A pair of orthonormal bases { | e 1 ⟩ ⟩ , … … , | e d ⟩ ⟩ } and { | f 1 ⟩ ⟩ , … … , | f d ⟩ ⟩ } in Hilbert space C are said to be mutually unbiased, if and only if the square of the magnitude of the inner product between any basis states | e j ⟩ ⟩ and | f k ⟩ ⟩ equals the inverse of the dimension d :