Quantum Flip-Floppers: Photon Findings Add to Mystery of Wave-Particle Duality
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.
Read more at http://www.scientificamerican.com/article.cfm?id=quantum-delayed-choice