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Quantum optics is a branch ofatomic, molecular, and optical physics andquantum chemistry that studies the behavior ofphotons (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 ofquantum mechanics, such asentanglement andteleportation, and are a useful resource forquantum information processing.
Light propagating in a restricted volume of space has itsenergy andmomentum quantized according to an integer number of particles known asphotons. 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 theblackbody radiation spectrum byMax Planck in 1899 under the hypothesis of light being emitted in discrete units of energy. Thephotoelectric effect was further evidence of this quantization as explained byAlbert Einstein in a 1905 paper, a discovery for which he was to be awarded theNobel Prize in 1921.Niels Bohr showed that the hypothesis of optical radiation being quantized corresponded to his theory of thequantized energy levels of atoms, and thespectrum ofdischarge emission fromhydrogen in particular. The understanding of the interaction between light andmatter following these developments was crucial for the development ofquantum 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 ofatom physics andquantum 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 –discuss], and the namequantum 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 ofDirac inquantum field theory,John R. Klauder,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 thestatistics of light (seedegree of coherence). This led to the introduction of thecoherent state as a concept that addressed variations between laser light, thermal light, exoticsqueezed states, etc. as it became understood that light cannot be fully described just referring to theelectromagnetic fields describing the waves in the classical picture. In 1977,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 assqueezed light were subsequently discovered.
Development of short andultrashort laser pulses—created byQ switching andmodelocking techniques—opened the way to the study of what became known as ultrafast processes.[1] 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 andSisyphus cooling, was the crucial technology needed to achieve the celebratedBose–Einstein condensation.
Other remarkable results are thedemonstration of quantum entanglement,quantum teleportation, andquantum logic gates. The latter are of much interest inquantum information theory, a subject that partly emerged from quantum optics, partly from theoreticalcomputer science.[2]
Today's fields of interest among quantum optics researchers includeparametric down-conversion,parametric oscillation, even shorter (attosecond) light pulses, use of quantum optics forquantum information, manipulation of single atoms,Bose–Einstein condensates, their application, and how to manipulate them (a sub-field often calledatom optics),coherent perfect absorbers, and many more. Topics classified under the term of quantum optics, especially as applied to engineering and technological innovation, often go under the modern termphotonics.
SeveralNobel Prizes have been awarded for work in quantum optics. These were awarded as follows:
According toquantum theory, light may be considered not only to be anelectro-magnetic wave but also a "stream" of particles calledphotons, which travel withc, thespeed of light in vacuum. These particles should not be considered to beclassical billiard balls, but quantum mechanical particles described by awavefunction spread over a finite region.
Each particle carries one quantum of energy, equal tohf, whereh is thePlanck constant andf 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 ofspontaneous emission also predicted the existence ofstimulated emission, the principle upon which thelaser rests. However, the actual invention of themaser (and laser) many years later was dependent on a method to produce apopulation inversion.
The use ofstatistical 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 ofquantum electrodynamics.
A frequently encountered state of the light field is thecoherent state, as introduced byE.C. George Sudarshan in 1960. This state, which can be used to approximately describe the output of a single-frequencylaser well above the laser threshold, exhibitsPoissonian photon number statistics. Via certainnonlinear interactions, a coherent state can be transformed into asqueezed coherent state, by applying a squeezing operator that can exhibitsuper- orsub-Poissonian photon statistics. Such light is calledsqueezed 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 mechanicaloscillators with adiscreteenergy spectrum, with the transitions between the energyeigenstates being driven by the absorption or emission of light according to Einstein's theory.
For solid state matter, one uses theenergy band models ofsolid state physics. This is important for understanding how light is detected by solid-state devices, commonly used in experiments.
Quantum electronics is a term that was used mainly between the 1950s and 1970s[8] to denote the area ofphysics dealing with the effects ofquantum mechanics on the behavior ofelectrons in matter, together with their interactions withphotons. 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 inelectronics is researched withinsemiconductor physics. The term also encompassed the basic processes oflaser operation, which is today studied as a topic in quantum optics. Usage of the term overlapped early work on thequantum Hall effect andquantum cellular automata.
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