where | n k i ⟩ ⟩ represents the state in which n k i photons are in the mode k i . In this notation, the creation of a new photon in mode k i (e.g., emitted from an atomic transition) is written as | n k i ⟩ ⟩ → → | n k i + 1 ⟩ ⟩ . This notation merely expresses the concept of Born, Heisenberg and Jordan described above, and does not add any physics.