Editing Dark state (section)

===In practice===
Experiments in atomic physics are often done with a laser of a specific frequency <math>\omega</math> (meaning the photons have a specific energy), so they only couple one set of states with a particular energy <math>E_1</math> to another set of states with an energy <math>E_2=E_1 + \hbar \omega</math>. However, the atom can still decay spontaneously into a third state by emitting a photon of a different frequency. The new state with energy <math>E_3<E_2</math> of the atom no longer interacts with the laser simply because no photons of the right frequency are present to induce a transition to a different level. In practice, the term dark state is often used for a state that is not accessible by the specific laser in use even though transitions from this state are in principle allowed.