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{{Short description|Sub-field of quantum physics and optics}}
{{Redirect|Quantum electronics|the journal|Quantum Electronics (journal)}}
'''Quantum optics''' is
==History==
Light propagating in a
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 [[Paul Dirac|Dirac]] in [[quantum field theory]], [[John R. Klauder]], [[George Sudarshan]], [[Roy J. Glauber]], and [[Leonard Mandel]] applied quantum theory to the electromagnetic field in the 1950s and 1960s to gain a more detailed understanding of photodetection and the [[statistical mechanics|statistics]] of light (see [[degree of coherence]]). This led to the introduction of the [[coherent state]] as a concept which addressed variations between laser light, thermal light, exotic [[squeezed state]]s, etc. as it became understood that light cannot be fully described just referring to the [[electromagnetic field]]s describing the waves in the classical picture. In 1977, [[H. Jeff Kimble|Kimble]] et al. demonstrated a single atom emitting one photon at a time, further compelling evidence that light consists of photons. Previously unknown quantum states of light with characteristics unlike classical states, such as [[Squeezed coherent state|squeezed light]] were subsequently discovered.
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Several [[Nobel prize]]s have been awarded for work in quantum optics. These were awarded:
* in 2022, [[Alain Aspect]], [[John Clauser]] and [[Anton Zeilinger]] "for experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science".<ref>[https://www.nobelprize.org/prizes/physics/2022/summary/ "The Nobel Prize in Physics 2022"]. Nobel Foundation. Retrieved 9 June 2023.</ref>
* in 2012, [[Serge Haroche]] and [[David J. Wineland]] "for ground-breaking experimental methods that enable measuring & manipulation of individual quantum systems".<ref>[https://www.nobelprize.org/nobel_prizes/physics/laureates/2012/index.html "The Nobel Prize in Physics 2012"]. Nobel Foundation. Retrieved 9 October 2012.</ref>
* in 2005, [[Theodor W. Hänsch]], [[Roy J. Glauber]] and [[John L. Hall]]<ref>{{cite web|url=https://www.nobelprize.org/nobel_prizes/physics/laureates/2005/ |title=The Nobel Prize in Physics 2005 |publisher=Nobelprize.org |access-date=2015-10-14}}</ref>
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==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 vacuum [[speed of light]]. 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 [[Planck's 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:
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 which 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.
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==Quantum electronics==
'''Quantum electronics''' is a term that was used mainly between the 1950s and 1970s<ref>{{Cite book |last=Brunner |first=Witlof |title=Quantenelektronik |last2=Radloff |first2=Wolfgang |last3=Junge |first3=Klaus |publisher=[[Deutscher Verlag der Wissenschaften]] |year=1975 |language=de}}</ref> to denote the area of [[physics]] dealing with the effects of [[quantum mechanics]] on the behavior of [[electron]]s in matter, together with their interactions with [[photon]]s. Today, it is rarely considered a sub-field in its own right, and it has been absorbed by other fields. [[Solid state physics]] regularly takes quantum mechanics into account, and is usually concerned with electrons. Specific applications of quantum mechanics in [[electronics]] is researched within [[semiconductor physics]]. The term also encompassed the basic processes of [[laser]] operation, which is today studied as a topic in quantum optics. Usage of the term overlapped early work on the [[quantum Hall effect]] and [[quantum cellular automata]].
==See also==
{{Portal|Physics}}
{{Div col|colwidth=20em}}
* [[Atomic, molecular, and optical physics]]
*[[Nonclassical light]]▼
* [[Attophysics]]
*[[Cavity optomechanics|Optomechanics]]▼
▲* [[Nonclassical light]]
*[[Coherent control|Quantum control]]▼
▲* [[Cavity optomechanics|Optomechanics]]
*[[Optical phase space]]▼
▲* [[Coherent control|Quantum control]]
*[[Optical physics]]▼
▲* [[Optical phase space]]
▲* [[Optical physics]]
*[[Quantization of the electromagnetic field]]▼
* [[
▲* [[Quantization of the electromagnetic field]]
*[[Valleytronics]]▼
* [[Two-state quantum system]]
* [[Spinplasmonics]]
▲* [[Valleytronics]]
{{Div col end}}
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==References==
* {{cite book|title=Introduction to Quantum Optics|last1=Gerry|first1=Christopher|last2=Knight|first2=Peter|year=2004|publisher=Cambridge University Press|isbn= 052152735X}}
* [https://www.nobelprize.org/nobel_prizes/physics/laureates/2005/ The Nobel Prize in Physics 2005]
==Further reading==
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==External links==
{{Spoken Wikipedia|QuantumOpticsFull.ogg|date=2009-08-11}}
* [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.
* [https://web.archive.org/web/20090924044639/http://qwiki.stanford.edu/wiki/Quantum_Optics 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]
{{Branches of physics}}
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