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Disentangling the wave-particle duality in the double-slit experiment

Interference patterns produced by double-slit experiments.

The subtlest experiment in quantum mechanics is also one of the simplest: send a stream of particles through two openings in a barrier, and you'll produce an interference pattern because the particles act as waves. Amazingly, this also works if you send the particles through one at a time—the interference pattern builds up slowly as more particles go through. The double-slit experiment has been replicated with photons, electrons, atoms, and even entire molecules.

Typically, the particle nature and the wave nature have to be observed separately; if you track the particles through a single slit, the interference pattern vanishes. However, Ralf Menzel, Dirk Puhlmann, Axel Heuer, and Wolfgang P. Schleich entangled two photons and allowed one to pass through a barrier with two slits. The entanglement enabled them to determine which opening the photon went through, but a detector on the other side still picked up an interference pattern, demonstrating light's wave- and particle-like characteristics simultaneously.

The key to the experiment is the particular state in which the photons were produced. The researchers started with a laser in a configuration known as TEM01 mode, which means the electric (E) and magnetic (M) fields are perpendicular (or transverse, T) to the direction the photons travel. The "01" means there are two distinct points of maximum intensity.

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from Ars Technica http://arstechnica.com/science/2012/05/disentangling-the-wave-particle-dualit...