Degrees of quantumness: shades of gray in particle-wave duality.

Is light made of particles or waves? The answer, according to quantum physics, is both. Depending on the situation, particles of light--and particles of matter too--sometimes contradict themselves and act like waves. But between these two extremes, there's a range of behaviors. Scientists have now demonstrated those intermediates in a conspicuous way.

The new research is a variation on the so-called double-slit experiment, a staple of introductory quantum theory courses. In the classic version, light passes through two slits in an opaque screen and hits another screen some distance away. Crests and troughs of light waves emerging from each slit add together or cancel each other out, depending on howthey overlap, and create an interference pattern of light and dark stripes on the screen. This phenomenon has been demonstrated not only with photons but also with electrons, and even whole atoms.

From the quantum perspective, however, light is a stream of photons. To explain the interference pattern, physicists say that each photon travels through both slits simultaneously and then interferes with itself on the other side.

The additional twist is that, according to quantum theory, the interference pattern--a wave phenomenon--would disappear if one knew for sure through which slit each photon went. In principle, detectors at the slits would register a photon's passage without capturing the particle. In that situation, the photon would have chosen one slit or the other, thereby behaving like an old-fashioned, classical-physics particle.

Physicists suspected that it's possible to extract only partial information about a particle's route. They predicted that different degrees of certainty about the path would blur the interference pattern by different amounts.

In the mid-1980s, theoretical physicist Wojciech Zurek of the Los Alamos (N.M.) National Laboratory proposed a way to use beams of electrons to explore this idea. More than 20 years later, Franz Hasselbach and Peter Sonnentag of the University of Tubingen in Germany have put Zurek's idea to the test.

In their setup, electric fields play the role of the slits, steering electrons along two possible paths parallel to an underlying horizontal plate. As each electron passes, its electrostatic field moves charges inside the plate. Those movements, acting against the plate's electrical resistance, generate a tiny amount of heat.

By detecting that heat, an experimenter could locate the electron's path and make it lose its wavelike behavior. But this detection can be accomplished to different degrees. The closer the beam is to the plate, the larger the dissipation, and the easier it will be to tell apart the two trajectories. The Tubingen team's images reveal that with increasing certainty, the interference fringes become progressively blurred.

"The visibility of the fringes changes," Sonnentag says. The results appear in an upcoming Physical Review Letters.

Zurek says that he's pleased to see his predictions confirmed. "The nice thing is that you can quantify this leakage of information," he says. "You can turn the knob and vary the quantumness of the system."