The complex coefficients { c i } in the expansion of a quantum state in terms of eigenstates { | ϕ ϕ i ⟩ ⟩ } , | ψ ψ ⟩ ⟩ = ∑ ∑ i c i | ϕ ϕ i ⟩ ⟩ . can be written as an (complex) overlap of the corresponding eigenstate and the quantum state: c i = ⟨ ⟨ ϕ ϕ i | ψ ψ ⟩ ⟩ . They are called the probability amplitudes. The square modulus | c i | 2 is the probability that a measurement of the observable yields the eigenstate | ϕ ϕ i ⟩ ⟩ . The sum of the probability over all possible outcomes must be one: