Quantum Flip-Floppers: Photon Findings Add to Mystery of Wave-Particle Duality

New experiments show that photons can traverse an optical obstacle course as both a wave and particle simultaneously

By John Matson

SHIFTY PHOTONS: New experiments explore the continuous transition from photons acting like particles to behaving as waves. Image: Image courtesy of S. Tanzilli, CNRS

Quantum objects are notoriously shifty. Take the photon, for example. The quantum of light can act as a particle one moment, following a well-defined path like a tiny projectile, and a wave the next, overlapping with its ilk to produce interference patterns, much like a ripple on the water.

Wave?particle duality is a key feature of quantum mechanics, one not easily understood in the intuitive terms of everyday experience. But the dual nature of quantum entities gets stranger still. New experiments demonstrate that photons not only switch from wave to particle and back again but can actually harbor both wave and particle tendencies at the same time. In fact, a photon can run through a complex optical apparatus and disappear for good into a detector without having decided on an identity?assuming a wave or particle nature only after it has been destroyed.

Physicists have shown in recent years that a photon "chooses" whether to act as a wave or a particle only when forced. If, for instance, a photon is steered by a beam splitter (a kind of fork in the optical road) onto one of two paths, each leading to a photon detector, the photon will appear at one or the other detector with equal probability. In other words, the photon simply chooses one of the routes and follows it to the end, like a marble rolling through a tube. But if the split paths recombine before the detectors, allowing the contents of the two channels to interfere like waves flowing around a pillar to meet on the other side, the photon demonstrates wavelike interference effects, having essentially traversed both paths at once. In other words, measure a photon like a particle, and it behaves like a particle. Measure a photon like a wave, and it acts like one.

One might suspect that photons simply assume one of two behaviors?wave or particle?beforehand, or when they hit the beam splitter. But a 2007 "delayed choice" experiment ruled out that possibility. Physicists using an interferometer, an experimental device that includes the beam splitter, toggled between combining the paths and leaving them separate. But they made the choice only after the photon had passed through the beam splitter. The photons still demonstrated interference effects when recombined, even though (in a simpler world, at least), the particles should already have been forced to decide which path to take.

Now, two research groups have implemented an even more bizarre version of the delayed choice experiment. In two studies in the November 2 issue of Science, a team based in France and a group in England each reported using a quantum switch to toggle the experimental device. Except in this experiment, the switch was not flipped?thus forcing the photon to act like a wave or like a particle?until the physicists had identified the photon in one of the detectors.

By changing the settings on the device, both teams could not only force the experimental photon to behave as a particle or as a wave, but could explore intermediate states as well. "We can continuously morph the behavior of the test photon from wavelike to particlelike behavior," says S?bastien Tanzilli, a study co-author and a quantum optical physicist with the National Center for Scientific Research in Paris who is based at the University of Nice Sophia Antipolis. "Between the two extremes, we have states that come with reduced interference. So we have a superposition of wave and particle."

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