If the measurement of the physical quantity A on the system in the state | ψ ψ ⟩ ⟩ gives the result a n , then the state of the system immediately after the measurement is the normalized projection of | ψ ψ ⟩ ⟩ onto the eigensubspace associated with a n | ψ ψ ⟩ ⟩ ⟹ ⟹ a n P n | ψ ψ ⟩ ⟩ ⟨ ⟨ ψ ψ | P n | ψ ψ ⟩ ⟩