Measurements retroactively force photons to be both wave and particle

Ars Technica » Scientific Method 2012-11-05

One of the devices in which photons will act as a particle and wave simultaneously.
Fernando Tarquin

One of the stranger features of the quantum world is that light—even individual photons—can behave as a wave or a particle, depending on how you measure it. But, according to papers released by Science today, the quantum weirdness doesn't end there. Researchers have now found a way to put a photon in a quantum superposition where it is both a wave and a particle at the same time. Worse still, one setup allows them to determine the photon's nature as a wave or particle after it has gone through an apparatus where it must act as one or the other.

Got that? Didn't think so, so let's go through it in more detail.

The two experiments use a similar design. Polarized photons are sent one at a time into a device, where they first encounter a beamsplitter, which has a 50/50 chance of sending them down one of two paths. On one of the paths, they will encounter a device that rotates the polarization a bit. Mirrors then send the photons toward an intersection flanked by two detectors.

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