text: Atomic coherence is the induced coherence between levels of a multi-level atomic system.
  The internal state of an atom is characterized by a superposition of excited states and their associated energy levels. In the presence of external electromagnetic fields, the atoms energy levels acquire perturbations to the excited states that describe the atoms internal state. When the acquired phase is the same over the range of internal states, the atom is coherent. Atomic coherence is characterized by the length of time over which the internal state of the atom can be reliably manipulated.