Electrons display their antisocial nature.

Soon after the birth of quantum mechanics early in this century, physicists theorized that force-carrying particles, known as bosons, tend to bunch together. By contrast, the elementary particles of matter, called fermions, demand a little elbowroom from their peers.

In a landmark 1950s experiment, researchers directly observed the predicted preference for bunching among photons--the bosons of electromagnetic radiation--and created a new research field. Called quantum optics, it eventually led to practical benefits such as the laser.

In the April 9 Science, two independent research groups report that they have finally performed a comparable experiment for fermions. Their results demonstrate the complementary, standoffish nature of electrons, one of the lighter members of the fermion clan, which also includes protons and neutrons.

In doing so, the investigators have confirmed the validity of the Pauli exclusion principle, part of the bedrock of quantum mechanics. This dictum states that no two identical fermions can occupy the same quantum state, such as a single atomic-energy level.

Although physicists commonly invoke the exclusion principle, "you have to measure [its effects] before people really believe it," says Stefan Oberholzer of the University of Basel in Switzerland, where he and his colleagues conducted one of the confirming tests. William D. Oliver and other researchers at Stanford University carried out an analogous experiment.

The new results "will certainly be the stuff of future textbook discussions," comments Marian O. Scully of Texas A&M University in College Station. Oberholzer notes that until recently scientists could not make dense enough fermion streams to explore the particles' chumminess.

In the new studies, both teams used electrons chilled to ultralow temperatures and confined to an extraordinarily thin layer between semiconductors. The scientists forced the particles into a narrow region blocked by an electrode's voltage so that only half of the electrons, on average, had enough energy to pop through. The others bounced off and exited via a different path.

The scientists measured currents from each arm of these beam splitters. As the currents fluctuated, the researchers consistently found that an increase in one arm was offset by a decrease in the other--the sign of fermionic, one-at-a-time, passage through the beam-splitter.

The experimental techniques may also help scientists probe the nature of mysterious quasiparticles, which can mix fermion and boson characteristics (SN: 10/17/98, p. 247), and test properties of very small electronic devices, the researchers say.