Expectation value (quantum mechanics)

Consider an operator ${\displaystyle A}$. The expectation value is then ${\displaystyle \langle A\rangle =\langle \psi |A|\psi \rangle }$ in Dirac notation.

In quantum theory, an experimental setup is described by the observable ${\displaystyle A}$ to be measured, and the state ${\displaystyle \sigma }$ of the system. The expectation value of ${\displaystyle A}$ in the state ${\displaystyle \sigma }$ is denoted as ${\displaystyle \langle A\rangle _{\sigma }}$.

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