Quantum entanglement is a weird concept in a theory full of weird concepts. A typical experiment goes as follows: you prepare a pair of photons such that their polarizations are complementary. A subsequent measurement on one photon will reveal the outcome of a related measurement on the other photon—no matter how widely they are separated. A “quantum eraser” takes that idea one step further: one of the photons is sent into an interference device, and measurement on the second determines whether or not interference actually takes place. Researchers constructed a quantum eraser experiment on two of the Canary Islands to show that the two measuring apparatuses can’t communicate with each other.

The quantum eraser experiment involves producing two sets of photons with correlated polarizations. One set, known as the system photons, are sent into a polarizing beam-splitter (PBS); as the name suggests, this directs light along different paths based on its polarization. The two possible paths for the system photon were then recombined, so they could either interfere (if the photon is behaving like a wave) or show up in one of two detectors (behaving like a particle). [Read more…]