physics Physics is the scientific study of matter, its Elementary particle, fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge whi ...

, superradiance, or superradiation, is the radiation enhancement effects in several contexts including

quantum mechanics Quantum mechanics is the fundamental physical Scientific theory, theory that describes the behavior of matter and of light; its unusual characteristics typically occur at and below the scale of atoms. Reprinted, Addison-Wesley, 1989, It is ...

astrophysics Astrophysics is a science that employs the methods and principles of physics and chemistry in the study of astronomical objects and phenomena. As one of the founders of the discipline, James Keeler, said, astrophysics "seeks to ascertain the ...

and relativity.

Quantum optics

quantum optics Quantum optics is a branch of atomic, molecular, and optical physics and quantum chemistry that studies the behavior of photons (individual quanta of light). It includes the study of the particle-like properties of photons and their interaction ...

, superradiance is a phenomenon that occurs when a group of ''N'' emitters, such as excited atoms, interact with a common light field. If the wavelength of the light is much greater than the separation of the emitters, then the emitters interact with the light in a collective and coherent fashion. This causes the group to emit light as a high-intensity pulse (with rate proportional to ''N''²). This is a surprising result, drastically different from the expected

exponential decay A quantity is subject to exponential decay if it decreases at a rate proportional to its current value. Symbolically, this process can be expressed by the following differential equation, where is the quantity and (lambda Lambda (; uppe ...

(with rate proportional to ''N'') of a group of independent atoms (see

spontaneous emission Spontaneous emission is the process in which a Quantum mechanics, quantum mechanical system (such as a molecule, an atom or a subatomic particle) transits from an excited state, excited energy state to a lower energy state (e.g., its ground state ...

). Superradiance has since been demonstrated in a wide variety of physical and chemical systems, such as

quantum dot Quantum dots (QDs) or semiconductor nanocrystals are semiconductor particles a few nanometres in size with optical and electronic properties that differ from those of larger particles via quantum mechanical effects. They are a central topic i ...

arrays and

J-aggregate A J-aggregate is a type of dye with an absorption band that shifts to a longer wavelength ( bathochromic shift) of increasing sharpness (higher absorption coefficient) when it aggregates under the influence of a solvent or additive or concentratio ...

s. This effect has been used to produce a

superradiant laser A superradiant laser is a laser that does not rely on a large population of photons within the laser cavity to maintain coherence. Rather than relying on photons to store phase coherence, it relies on collective effects in an atomic medium to stor ...

Rotational superradiance

Rotational superradiance is associated with the acceleration or motion of a nearby body (which supplies the energy and momentum for the effect). It is also sometimes described as the consequence of an "effective" field differential around the body (e.g. the effect of

tidal forces The tidal force or tide-generating force is the difference in gravitational attraction between different points in a gravitational field, causing bodies to be pulled unevenly and as a result are being stretched towards the attraction. It is the d ...

). This allows a body with a concentration of angular or linear momentum to move towards a lower energy state, even when there is no obvious classical mechanism for this to happen. In this sense, the effect has some similarities with

quantum tunnelling In physics, quantum tunnelling, barrier penetration, or simply tunnelling is a quantum mechanical phenomenon in which an object such as an electron or atom passes through a potential energy barrier that, according to classical mechanics, shoul ...

(e.g. the tendency of waves and particles to "find a way" to exploit the existence of an energy potential, despite the absence of an obvious classical mechanism for this to happen). * In classical physics, the motion or rotation of a body in a particulate medium will normally be expected to result in

momentum In Newtonian mechanics, momentum (: momenta or momentums; more specifically linear momentum or translational momentum) is the product of the mass and velocity of an object. It is a vector quantity, possessing a magnitude and a direction. ...

and

energy Energy () is the physical quantity, quantitative physical property, property that is transferred to a physical body, body or to a physical system, recognizable in the performance of Work (thermodynamics), work and in the form of heat and l ...

being transferred to the surrounding particles, and there is then an increased statistical likelihood of particles being discovered following trajectories that imply removal of momentum from the body. * In quantum mechanics, this principle is extended to the case of bodies moving, accelerating or rotating in a

vacuum A vacuum (: vacuums or vacua) is space devoid of matter. The word is derived from the Latin adjective (neuter ) meaning "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressur ...

– in the quantum case,

quantum fluctuation In quantum physics, a quantum fluctuation (also known as a vacuum state fluctuation or vacuum fluctuation) is the temporary random change in the amount of energy in a point in space, as prescribed by Werner Heisenberg's uncertainty principle. ...

s with appropriate vectors are said to be stretched and distorted and provided with energy and momentum by the nearby body's motion, with this selective amplification generating real physical radiation around the body. Where a classical description of a rotating isolated weightless sphere in a vacuum will tend to say that the sphere will continue to rotate indefinitely, due to the lack of

friction Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. Types of friction include dry, fluid, lubricated, skin, and internal -- an incomplete list. The study of t ...

al effects or any other form of obvious coupling with its smooth empty environment, under quantum mechanics the surrounding region of vacuum is not entirely smooth, and the sphere's field can couple with quantum fluctuations and accelerate them to produce real radiation. Hypothetical virtual wavefronts with appropriate paths around the body are stimulated and amplified into real physical wavefronts by the coupling process. Descriptions sometimes refer to these fluctuations "tickling" the field to produce the effect.

In black holes

In theoretical studies of black holes, the effect is also sometimes described as the consequence of the gravitational

around a strongly gravitating body pulling apart virtual particle pairs that would otherwise quickly mutually annihilate, to produce a population of real particles in the region outside the horizon. The

black hole bomb A black hole bomb is the name given to a physical effect utilizing how a bosonic field impinging on a rotating black hole can be amplified through superradiant scattering. If the amplified field is reflected back towards the black hole, the ampl ...

is an exponentially growing instability in the interaction between a massive

bosonic field In quantum field theory, a bosonic field is a quantum field whose quanta are bosons; that is, they obey Bose–Einstein statistics. Bosonic fields obey canonical commutation relations, as distinct from the canonical anticommutation relations obey ...

and a rotating black hole.

Astrophysics and relativity

, a potential example of superradiance is Zeldovich radiation. It was

Yakov Zeldovich Yakov Borisovich Zeldovich (, ; 8 March 1914 – 2 December 1987), also known as YaB, was a leading Soviet people, Soviet Physics, physicist of Belarusians, Belarusian origin, who is known for his prolific contributions in physical Physical c ...

who first described this effect in 1971, Igor Novikov at the University of Moscow further developed the theory. Zeldovich picked the case under

quantum electrodynamics In particle physics, quantum electrodynamics (QED) is the Theory of relativity, relativistic quantum field theory of electrodynamics. In essence, it describes how light and matter interact and is the first theory where full agreement between quant ...

(QED) where the region around the equator of a spinning metal sphere is expected to throw off

electromagnetic radiation In physics, electromagnetic radiation (EMR) is a self-propagating wave of the electromagnetic field that carries momentum and radiant energy through space. It encompasses a broad spectrum, classified by frequency or its inverse, wavelength ...

tangent In geometry, the tangent line (or simply tangent) to a plane curve at a given point is, intuitively, the straight line that "just touches" the curve at that point. Leibniz defined it as the line through a pair of infinitely close points o ...

ially, and suggested that the case of a spinning gravitational mass, such as a

Kerr black hole The Kerr metric or Kerr geometry describes the geometry of empty spacetime around a rotating uncharged axially symmetric black hole with a quasispherical event horizon. The Kerr metric is an exact solution of the Einstein field equations of gen ...

ought to produce similar coupling effects, and ought to radiate in an

analogous Analogy is a comparison or correspondence between two things (or two groups of things) because of a third element that they are considered to share. In logic, it is an inference or an argument from one particular to another particular, as oppose ...

way. This was followed by arguments from

Stephen Hawking Stephen William Hawking (8January 194214March 2018) was an English theoretical physics, theoretical physicist, cosmologist, and author who was director of research at the Centre for Theoretical Cosmology at the University of Cambridge. Between ...

and others that an accelerated observer near a black hole (e.g. an observer carefully lowered towards the horizon at the end of a rope) ought to see the region inhabited by "real" radiation, whereas for a distant observer this radiation would be said to be "virtual". If the accelerated observer near the

event horizon In astrophysics, an event horizon is a boundary beyond which events cannot affect an outside observer. Wolfgang Rindler coined the term in the 1950s. In 1784, John Michell proposed that gravity can be strong enough in the vicinity of massive c ...

traps a nearby particle and throws it out to the distant observer for capture and study, then for the distant observer, the appearance of the particle can be explained by saying that the physical acceleration of the particle has turned it from a

virtual particle A virtual particle is a theoretical transient particle that exhibits some of the characteristics of an ordinary particle, while having its existence limited by the uncertainty principle, which allows the virtual particles to spontaneously emer ...

into a "real" particle (see

Hawking radiation Hawking radiation is black-body radiation released outside a black hole's event horizon due to quantum effects according to a model developed by Stephen Hawking in 1974. The radiation was not predicted by previous models which assumed that onc ...

). Similar arguments apply for the cases of observers in accelerated frames ( Unruh radiation).

Cherenkov radiation Cherenkov radiation () is electromagnetic radiation emitted when a charged particle (such as an electron) passes through a dielectric medium (such as distilled water) at a speed greater than the phase velocity (speed of propagation of a wavefro ...

, electromagnetic radiation emitted by charged particles travelling through a particulate medium at more than the nominal

speed of light The speed of light in vacuum, commonly denoted , is a universal physical constant exactly equal to ). It is exact because, by international agreement, a metre is defined as the length of the path travelled by light in vacuum during a time i ...

in that medium, has also been described as "inertial motion superradiance". Additional examples of superradiance in astrophysical environments include the study of radiation flares in maser-hosting regions and

fast radio burst In radio astronomy, a fast radio burst (FRB) is a transient radio wave of length ranging from a fraction of a millisecond, for an ultra-fast radio burst, to 3 seconds, caused by a high-energy astrophysical process as yet not understood. Astronome ...

s. Evidence of superradiance in these settings suggests the existence of intense emissions from entangled quantum mechanical states, involving a very large number of molecules, ubiquitously present across the universe and spanning large distances (e.g. from a few kilometres in the interstellar medium to possibly over several billion kilometres).

Instruments

Instruments that uses the superradiant emission. *

Free-electron laser A free-electron laser (FEL) is a fourth generation light source producing extremely brilliant and short pulses of radiation. An FEL functions much as a laser but employs relativistic electrons as a active laser medium, gain medium instead of using ...

(FEL) * Nitrogen lasers using transverse excitation at atmospheric pressure (TEA) * Far-infrared (FIR) laser *

Undulator An undulator is an insertion device from high-energy physics and usually part of a larger installation, a synchrotron storage ring, or it may be a component of a free electron laser. It consists of a periodic structure of dipole magnets. These ca ...

allows to obtain the superradiant emission.

References