The feature of a quantum system whereby it exists in several separate quantum states at the same time. For example, electrons possess a quantum feature called spin, a type of intrinsic angular momentum. In the presence of a magnetic field, the electron may exist in two possible spin states, usually referred to as spin up and spin down. Each electron, until it is measured, will have a finite chance of being in either state. Only when measured is it observed to be in a specific spin state. In common experience a coin facing up has a definite value: it is a head or a tail. Even if you don’t look at the coin you trust that it must be a head or tail. In quantum experience the situation is more unsettling: material properties of things do not exist until they are measured. Until you “look” (measure the particular property) at the coin, as it were, it has no fixed face up.
To learn more about quantum superposition, please visit the Superposition entry at The Quantum Atlas.
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