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2 Concepts of Quantum Optics
According to quantum mechanics, light may be considered not only as electro-magnetic wave but also as a "stream" of particles called photons which travel with c, the vacuum speed of light.
Do not imagine photons as tiny little balls. They should rather be considered as spread-out and moving regions in space which have a non-zero quantum-mechanical amplitude of an elementary excitation of the electromagnetic field. (Speaking very simplifying this means: If you put an piece of matter into a region that is passed by a certain number of photons, there is a certain probability to detect by certain means that your piece of matter has absorbed up to that number of quanta of energy.)
The postulation of the quantization of light by Max Planck in 1899 and the discovery of the general validity of this idea in Albert Einstein's 1905 explanation of the photoelectric effect soon led physicists to realize the possibility of population inversion and the possibility of the laser.
This kind of use of statistical mechanics is the fundament of most 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. For a coarser view, one considers coherent states with Poissonean statistics as the normal state of light and deviations from it (super- or sub-Poissonean photon statistics in a squeezed state) are called squeezed light .
Atoms are considered as quantum mechanical oscillators with a discrete energy spectrum with the transitions between the energy eigenstates being driven by the absorption or emission of light according to Einstein's theory with the oscillator strength depending on the quantum numbers of the states.
For solid state matter one uses the energy band models of solid state physics. This is important as understanding how light is detected (typically by a solid-state device that absorbs it) is crucial for understanding experiments.
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