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{{Short description|Sub-field of quantum physics and optics}}
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'''Quantum optics''' is a branch of [[atomic, molecular, and optical physics]] and [[quantum chemistry]] that studies the behavior of [[photon]]s (individual quanta of light). It includes the study of the particle-like properties of photons and their interaction with, for instance, atoms and molecules. Photons have been used to test many of the counter-intuitive predictions of [[Quantum Mechanics|quantum mechanics]], such as [[Quantum entanglement|entanglement]] and [[Quantum teleportation|teleportation]], and are a useful resource for [[Quantum information science|quantum information processing]].

== History ==
Light propagating in a restricted volume of space has its [[energy]] and [[momentum]] quantized according to an integer number of particles known as [[photons]].  Quantum optics studies the nature and effects of light as quantized photons. The first major development leading to that understanding was the correct modeling of the [[blackbody radiation]] spectrum by  [[Max Planck]] in 1899 under the hypothesis of light being emitted in discrete units of energy. The [[photoelectric effect]] was further evidence of this quantization as explained by [[Albert Einstein]] in a 1905 paper, a discovery for which he was to be awarded the [[Nobel Prize]] in 1921. [[Niels Bohr]] showed that the hypothesis of optical radiation being quantized corresponded to his theory of the [[quantized energy levels of atoms]], and the [[spectrum]] of [[Gas-discharge lamp|discharge emission]] from [[hydrogen]] in particular. The understanding of the interaction between light and [[matter]] following these developments was 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; hence one rather spoke of [[atom physics]] and [[quantum electronics]] 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{{dubious|date=May 2013}}, 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 [[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 that 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.

Development of short and [[ultrashort pulse|ultrashort]] laser pulses—created by [[Q switching]] and [[modelocking]] techniques—opened the way to the study of what became known as 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 an [[optical trap]] or [[optical tweezers]] by laser beam. This, along with [[Doppler cooling]] and [[Sisyphus cooling]], was the crucial technology needed to achieve the celebrated [[Bose–Einstein condensation]].

Other remarkable results are the [[Bell test experiments|demonstration of quantum entanglement]], [[quantum teleportation]], and [[quantum logic gate]]s. The latter are of much interest in [[quantum information theory]], a subject that partly emerged from quantum optics, partly from theoretical [[computer science]].<ref>{{cite book|last1=Nielsen|first1=Michael A.|last2=Chuang|first2=Isaac L.|title=Quantum computation and quantum information|date=2010|publisher=Cambridge University Press|location=Cambridge|isbn=978-1107002173|edition=10th anniversary}}</ref>

Today's fields of interest among quantum optics researchers include [[parametric down-conversion]], [[Optical parametric oscillator|parametric oscillation]], even shorter (attosecond) light pulses, use of quantum optics for [[quantum information]], manipulation of single atoms, [[Bose–Einstein condensate]]s, their application, and how to manipulate them (a sub-field often called [[atom optics]]), [[coherent perfect absorber]]s, and many more. Topics classified under the term of quantum optics, especially as applied to engineering and technological innovation, often go under the modern term [[photonics]].

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>
* in 2001, [[Wolfgang Ketterle]], [[Eric Allin Cornell]] and [[Carl Wieman]]<ref>{{cite web|url=https://www.nobelprize.org/nobel_prizes/physics/laureates/2001/ |title=The Nobel Prize in Physics 2001 |publisher=Nobelprize.org |access-date=2015-10-14}}</ref>
* in 1997, [[Steven Chu]], [[Claude Cohen-Tannoudji]] and [[William Daniel Phillips]] for [[laser cooling]]<ref>{{cite web|url=https://www.nobelprize.org/prizes/physics/1997/summary/ |title=The Nobel Prize in Physics 1997 |publisher=Nobelprize.org |access-date=2015-10-14}}</ref>

== 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.

== 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]]
* [[Attophysics]]
* [[Nonclassical light]]
* [[Cavity optomechanics|Optomechanics]]
* [[Coherent control|Quantum control]]
* [[Optical phase space]]
* [[Optical physics]]
* [[Optics]]
* [[Quantization of the electromagnetic field]]
* [[Two-state quantum system]]
* [[Spinplasmonics]]
* [[Valleytronics]]
{{div col end}}

== Notes ==
{{reflist<!--as yet, too few references to need: |colwidth=30em-->}}

== 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 ==
* [[Leonard Mandel|L. Mandel]], [[Emil Wolf|E. Wolf]] ''Optical Coherence and Quantum Optics'' (Cambridge 1995).
* [[Daniel Frank Walls|D. F. Walls]] and [[Gerard J. Milburn|G. J. Milburn]] ''Quantum Optics'' (Springer 1994).
* [[Crispin Gardiner]] and [[Peter Zoller]], ''Quantum Noise'' (Springer 2004).
* [[Héctor Manuel Moya Cessa|H.M. Moya-Cessa]] and F. Soto-Eguibar, ''Introduction to Quantum Optics'' (Rinton Press 2011).
* [[Marlan O. Scully|M. O. Scully]] and  [[Muhammad Suhail Zubairy|M. S. Zubairy]] ''Quantum Optics'' (Cambridge 1997).
* [[Wolfgang P. Schleich|W. P. Schleich]] ''Quantum Optics in Phase Space'' (Wiley 2001).
* {{cite book|last1=Kira|first1=M.|last2=Koch|first2=S. W.|title=Semiconductor Quantum Optics|year=2011|publisher=Cambridge University Press|isbn=978-0521875097}}
* {{cite book |title=  Quantum Optics for Engineers | publisher= CRC |location=New York |year=2014 |isbn=978-1439888537 | author = F. J. Duarte | author-link = F. J. Duarte }}

== 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}}
{{Physics operator}}
{{Quantum mechanics topics}}
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[[Category:Quantum optics| ]]
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