Experiments in atomic physics are often done with a laser of a specific frequency ω ω (meaning the photons have a specific energy), so they only couple one set of states with a particular energy E 1 to another set of states with an energy E 2 = E 1 + ℏ ℏ ω ω . However, the atom can still decay spontaneously into a third state by emitting a photon of a different frequency. The new state with energy E 3 < E 2 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.