Number 268 (Story #1), May 1, 1996 by Phillip F. Schewe and Ben Stein CHAOTIC QUANTUM PINBALL. If struck correctly, a ball on a rectangular billiard table will follow a reproducible trajectory. Changing the table from a rectangular to a stadium shape causes the trajectory to become essentially unpredictable; the ball's motion is chaotic. A quantum mechanical analog of this has been devised in the form of electrons confined in a quantum well, a semiconductor arrangement consisting of a thin layer (22 nm) of GaAs sandwiched between layers of AlGaAs. The quantum well, part of a tunnel diode, acts as a box inside of which electrons knock around like billiard balls. If in addition the electrons are exposed to a high magnetic field (up to 37 T) tilted relative to the walls of the box, the electrons' motions become chaotic. Physicists from a Nottingham-Tokyo collaboration (L. Eaves, 44-115-951-5165) sample the current from the junction as the magnetic field and the voltage across the well are varied. In this way the researchers can observe the onset of chaos and can map out the wavefunction (the probability) for the electron to be at certain positions. Despite the unstable nature of the electron's orbit, the maps did exhibit concentrations (enhancements in the probability amplitude) in certain parts of the well. These "scars" in the electron wavefunction had been predicted but never before seen in a quantum system (although scarred states had been observed for microwaves confined in an irregular cavity). Besides serving as a laboratory for studying quantum chaos, the tunnel diode is a workable electronic device, and it might be possible to exploit the current fluctuations which come about because of the scarring effect. (P.B. Wilkinson et al., Nature 18 April 1996; the group has also published in T.M. Fromhold et al., Physical Review Letters, 7 August 1995.)