a physicist at JILA, a laboratory run jointly by the National Institute of Standards and Technology and the University of Colorado, Boulder. Physicists already use a similar technique for cooling magnetically trapped atoms, exposing them to radio waves that rearrange the electrons inside the most energetic atoms in such a way that the magnetic field will no longer hold them. However, the shaking technique might work when the radio waves won't, Cornell says. In particular, it might serve to cool atoms trapped in an intense beam of laser light, a situation in which radio waves have no effect.