Editing Dark state (section)

{{Short description|State of an atom or molecule}}
{{About||the 2021 film|Dark State (film)}}
In [[atomic physics]], a '''dark state''' refers to a state of an atom or [[molecule]] that cannot absorb (or emit) photons. All atoms and molecules are described by [[quantum state]]s; different states can have different energies and a system can make a transition from one [[energy level]] to another by emitting or absorbing one or more [[photon]]s. However, not all transitions between arbitrary states are allowed. A state that cannot absorb an incident photon is called a dark state. This can occur in experiments using [[laser]] light to induce transitions between energy levels, when atoms can spontaneously decay into a state that is not coupled to any other level by the laser light, preventing the atom from absorbing or emitting light from that state.  

A dark state can also be the result of [[quantum interference]] in a three-level system, when an atom is in a [[quantum coherence|coherent]] superposition of two states, both of which are coupled by lasers at the right frequency to a third state.  With the system in a particular superposition of the two states, the system can be made dark to both lasers as the probability of absorbing a photon goes to 0.