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Superposition
In quantum mechanics, superposition describes how two (or more) quantum states can be added together to produce another valid quantum state.
Explanation
It means that every quantum state can be represented as a sum of two or more other distinct states.
Decay time
Quantum superpositions do not last forever. For an unstable particle the superposition |ψ⟩ would decay to either |1⟩ or |2⟩ in an everage time τ of the order
where
τ Imaginary time ℏ Dirac constant EG Gravitational self-energy
Example 1
An electron in the state |1⟩ + i |2⟩ is in a quantum superposition of the states |1⟩ and |2⟩.
Example 2
A vector PR between two points P and R is equal to the sum of the two vectors PQ and QR via a third point Q
PQ + QR = PR
This is called the head-tail method, and is a form of superposition in physics.
Example 3
An exciton-polariton is a superposition of an exciton and a photon; a phonon-polariton is a superposition of a phonon and a photon.