Quantum optics
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.
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 this. The first indication that light might be quantized came from
Max Planck in
1899 when he correctly modelled
blackbody 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 the
photoelectric effect. It appeared that the only possible explanation for the effect was the existence of particles of light called photons. Later,
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 and
matter 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 of
atom physics and
quantum electronics.
This changed with the invention of the
maser in
1953 and the
laser in 1960.
Laser scienceâ€"i.e., research into principles, design and application of these devicesâ€"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, 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 pulsesâ€"created by
Q switching and
modelocking techniquesâ€"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
Bose-Einstein 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 down-conversion,
parametric oscillation, even shorter (
attosecond) light pulses, use of quantum optics for
quantum information, manipulation of single atoms,
Bose-Einstein condensates, their application, and how to manipulate them (a sub-field 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.
According 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 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 photonsâ€"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 single-frequency
laser well above the laser threshold, exhibits
Poissonean 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 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
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 solid-state device that
absorbs it) is crucial for understanding experiments.
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Optics*
Optical physics*
An introduction to quantum optics of the light field*
Encyclopedia of laser physics and technology, with content on quantum optics (particularly quantum noise in lasers), by RĂĽdiger Paschotta.
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Qwiki - A quantum physics wiki devoted to providing technical resources for practicing quantum physicists.
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An introduction to quantum optics, with a description of optical cavities, quantum dots, vacuum-field Rabi oscillations, the semi-classical Rabi model and the Jaynes-Cummings model.
