Superradiance

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In physics, superradiance is the radiation enhancement effects in several contexts including quantum mechanics, astrophysics and relativity.

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Quantum optics

For want of a better term, a gas which is radiating strongly because of coherence will be called "super-radiant".

—  Robert H. Dicke, 1954, [1]

In quantum optics, 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, [2] then the emitters interact with the light in a collective and coherent fashion. [3] This causes the group to emit light as a high-intensity pulse (with rate proportional to N2). This is a surprising result, drastically different from the expected exponential decay (with rate proportional to N) of a group of independent atoms (see spontaneous emission). Superradiance has since been demonstrated in a wide variety of physical and chemical systems, such as quantum dot arrays [4] and J-aggregates. [5] This effect has been used to produce a superradiant laser.

Rotational superradiance

Rotational superradiance [6] 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). 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 (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).

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 frictional 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 theoretical studies of black holes, the effect is also sometimes described as the consequence of the gravitational tidal forces 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 is an exponentially growing instability in the interaction between a massive bosonic field and a rotating black hole.

Astrophysics and relativity

In astrophysics, a potential example of superradiance is Zeldovich radiation. [7] It was Yakov Zeldovich who first described this effect in 1971, [8] Igor Novikov at the University of Moscow further developed the theory. Zeldovich picked the case under quantum electrodynamics (QED) where the region around the equator of a spinning metal sphere is expected to throw off electromagnetic radiation tangentially, and suggested that the case of a spinning gravitational mass, such as a Kerr black hole ought to produce similar coupling effects, and ought to radiate in an analogous way.

This was followed by arguments from Stephen Hawking 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 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 into a "real" particle [9] (see Hawking radiation).

Similar arguments apply for the cases of observers in accelerated frames (Unruh radiation). Cherenkov radiation, electromagnetic radiation emitted by charged particles travelling through a particulate medium at more than the nominal speed of light in that medium, has also been described as "inertial motion superradiance". [6]

Additional examples of superradiance in astrophysical environments include the study of radiation flares in maser-hosting regions [10] [11] and fast radio bursts. [12] 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 [13] to possibly over several billion kilometres [12] ).

Instruments

Instruments that uses the super radiant emission.

See also

Related Research Articles

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<span class="mw-page-title-main">Photoelectric effect</span> Emission of electrons when electromagnetic radiation hits a material

The photoelectric effect is the emission of electrons from a material caused by electromagnetic radiation such as ultraviolet light. Electrons emitted in this manner are called photoelectrons. The phenomenon is studied in condensed matter physics, solid state, and quantum chemistry to draw inferences about the properties of atoms, molecules and solids. The effect has found use in electronic devices specialized for light detection and precisely timed electron emission.

<span class="mw-page-title-main">Radiation pressure</span> Pressure exerted upon any surface exposed to electromagnetic radiation

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<span class="mw-page-title-main">Spectral line</span> A distinctive narrow spectral feature of chemical species

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<span class="mw-page-title-main">W. G. Unruh</span> Canadian physicist

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<span class="mw-page-title-main">Quantum field theory in curved spacetime</span> Extension of quantum field theory to curved spacetime

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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 amplification can be repeated, leading to a run-away growth of the field, i.e. an explosion. This explosion can be as powerful as a supernova. One way this reflection could be realized in nature is if the bosonic field has mass. The mass of the field can then cause the amplified modes to be trapped around the black hole, leading to an endless cycle of self-amplification. The mechanism by which the black hole bomb functions is called superradiant instability. It can also refer to one such method of creating such a runaway effect, a Penrose sphere with no means for energy to passively escape.

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The Dicke model is a fundamental model of quantum optics, which describes the interaction between light and matter. In the Dicke model, the light component is described as a single quantum mode, while the matter is described as a set of two-level systems. When the coupling between the light and matter crosses a critical value, the Dicke model shows a mean-field phase transition to a superradiant phase. This transition belongs to the Ising universality class and was realized in cavity quantum electrodynamics experiments. Although the superradiant transition bears some analogy with the lasing instability, these two transitions belong to different universality classes.

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References

  1. ↑ Dicke, Robert H. (1954). "Coherence in Spontaneous Radiation Processes". Physical Review. 93 (1): 99–110. Bibcode:1954PhRv...93...99D. doi: 10.1103/PhysRev.93.99 .
  2. ↑ Y. Pinhasi (2002). "Generalized theory and simulation of spontaneous and super-radiant emissions in electron devices and free-electron lasers". Physical Review. 65 (2): 1–8. Bibcode:2002PhRvE..65b6501P. doi:10.1103/PhysRevE.65.026501. PMID   11863669.
  3. ↑ Gross, M.; Haroche, S. (1 December 1982). "Superradiance: An essay on the theory of collective spontaneous emission". Physics Reports. 93 (5): 301–396. Bibcode:1982PhR....93..301G. doi:10.1016/0370-1573(82)90102-8.
  4. ↑ Scheibner, Michael; Schmidt, T.; Worschech, L.; Forchel, A.; Bacher, G.; Passow, T.; Hommel, D. (2007). "Superradiance of quantum dots". Nature Physics. 3 (2): 106–110. Bibcode:2007NatPh...3..106S. doi: 10.1038/nphys494 .
  5. ↑ Benedict, M. G. (1996). Super-radiance: multiatomic coherent emission. Bristol [u.a.]: Inst. of Physics Publ. ISBN   0750302836.
  6. 1 2 Bekenstein, Jacob; Schiffer, Marcelo (1998). "The many faces of superradiance". Physical Review D. 58 (6): 064014. arXiv: gr-qc/9803033 . Bibcode:1998PhRvD..58f4014B. doi:10.1103/PhysRevD.58.064014. S2CID   14585592.
  7. ↑ Thorne, Kip S. (1994). Black holes and timewarps: Einstein's outrageous legacy. p. 432.
  8. ↑ Zeldovich, Yakov Borisovich (1971). "Generation of waves by a rotating body" (PDF). ZhETF Pisma Redaktsiiu. 14: 270. Bibcode:1971ZhPmR..14..270Z. Archived from the original (PDF) on 2018-05-20. Retrieved 2018-05-20.
  9. ↑ Thorne, K.S. (1986). Black holes: the membrane paradigm. New Haven: Yale University Press. ISBN   978-0300037692.
  10. ↑ Rajabi, F.; Houde, M. (2016). "DICKE'S SUPERRADIANCE IN ASTROPHYSICS. I. THE 21 cm LINE". The Astrophysical Journal. 826 (2): 216. arXiv: 1601.01717 . Bibcode:2016ApJ...826..216R. doi: 10.3847/0004-637X/826/2/216 . S2CID   28730845.
  11. ↑ Rajabi, Fereshteh (2016). "DICKE'S SUPERRADIANCE IN ASTROPHYSICS. II. THE OH 1612 MHz LINE". The Astrophysical Journal. 828 (1): 57. arXiv: 1601.01718 . Bibcode:2016ApJ...828...57R. doi: 10.3847/0004-637X/828/1/57 . S2CID   20321318.
  12. 1 2 Houde, M.; Mathews, A.; Rajabi, F. (12 December 2017). "Explaining fast radio bursts through Dicke's superradiance". Monthly Notices of the Royal Astronomical Society. 475 (1): 514. arXiv: 1710.00401 . Bibcode:2018MNRAS.475..514H. doi: 10.1093/mnras/stx3205 . S2CID   119240095.
  13. ↑ Rajabi, F.; Houde, M. (2017). "Explaining recurring maser flares in the ISM through large-scale entangled quantum mechanical states". Science Advances. 3 (3): e1601858. arXiv: 1704.01491 . Bibcode:2017SciA....3E1858R. doi:10.1126/sciadv.1601858. PMC   5365248 . PMID   28378015.
  14. ↑ D. P. Scherrer; F. K. Kneubuhl (1993). "New phenomena related to pulsed far-infrared superradiant and Raman emission". Infrared Physics. 34 (3): 227–267. Bibcode:1993InfPh..34..227S. doi:10.1016/0020-0891(93)90013-W.
  15. ↑ M. Arbel; A. Abramovich; A. L. Eichenbaum; A. Gover; H. Kleinman; Y. Pinhasi; I. M. Yakover1 (2001). "Superradiant and Stimulated Superradiant Emission in a Prebunched Beam Free-Electron Maser". Physical Review. 86 (12): 2561–2564. Bibcode:2001PhRvL..86.2561A. doi:10.1103/PhysRevLett.86.2561. PMID   11289980.{{cite journal}}: CS1 maint: numeric names: authors list (link)
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