" Curiously, though, we only have to look at one of the two slits for the outcome of the whole experiment to be affected, as if the electrons passing through the other slit also knew what we were doing. This is an example of quantum “non-locality,” which means that what happens in one location seems to affect events in another location instantly. Non-locality is a key feature of the central mystery of quantum mechanics, and a vital ingredient in quantum computers. "

― John Gribbin , Computing with Quantum Cats: From Colossus to Qubits