Consider what happens when a positron and electron annihilate. Two gammas (high energy photons) are produced that go off in opposite directions, 180° apart. They can be detected many meters from their source and have opposite spins, but which gamma has which spin is a random choice. That is, for each gamma there is a 50/50 chance that it will be in a particular direction. Suppose you find that the spin of one gamma is pointing up. But, the moment you detect the spin of that gamma, the spin of the other gamma must be pointing down. The result of one measurement determines the results of the other. The two detectors would measure the spins of their gammas at the same time, so there is no way that one gamma could communicate with the other gamma.

Physicists say that the spins of the two photons are entangled and that the spin state of each photon is the superposition of the two possible spin directions. When the spin is measured the “wavefunction collapses” and gives a definitive result. Albert Einstein (1879-1955) called results like this “Spooky action at a distance.” Others have called it quantum weirdness.

Similar results can be obtained with atoms or ions, in which case the photons are light quanta and may be transported through space (or the air) or by optical fibers. For example, if an atom is excited by the absorption of a photon it can emit two photons that are entangled the same way the gammas are in the example above.