Electron waves: interference in an atom.

A classic demonstration of the wave nature of light involves sending a light beam through a pair of slits, which then serve as closely spaced light sources. At some distance from the slits, the resulting beams overlap to create an interference pattern of alternating dark and bright bars, visible on a screen or photographic film (see illustration). Acting like water waves, the overlapping crests and troughs of the light waves cancel or reinforce each other to produce such a pattern.

Taking advantage of the fact that electrons behave not only like particles but also like waves, researchers have now used finely tuned, precisely timed laser pulses to smear a single electron within an atom into interfering with itself. This situation is roughly analogous to the optical double-slit experiment with the two slits on the opposite sides of an electron's orbit.

Michael W. Noel and Carlos R. Stroud Jr. of the Institute of Optics at the University of Rochester in New York describe their experiment in the Aug. 14 Physical Review Letters.

"We're trying to understand the boundary between classical and quantum mechanics, and part of that is learning how to control an electron within an atom as completely as possible," Stroud says.

The researchers begin by using a short laser pulse to excite an electron in a potassium atom into an orbit about 0.5 micrometers wide, placing the electron more than 1,000 times farther from the atom's nucleus than normal. In this excited state, the electron moves initially as a localized wave packet, behaving much like a planet orbiting the sun.

A second, identical laser pulse, delayed so that the initial wave packet has time to move to the opposite side of its orbit, creates another wave packet in the same orbit. In effect, after the two laser pulses, the probability of finding the electron becomes highest at two positions along its atom-girdling path.

Over time, each of these circulating wave packets spreads out and ultimately evolves into a pair of smaller wave packets on opposite sides of the orbit. The two sets of packets overlap to create an interference pattern. Depending on the phase relationship between the initial laser pulses, the researchers can detect various configurations (see illustration).

"The interference is quite dramatic," the researchers note.

This experiment represents one step in a much longer campaign aimed at manipulating atomic states. Such wave packet shaping may eventually prove useful in controlling chemical reactions and in quantum computing (SN: 1/14/95, p.30).