(color online) Space–time diagram for the Mach–Zehnder type atom interferometry. The pulse durations of the π / 2 − π − π / 2 sequences are τ 1 , τ 2 , and τ₃, respectively. The time of free propagation between the pulses is T and T ′ , respectively. The time propagation operator U(t) for an atom interferometer can be decomposed into five parts: U laser ( τ 1 ) , U prop ( T + τ 1 ) , U laser ( τ 2 ) , U prop ( T ′ + τ 2 ) , and U laser ( τ 3 ) .