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Atomic or molecular gases in low concentration show sharp energy eigen spectra. This was shown for the hydrogen atom. Usually, there are infinitely many energy eigenstates in an atomic, molecular or solid-state medium and the spectral lines are associated with allowed transitions between two of these energy eigenstates. For many physical considerations it is already sufficient to take only two of these possible energy eigenstates into account, for example those which are related to the laser transition. The pumping of the laser can be later described by phenomenological relaxation processes into the upper laser level and out of the lower laser level. The resulting simple model is often called a two-level atom, which is mathematically also equivalent to a spin 1 /2 particle in an external magnetic field, because the spin can only be parallel or anti-parallel to the field, i. e. it has two energy levels and energy eigenstates 4. The interaction of the two-level atom with the electric field of an electromagnetic wave is described by the Bloch equations.