In a tangle
Entangled photons can be produced when an ultraviolet laser beam strikes a crystal with nonlinear optical properties. There is a small probability that one of the photons will spontaneously decay into a pair of photons with longer wavelengths (the sets of concentric rings above). Under certain conditions one of the "down-converted" photons will be polarized horizontally and the other will be polarized vertically. If the down-converted beams are made to overlap, the photons carry no individual polarizations all we know is that the polarizations are different. This is an entangled state and such states can be used both to test Bell's theorem and in various applications in quantum information (see boxes 1 and 2). (Picture credit: University of Innsbruck)