Dark state
Encyclopedia
The term dark state refers to state of an atom or molecule which can no longer absorb (or emit) photons and therefore appears dark. All atoms and molecules are described by quantum states -- different states can have different energies and a system can make a transition from one energy level
to another by, for example, emitting or absorbing a photon
. However, not all transitions between arbitrary states are allowed. A state that cannot be accessed by absorbing a 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.
(meaning the photons have a specific energy), so they only couple one set of states with a particular energy 
to another set of states with an energy 
. The atom can however still decay spontaneously into a third state by emitting a photon of a different frequency. The new state with energy 
of the atom no longer interacts with the laser simply because no photons of the right energy 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.
and a state 
is allowed often depends on how detailed the model is that we use for the atom-light interaction. From a particular model follow a set of selection rules that determine which transitions are allowed and which are not. Often these selection rules can be boiled down to conservation of angular momentum (the photon has angular momentum). In most cases we only consider an atom interacting with the electric dipole field of the photon. Then some transitions are not allowed at all, others are only allowed for photons of a certain polarization.
Let's consider for example the hydrogen atom. The transition from the state
with mj=-1/2 to the state 
with mj=-1/2 is only allowed for light with polarization along the z axis (quantization axis) of the atom. The state 
with mj=-1/2 therefore appears dark for light of other polarizations.
Transitions from the 2S level to the 1S level are not allowed at all. The 2S state can not decay to the ground state by emitting a single photon. It can only decay by collisions with other atoms or by emitting multiple photons. Since these events are rare, the atom can remain in this excited state for a very long time, such an excited state is called a metastable state.
Energy level
A quantum mechanical system or particle that is bound -- that is, confined spatially—can only take on certain discrete values of energy. This contrasts with classical particles, which can have any energy. These discrete values are called energy levels...
to another by, for example, emitting or absorbing a photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
. However, not all transitions between arbitrary states are allowed. A state that cannot be accessed by absorbing a 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.
In practice
Experiments in atomic physics are often done with a laser of a specific frequency (meaning the photons have a specific energy), so they only couple one set of states with a particular energy to another set of states with an energy . The atom can however still decay spontaneously into a third state by emitting a photon of a different frequency. The new state with energy of the atom no longer interacts with the laser simply because no photons of the right energy 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.In theory
Whether or not we say a transition between a state and a state is allowed often depends on how detailed the model is that we use for the atom-light interaction. From a particular model follow a set of selection rules that determine which transitions are allowed and which are not. Often these selection rules can be boiled down to conservation of angular momentum (the photon has angular momentum). In most cases we only consider an atom interacting with the electric dipole field of the photon. Then some transitions are not allowed at all, others are only allowed for photons of a certain polarization.Let's consider for example the hydrogen atom. The transition from the state
with mj=-1/2 to the state with mj=-1/2 is only allowed for light with polarization along the z axis (quantization axis) of the atom. The state with mj=-1/2 therefore appears dark for light of other polarizations.Transitions from the 2S level to the 1S level are not allowed at all. The 2S state can not decay to the ground state by emitting a single photon. It can only decay by collisions with other atoms or by emitting multiple photons. Since these events are rare, the atom can remain in this excited state for a very long time, such an excited state is called a metastable state.