IT IS the ultimate miniature experiment in the world of the infinitesimally small. Two American physicists have recreated one of the classic experiments of quantum mechanics inside a single atom. They have split an electron into two portions and watched as these portions interfere with each other.
In the early 19th century, the English physicist Thomas Young passed a beam of light through two narrow slits in a screen, and demonstrated the wave-like properties of light. As the waves spread out beyond the slits they interfere, giving a distinctive pattern of pale and dark bands when the light strikes a background screen. More recently, physicists have shown that this interference pattern occurs even when the light source is so dim that single photons pass through the apparatus one at a time. In this case, each photon is effectively split into two "wave packets" which then interfere with one another.
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