Introduction to Quantum Mechanics

1. Electron Interference Most textbooks of quantum mechanics start with Max Planck’s introduction of the “quantum” in the context of blackbody radiation in 1900. Let us break with tradition and start with the discussion of an experiment carried out in 1989. The experiment was done at the Hitachi Advanced Research Laboratory and Gakushuin University in Tokyo, by Tonomura et al. It consisted of a beam of electrons being split into two by an electric field, and then allowed to recombine, very similar to Young’s double slit experiment with light in which a beam of light is split into two by its passage through two closely spaced slits and allowed to recombine on a photographic plate. For convenience, let us call the apparatus a double-slit apparatus for electrons. The results from this experiment, taken at various times, are shown in the figure. At first, only a few electrons have reached the photographic plate and no pattern is discernible. However, as time passes and more and more “events” are recorded, a definite pattern emerges. The pattern consists of light and dark bands which indicate that the electrons, after passing through (one of) the “slits,” prefer certain regions of the photographic plate and avoid the others.