Editing Quantum optics (section)

== Concepts ==
According to [[Quantum mechanics|quantum theory]], light may be considered not only to be as an [[electro-magnetism|electro-magnetic wave]] but also as a [[stream of particles|"stream" of particles]] called [[photons]], which travel with ''c'', the [[speed of light]] in vacuum. These particles should not be considered to be [[classical billiard balls]], but as quantum mechanical particles described by a [[wavefunction]] spread over a finite region.

Each particle carries one quantum of energy, equal to ''hf'', where ''h'' is the [[Planck constant]] and ''f'' is the frequency of the light. That energy possessed by a single photon corresponds exactly to the transition between discrete energy levels in an atom (or other system) that emitted the photon; material absorption of a photon is the reverse process. Einstein's explanation of [[spontaneous emission]] also predicted the existence of [[stimulated emission]], the principle upon which the [[laser]] rests. However, the actual invention of the [[maser]] (and laser) many years later was dependent on a method to produce a [[population inversion]].

The use of [[statistical mechanics]] is fundamental to the concepts of quantum optics: light is described in terms of field operators for creation and annihilation of photons—i.e. in the language of [[quantum electrodynamics]].

A frequently encountered state of the light field is the [[coherent state]], as introduced by [[E. C. George Sudarshan|E.C. George Sudarshan]] in 1960. This state, which can be used to approximately describe the output of a single-frequency [[laser]] well above the laser threshold, exhibits [[Poisson distribution|Poissonian]] photon number statistics. Via certain [[Nonlinear optics|nonlinear]] interactions, a coherent state can be transformed into a [[squeezed coherent state]], by applying a squeezing operator that can exhibit [[super-Poissonian|super]]- or [[sub-Poissonian]] photon statistics. Such light is called [[Squeezed coherent state|squeezed light]]. Other important quantum aspects are related to correlations of photon statistics between different beams. For example, [[spontaneous parametric down-conversion]] can generate so-called 'twin beams', where (ideally) each photon of one beam is associated with a photon in the other beam.

Atoms are considered as quantum mechanical [[oscillator]]s with a [[discrete space|discrete]] [[energy spectrum]], with the transitions between the energy [[eigenstate]]s being driven by the absorption or emission of light according to Einstein's theory.

For solid state matter, one uses the [[energy band]] models of [[solid state physics]]. This is important for understanding how light is detected by solid-state devices, commonly used in experiments.