Quantum superposition occurs when an object simulataneously "possesses" two or more values for an observable quantity (e.g. the position or energy of a particle).
More specifically, in quantum mechanics, any observable quantity corresponds to an eigenstate of a Hermitian linear operator. The linear combination of two or more eigenstates results in quantum superposition of two or more values of the quantity. If one attempts to measure the quantity, the projection postulate states that the state will be randomly collapsed onto one of the values in the superposition (with a probability proportional to the amplitude of that eigenstate in the linear combination).
Quantum superposition results in many directly observable effects, such as interference peaks from an electron wave in a singleslit experiment.
If two observables correspond to noncommuting operators, they obey an uncertainty principle and a distinct state of one observable corresponds to a superposition of many states for the other observable.
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