- Quantum optics
**Quantum optics**is a field of research inphysics , dealing with the application ofquantum mechanics to phenomena involvinglight and its interactions withmatter .**History of quantum optics**Light is made up of particles called

photons and hence inherently is "grainy" (quantized). Quantum optics is the study of the nature and effects of light as quantized photons. The first indication that light might be quantized came fromMax Planck in 1899 when he correctly modelledblackbody radiation by assuming that the exchange of energy between light and matter only occurred in discrete amounts he called quanta. It was unknown whether the source of this discreteness was the matter or the light. In 1905,Albert Einstein published the theory of thephotoelectric effect . It appeared that the only possible explanation for the effect was the existence of particles of light called photons. Later,Niels Bohr showed that the atoms were also quantized, in the sense that they could only emit discrete amounts of energy. The understanding of the interaction between light andmatter following from these developments not only formed the basis of quantum optics but also were crucial for the development of quantum mechanics as a whole. However, the subfields of quantum mechanics dealing with matter-light interaction were principally regarded as research into matter rather than into light and hence, one rather spoke ofatom physics andquantum electronics .This changed with the invention of the

maser in 1953 and thelaser in 1960.Laser science —i.e., research into principles, design and application of these devices—became an important field, and the quantum mechanics underlying the laser's principles was studied now with more emphasis on the properties of light, and the name "quantum optics" became customary.As laser science needed good theoretical foundations, and also because research into these soon proved very fruitful, interest in quantum optics rose. Following the work of Dirac in

quantum field theory ,George Sudarshan ,Roy J. Glauber , andLeonard Mandel applied quantum theory to the electromagnetic field in the 1950s and 1960s to gain a more detailed understanding of photodetection and the statistics of light (seedegree of coherence ). This led to the introduction of thecoherent state as a quantum description of laser light and the realization that some states of light could not be described with classical waves. In 1977, Kimble et al. demonstrated the first source of light which required a quantum description: a single atom that emitted one photon at a time. This was the first conclusive evidence that light was made up of photons. Another quantum state of light with certain advantages over any classical state, squeezed light, was soon proposed. At the same time, development of short and ultrashort laser pulses—created byQ switching andmodelocking techniques—opened the way to the study of unimaginably fast ("ultrafast ") processes. Applications for solid state research (e.g.Raman spectroscopy ) were found, and mechanical forces of light on matter were studied. The latter led to levitating and positioning clouds of atoms or even small biological samples in anoptical trap oroptical tweezers by laser beam. This, along withDoppler cooling was the crucial technology needed to achieve the celebratedBose-Einstein condensation .Other remarkable results are the demonstration of quantum entanglement,

quantum teleportation , and (recently, in 1995)quantum logic gate s. The latter are of much interest inquantum information theory , a subject which partly emerged from quantum optics, partly from theoreticalcomputer science .Today's fields of interest among quantum optics researchers include

parametric down-conversion , parametric oscillation, even shorter (attosecond) light pulses, use of quantum optics forquantum information , manipulation of single atoms,Bose-Einstein condensate s, their application, and how to manipulate them (a sub-field often calledatom optics ), and much more.Research into quantum optics that aims to bring photons into use for information transfer and computation is now often called

photonics to emphasize the claim that photons and photonics will take the role thatelectron s andelectronics now have.**Concepts of quantum optics**Quantum optics operatorsAccording to quantum mechanics, light may be considered not only as an electro-magnetic wave but also as a "stream" of particles called

photons which travel with "c", the vacuumspeed of light . These particles should not be considered to be classical billiard balls, but as quantum mechanical particles described by awavefunction spread over a finite region. Each particle carries one quantum of energy equal to "hf", where h isPlanck's constant and f is the frequency of the light. The postulation of thequantization of light byMax Planck in 1899 and the discovery of the general validity of this idea inAlbert Einstein 's 1905 explanation of thephotoelectric effect soon led physicists to realize the possibility ofpopulation inversion and the possibility of thelaser .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 ofquantum electrodynamics .A frequently encountered state of the light field is the

coherent state as introduced byRoy J. Glauber in 1963. This state, which can be used to approximately describe the output of a single-frequencylaser well above the laser threshold, exhibits Poissonian photon number statistics. Via certain nonlinear interactions, a coherent state can be transformed into asqueezed coherent state , which can exhibit super- or sub- Poissonean photon statistics. Such light is called squeezed light. Other important quantum aspects are related to correlations of photon statistics between different beams. For example, parametric nonlinear processes 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 discreteenergy spectrum with the transitions between the energyeigenstate s being driven by the absorption or emission of light according to Einstein's theory with the oscillator strength depending on thequantum number s of the states.For solid state matter one uses the

energy band models ofsolid 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.**ee also***

Optics

*Optical physics

*Nonclassical light **References*** L. Mandel, E. Wolf "Optical Coherence and Quantum Optics" (Cambridge 1995)

* D. F. Walls and G. J. Milburn "Quantum Optics" (Springer 1994)

* C. W. Gardiner andPeter Zoller , "Quantum Noise", (Springer 2004).

* M. O. Scully and M. Zubairy "Quantum Optics" (Cambridge 1997)

* W. P. Schleich "Quantum Optics in Phase Space" (Wiley 2001)**External links*** [

*http://gerdbreitenbach.de/gallery An introduction to quantum optics of the light field*]

* [*http://www.rp-photonics.com/encyclopedia.html Encyclopedia of laser physics and technology*] , with content on quantum optics (particularly quantum noise in lasers), by Rüdiger Paschotta.

* [*http://qwiki.caltech.edu/ Qwiki*] - A quantum physics wiki devoted to providing technical resources for practicing quantum physicists.

* [*http://www.quantiki.org/ Quantiki*] - a free-content WWW resource in quantum information science that anyone can edit.

* [*http://www.physics.drexel.edu/~tim/Decoherence/index.html Various Quantum Optics Reports*]

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