In the field of atomic, molecular, and optical physics, a branching fraction refers to the probability of decay to a specific lower-lying energy states from some excited state. Suppose we drive a transition in an atomic system to an excited state |e⟩, which can decay into either the ground state |g⟩ or a long-lived state |d⟩. If the probability to decay (the branching fraction) into the |g⟩ state is ⁠ p ⁠, then the probability to decay into the other state |d⟩ would be ⁠ 1 − − p ⁠. Further possible decays would split appropriately, with their probabilities summing to 1.