Slow down atoms enough and they begin to behave like light waves rather than particles. Working with atoms in this sluggish condition, physicists have fashioned the first solid mirror that can reflect atoms much as a normal mirror reflects light. A physicist reports in the 5 February issue of Physical Review Letters that he achieved this effect with a slow-moving beam of neon atoms and a silicon reflector. Scientists hope to use the technique to develop new atomic optical devices--tools that make use of atoms behaving like light waves.
The phenomenon, called quantum reflection, happens this way: As atoms slowly approach a solid surface, they feel an attractive force called the van der Waals-Casimir potential. Sometimes, the atoms rebound in the same way light reflects off a nearly transparent pool of water. Scientists observed the quantum reflection of hydrogen and helium atoms off a liquid helium surface in the early 1990s, but they had never achieved the slow speeds needed to reflect an atom from a solid surface. "It is hard to do this on Earth because of gravity," says physicist Fujio Shimizu of the Institute for Laser Science at the University of Electro-Communications in Tokyo. "Even if you stop the atom completely at some point, it will be accelerated again."
So Shimizu used a trick. After cooling a gas of neon atoms to a near standstill, he dropped the cold neon atoms so that they grazed a nearly vertical silicon plate. The sleight of hand is that even though the atoms were whizzing downward at about 3 meters per second, they were approaching the silicon plate much more slowly. (Imagine parachuting onto a steep mountainside. As you look horizontally, the mountain appears to be slowly getting closer to you). Traveling at a few millimeters per second, almost 30% of the atoms reflected, enough to form a perfect image of the neon source on a detector.
"This is important because now we can direct these ultracold atom beams wherever we want them to go," says chemical engineer Carlos Carraro of the University of California, Berkeley. And that is just what Shimizu hopes to do. His team plans to use solid reflection in vital components of atom optics such as mirrors, beam splitters, gratings, and holograms that work just like their familiar light-reflecting counterparts.