Watching washes out interference.

Quantum mechanics posits that a particle such as an electron can also behave like a wave. Thus, electrons that pass through a pair of narrow slits create an interference pattern--in the form of an oscillating signal--analogous to that generated by overlapping ripples on the surface of a body of water. Simultaneous observation of an electron's particle and wave aspects, however, is forbidden. Determining the specific path taken by an electron, for example, inevitably suppresses any wavelike behavior, such as interference.

Mordehai Heiblum and his colleagues at the Weizmann Institute of Science in Rehovot, Israel, have now demonstrated experimentally that a detector's sensitivity to electrons has a dramatic effect on the observed results. Increasing the ability of a detector to determine whether an electron has passed through one slit or the other reduces the amount of interference observed. "We find that by varying the sensitivity of the detector, we can affect the visibility of the oscillatory interference signal," the physicists report in the Feb. 26 Nature.

To demonstrate the effect, Heiblum and his coworkers constructed a miniature device, less than 1 micrometer across, that incorporates an electron source, an interferometer, and a detector. By changing the tiny detector's electrical conductivity, the researchers could alter its sensitivity and observe its effect on electrons streaming through two microscopic openings in a semiconductor barrier. The results showed that by adjusting the properties of a quantum observer, it's possible to control the extent of the observer's influence on electron behavior.

"We believe that similar experimental setups, but with higher detector sensitivity, may be used to study other fundamental problems in quantum mechanics," the researchers conclude.