Quantum optics is a field of research in physics, dealing with the application of quantum mechanics to phenomena involving light and its interactions with matter.
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Light is made up of particles that tah hjbdfashdvasdm,vef,gnvjkae sdvljasdnjafnaj,.sgfrn. asvj jfnaler jnvrjk egargcalled 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 from Max Planck in 1899 when he correctly modelled blackbody radiation by assuming thr 4vtBohr]] showed that the atoms vtwere also quantized, in the sense that they could only emit discrete amounts of energy. The understanding of the interaction between light and matter fol3tlowing from these developments not only formed the basis of quantum optics but also were crucial for the development of quantum mechanics as a whole. Howe yewbvyegrcver, the subfields of quantum mechanics dealing with matterlight interaction were principally regarded as research into matter rather than into light and hence, one rather spoke of atom physics and quantum electronics.5vaser]] 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 resetx4q234ctvarch 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, 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 statistics of light (see degree of coherence). This led to the introduction of the coherent 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 by Q switching and modelocking 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 an optical trap or optical tweezers by laser beam. This, along with Doppler cooling was the crucial technology needed to achieve the celebrated BoseEinstein condensation.
Other remarkable results are the demonstration of quantum entanglement, quantum teleportation, and (recently, in 1995) quantum logic gates. The latter are of much interest in quantum information theory, a subject which partly emerged from quantum optics, partly from theoretical computer science.
Today's fields of interest among quantum optics researchers include parametric downconversion, parametric oscillation, even shorter (attosecond) light pulses, use of quantum optics for quantum information, manipulation of single atoms, BoseEinstein condensates, their application, and how to manipulate them (a subfield often called atom 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 that electrons and electronics now have.
Quantum optics operators 

Ladder operators 
Creation and annihilation operators 
Displacement operator 
Rotation operator (quantum mechanics) 
Squeeze operator 
Antisymmetric operator 
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According to quantum theory, light may be considered not only as an electromagnetic wave but also as a "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. 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.
A frequently encountered state of the light field is the coherent state as introduced by Roy J. Glauber in 1963. This state, which can be used to approximately describe the output of a singlefrequency laser well above the laser threshold, exhibits Poissonian photon number statistics. Via certain nonlinear interactions, a coherent state can be transformed into a squeezed 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 socalled twin beams, where ideally each photon of one beam is associated with a photon in the other beam.
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 solidstate device that absorbs it) is crucial for understanding experiments.
