In physics, atomic coherence is the induced coherence between levels of a multi-level atomic system sometimes observed when it interacts with a coherent electromagnetic field. If a coherent, narrow bandwidth laser is applied to a two-level system, the wave function will undergo Rabi flopping (Rabi oscillation) between the ground and excited states. At some point in time the system will undergo spontaneous decay and its wave function will collapse to the ground-state wave function. From there on, a new Rabi oscillation will start until the next spontaneous decay. Each spontaneous decay essentially changes the phase of the Rabi oscillation. If instead of single two-level system there is a large collection of identical two-level systems (like a lot of the same species atoms), then all of them