Number 94 (Story #4), September 16, 1992 by Phillip F. Schewe and Ben Stein ELECTRONS HAVE BEEN TRAPPED ABOVE POTENTIAL ENERGY WELLS , confirming a prediction made in the early days of quantum mechanics by Eugene Wigner and Max von Neumann. Usually, electrons are trapped inside potential wells; that is, they are trapped because they do not have enough energy to escape the attractive forces binding them. But if the potential wells are specially designed, Wigner and von Neumann argued, the electrons can interfere in such a way as to be in a bound state--but have more than enough energy to escape the well. Because these setups require a specific, rigorous geometry, the proposal was regarded at first as an abstract theoretical scenario instead of a physical possibility. But the advent of molecular beam epitaxy (MBE) techniques has changed that. Using MBE, Federico Capasso (908-582-7737) and his co-workers at AT&T Bell Labs constructed a semiconductor device made of alternating GaInAs and AlInAs layers and impurity-doped GaInAs layers. The alternating layers create a row of narrow wells which serve as mirrors, and they surround a wide well constructed with the doped layers. The mirrors reflect the electron waves, which interfere so that a bound state is created above the wide well. The researchers shone infrared light on the device and the absorption patterns reveal a transition between a bound state inside the potential well and a bound state above the energy well. (Nature, 13 August 1992.)