What in fact we’re dealing with here is another kind of quantum superposition. We’ve seen that superposition refers to a situation in which a measurement on a quantum object could produce two or more possible outcomes, but we don’t know which it will be, only their relative probabilities. Entanglement is that same idea applied to two or more particles: a superposition of the state in which particle A has spin up and B spin down, say, and the state with the opposite configuration. Although the particles are separated, they must be described by a single wavefunction. We can’t untangle that
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