You've seen the game of pachinko—a small metal ball dropped into a thicket of horizontal pegs rattles its way to the bottom. What if the pachinko ball suddenly stopped halfway down, without any force holding it in place? That's essentially what researchers have now done using wavy quantum particles instead of metal balls and specially prepared light instead of a pachinko board.
Researchers are pros at using laser beams to forcefully stop atoms in their tracks. But the effect demonstrated in the new experiments is different. According to quantum mechanics, a particle splits like a wave when it runs into a small enough object or other obstacle.
The particle has a certain probability of taking each path around the obstacle. Those paths can then interfere with each other like overlapping water waves created by stones in a fast-moving stream.
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If the obstacles are sufficiently jumbled, the resulting interference may restrict the wave to a small spot in an effect known as Anderson localization, for Princeton University physicist Philip Anderson, winner of the 1977 Nobel Prize in Physics, who predicted it in 1958. "There is no actual force here. It's different possible paths that interfere destructively," says Philippe Bouyer, a senior researcher with the CNRS at the Institute of Optics in Paris.
Although Anderson predicted the effect for electrons in a certain class of semiconductor, researchers have had a hard time observing it directly using matter. Instead they have seen it in systems such as light blocked by stacks of glass slides of slightly different thicknesses.
To observe localization of matter waves, Bouyer and his colleagues as well as, separately, a team from the University of Florence in Italy, set out to detect it by forcing atoms through scrambled light beams that scattered them wherever the light was concentrated.
Each group chilled a microscopic cloud of atoms to a temperature close to absolute zero (-459.67 degrees Fahrenheit) to generate a Bose-Einstein condensate (BEC), in which all the atoms share a single, wavy quantum state. The researchers allowed their BECs to expand from a small starting spot along a line created by laser beams.
They then either shined laser light through finely ground glass or combined multiple wavelengths of light to generate a random pattern across the line. As a result, the atoms expanded only up to about a tenth of an inch (a couple of millimeters) and then stopped, they report in Nature.
Bouyer says the result show that using BECs, "we can really simulate some very complex systems" such as semiconductors in this case, adding that the next steps are to extend the technique to two and three dimensions, to better mimic real materials. There's even a slim chance of practical applications for semiconductors. "If you understand how they behave," he says, "you might improve them."