It is this waviness that gives rise to distinctly quantum phenomena like interference, superposition and entanglement. These behaviours become possible when there is a well-defined relationship between the quantum ‘waves’: in effect, when they are in step. This co-ordination is called coherence. It’s a concept that comes from the science of ordinary waves. Here too, orderly wave interference (like that from double slits) happens only if there’s coherence in the oscillations of the interfering waves. If there is not, there can be no systematic coincidence of peaks and troughs and no regular
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