Number 234 (Story #2), July 21, 1995 by Phillip F. Schewe and Ben Stein INTERFEROMETRY OF ELECTRON WAVES WITHIN AN ATOM. Light waves passing through a pair of slits in a screen will interfere with each other and produce a characteristic pattern of light and dark areas on a photographic film. Exploiting the wave nature of particles, physicists have also performed this experiment ("Young's double-slit experiment") with electrons and even atoms. Carlos Stroud ((716-275-2598) and Michael Noel at the University of Rochester have now carried out interferometry within a single atom by causing a single electron to interfere with itself. First, they use a short laser pulse to excite a potassium atom into a so-called Rydberg wave packet state, a condition in which the outermost electron (although still attached to the nucleus) has a finite probability of residing in a classical Keplerian orbit (i.e., at times the electron's orbit is like that of a planet around the sun) as far away as half a micron (compared to a more typical distance of an angstrom). A second laser pulse, related to but delayed relative to the first pulse, further enhances the probability of the electron to be in the outer-lying orbit, although in a different location. One can think of this sequential laser excitation as establishing two spatially separate wave packets, which according to quantum mechanics correspond to the probability that the electron will be located in two particular locations in space. In the course of time, the waves in these packets spread out and interfere, just as the light waves emerging from the Youngs' slits interfere. The Rochester physicists then probe the interference pattern at later times with yet a third laser pulse. They find that by varying the relative phases of the laser pulses they can control whether the electron is on one side of its orbit or the other, a half micron away. Although they still possess quantum properties, the electrons in a Rydberg wave packet state also behave in a sort of quasi-classical way like particles traveling in large elliptical orbits. Scientists in this research area are seeking to learn more about the frontier between quantum and classical physics. (Michael W. Noel and C.R. Stroud, 14 Aug. 95, Physical Review Letters; journalists can obtain the text and figures by contacting AIP Public Information at 301-209-3091 or physnews@aip.org. Unfortunately, we cannot supply the article to non-journalists.)