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{{Short description|Elementary quantum of the electromagnetic field}}

{{Quantum matter backlink|Particles}}

A '''quantum photon''' is the elementary quantum of the [[Physics:Quantum electromagnetic field|electromagnetic field]]. It is the particle-like unit of electromagnetic radiation, including visible light, radio waves, X-rays, and gamma rays. In the [[Physics:Quantum Standard Model|Standard Model]], the photon is an elementary [[Physics:Quantum boson|boson]] and the gauge boson of the electromagnetic interaction.<ref name="pdg2008">{{cite journal |last1=Amsler |first1=C. |display-authors=etal |collaboration=Particle Data Group |year=2008 |title=Review of Particle Physics |journal=Physics Letters B |volume=667 |issue=1 |pages=1–134 |doi=10.1016/j.physletb.2008.07.018 |bibcode=2008PhLB..667....1A |hdl=1854/LU-685594 |s2cid=227119789 |hdl-access=free}}</ref>

Photons are usually described as massless, electrically neutral, stable spin-1 particles. In vacuum they propagate at the speed of light and have two physical polarization states. A photon carries energy and momentum even though it has no rest mass, and its energy is proportional to its frequency.<ref name="hecht">{{Cite book |last=Hecht |first=Eugene |title=Optics |edition=3rd |publisher=Addison-Wesley |year=1998 |isbn=978-0-201-83887-9}}</ref><ref name="schwartz">{{cite book |first=Matthew D. |last=Schwartz |title=Quantum Field Theory and the Standard Model |publisher=Cambridge University Press |year=2014 |isbn=978-1-107-03473-0}}</ref>

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[[File:Quantum_photon_yellow.png|400px|center]]
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A photon is the quantum particle of electromagnetic radiation.
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== Quantum description ==

In quantum physics, a photon is not simply a tiny classical object. It is a quantum excitation of an electromagnetic mode. When light is emitted or absorbed, energy is transferred in discrete units called photons. The energy of one photon is

:<math>E = h\nu = \hbar\omega</math>

where <math>h</math> is the Planck constant, <math>\nu</math> is the frequency, <math>\hbar</math> is the reduced Planck constant, and <math>\omega</math> is the angular frequency. In terms of wavelength,

:<math>E = \frac{hc}{\lambda}</math>

where <math>c</math> is the speed of light in vacuum and <math>\lambda</math> is the wavelength.<ref name="hecht" />

The momentum of a photon is

:<math>p = \frac{h}{\lambda} = \frac{E}{c}</math>.

This relation is consistent with the relativistic energy–momentum relation for a particle with zero rest mass.

== Bosonic nature ==

A photon is a spin-1 boson. Because photons obey Bose–Einstein statistics, more than one photon can occupy one shared quantum state. This property is central to coherent light, lasers, black-body radiation, and the quantum theory of electromagnetic fields.<ref name="bose">{{cite journal |last=Bose |first=Satyendra Nath |year=1924 |title=Plancks Gesetz und Lichtquantenhypothese |journal=Zeitschrift für Physik |volume=26 |issue=1 |pages=178–181 |doi=10.1007/BF01327326 |bibcode=1924ZPhy...26..178B |s2cid=186235974}}</ref><ref name="einstein1924">{{cite journal |last=Einstein |first=Albert |year=1924 |title=Quantentheorie des einatomigen idealen Gases |journal=Sitzungsberichte der Preussischen Akademie der Wissenschaften |volume=1924 |pages=261–267}}</ref>

Unlike [[Physics:Quantum fermion|fermions]], photons do not obey the Pauli exclusion principle. This is why an electromagnetic mode can contain many photons, and why coherent optical fields can be built from large numbers of photons in related quantum states.

== Wave–particle duality ==

Photons show [[Physics:Quantum wave-particle duality|wave–particle duality]]. In experiments such as interference and diffraction, photon probabilities follow wave-like patterns. Yet when a photon is detected, it is registered as a discrete event. A single photon can therefore produce an interference pattern only statistically, after many repeated detections.<ref name="taylor">{{cite journal |title=Interference Fringes with Feeble Light |journal=Proceedings of the Cambridge Philosophical Society |volume=15 |page=114 |year=1909}}</ref><ref name="grangier">{{cite journal |last1=Grangier |first1=P. |last2=Roger |first2=G. |last3=Aspect |first3=A. |year=1986 |title=Experimental Evidence for a Photon Anticorrelation Effect on a Beam Splitter: A New Light on Single-Photon Interferences |journal=Europhysics Letters |volume=1 |issue=4 |pages=173–179 |doi=10.1209/0295-5075/1/4/004 |bibcode=1986EL......1..173G |s2cid=250837011}}</ref>

This dual behavior is one reason photons are important in the foundations of [[Physics:Quantum mechanics|quantum mechanics]]. They connect the wave theory of light with the particle-like transfer of energy, momentum, and angular momentum.

== Physical properties ==

The photon has zero electric charge and is normally treated as having zero rest mass. Experimental tests place extremely small upper limits on any possible photon mass, but no nonzero photon mass has been established.<ref name="tu">{{Cite journal |last1=Tu |first1=Liang-Cheng |last2=Luo |first2=Jun |last3=Gillies |first3=George T. |year=2005 |title=The mass of the photon |journal=Reports on Progress in Physics |volume=68 |issue=1 |pages=77–130 |doi=10.1088/0034-4885/68/1/R02 |bibcode=2005RPPh...68...77T}}</ref><ref name="goldhaber">{{Cite journal |last1=Goldhaber |first1=Alfred Scharff |last2=Nieto |first2=Michael Martin |year=2010 |title=Photon and graviton mass limits |journal=Reviews of Modern Physics |volume=82 |issue=1 |pages=939–979 |doi=10.1103/RevModPhys.82.939 |arxiv=0809.1003 |bibcode=2010RvMP...82..939G}}</ref>

A real photon in vacuum has two polarization states. These may be described in terms of helicity, corresponding to spin angular momentum components <math>+\hbar</math> and <math>-\hbar</math> along the direction of propagation.<ref name="schwartz" /> Polarization links the quantum description of photons to the classical polarization of electromagnetic waves.

== Historical development ==

The modern photon concept developed from the study of black-body radiation, the photoelectric effect, and the particle-like behavior of light. Max Planck introduced quantized energy elements in his work on black-body radiation. Albert Einstein then proposed in 1905 that light itself could be treated as localized energy quanta in order to explain the photoelectric effect.<ref name="planck1901">{{cite journal |last=Planck |first=Max |year=1901 |title=Über das Gesetz der Energieverteilung im Normalspectrum |journal=Annalen der Physik |volume=4 |issue=3 |pages=553–563 |doi=10.1002/andp.19013090310 |bibcode=1901AnP...309..553P |doi-access=free}}</ref><ref name="einstein1905">{{Cite journal |last=Einstein |first=Albert |year=1905 |title=Über einen die Erzeugung und Verwandlung des Lichtes betreffenden heuristischen Gesichtspunkt |journal=Annalen der Physik |volume=17 |issue=6 |pages=132–148 |doi=10.1002/andp.19053220607 |bibcode=1905AnP...322..132E |doi-access=free}}</ref>

The name ''photon'' was popularized by Gilbert N. Lewis in 1926 for the quantum of light.<ref name="lewis">{{cite journal |last=Lewis |first=Gilbert N. |date=18 December 1926 |title=The conservation of photons |journal=Nature |volume=118 |issue=2981 |pages=874–875 |doi=10.1038/118874a0 |bibcode=1926Natur.118..874L |s2cid=4110026}}</ref><ref name="kragh">{{cite arXiv |last=Kragh |first=Helge |date=2014 |title=Photon: New light on an old name |eprint=1401.0293 |class=physics.hist-ph}}</ref> Arthur Compton's X-ray scattering experiments later provided strong evidence that light quanta carry momentum.<ref name="compton">{{cite journal |last=Compton |first=Arthur H. |year=1923 |title=A quantum theory of the scattering of X-rays by light elements |journal=Physical Review |volume=21 |issue=5 |pages=483–502 |doi=10.1103/PhysRev.21.483 |bibcode=1923PhRv...21..483C |doi-access=free}}</ref>

== Photon as a gauge boson ==

In quantum field theory, the photon is the gauge boson of electromagnetism. It is associated with a U(1) gauge symmetry and appears as the quantum carrier of the electromagnetic interaction.<ref name="ryder">{{cite book |last=Ryder |first=L. H. |title=Quantum Field Theory |edition=2nd |publisher=Cambridge University Press |year=1996 |isbn=978-0-521-47814-4}}</ref>

Virtual photons appear in quantum electrodynamics as intermediate carriers of electromagnetic interactions. They are not directly observed as free particles, but they are part of the mathematical description used to calculate electromagnetic scattering, radiative corrections, and forces between charged particles.<ref name="feynman">{{cite book |last=Feynman |first=Richard P. |title=QED: The Strange Theory of Light and Matter |publisher=Princeton University Press |year=1985 |isbn=978-0-691-12575-6}}</ref>

== Emission and absorption ==

Photons are emitted or absorbed when quantum systems change energy states. Atoms, molecules, nuclei, and solids can emit photons during transitions from higher to lower energy levels. Conversely, absorption of a photon can raise a system to a higher energy state or trigger processes such as ionization, photochemistry, or electronic excitation.

Einstein's theory of spontaneous and stimulated emission provided a statistical description of radiation interacting with matter. Stimulated emission later became the physical basis of the laser.<ref name="einstein1916">{{cite journal |last=Einstein |first=Albert |year=1916 |title=Strahlungs-emission und -absorption nach der Quantentheorie |journal=Verhandlungen der Deutschen Physikalischen Gesellschaft |volume=18 |pages=318–323 |bibcode=1916DPhyG..18..318E}}</ref><ref name="dirac1927">{{cite journal |last=Dirac |first=Paul A. M. |year=1927 |title=The Quantum Theory of the Emission and Absorption of Radiation |journal=Proceedings of the Royal Society A |volume=114 |issue=767 |pages=243–265 |doi=10.1098/rspa.1927.0039 |bibcode=1927RSPSA.114..243D |doi-access=free}}</ref>

== In matter ==

When light travels through transparent matter, its effective speed is reduced by interactions with the material. In a quantum description, photons may couple to excitations of matter to form quasiparticles such as polaritons. Photons can also be scattered, absorbed, or re-emitted by atoms, molecules, and solids.

Photon interactions with matter are central to spectroscopy, photochemistry, microscopy, solar radiation transport, optical communication, and quantum technologies. In many of these applications, the photon picture is essential because energy is exchanged in discrete units.

== Quantum information and optics ==

Photons are widely used in [[Physics:Quantum optics|quantum optics]] and quantum information. Their polarization, path, phase, arrival time, or photon number can encode quantum states. Single photons and entangled photon pairs are used in tests of quantum mechanics, quantum cryptography, optical communication, and photonic approaches to quantum computing.<ref name="scully">{{cite book |last1=Scully |first1=M. O. |last2=Zubairy |first2=M. S. |title=Quantum Optics |publisher=Cambridge University Press |year=1997 |isbn=978-0-521-43595-6}}</ref><ref name="fox">{{cite book |last=Fox |first=Mark |title=Quantum Optics: An Introduction |publisher=Oxford University Press |year=2006 |isbn=978-0-19-856673-1}}</ref>

Single-photon experiments, photon antibunching, and beam-splitter tests show that light cannot always be explained as a classical electromagnetic wave. These experiments support the quantum view of photons as individual excitations of electromagnetic modes.<ref name="kimble">{{cite journal |last1=Kimble |first1=H. J. |last2=Dagenais |first2=M. |last3=Mandel |first3=L. |year=1977 |title=Photon Antibunching in Resonance Fluorescence |journal=Physical Review Letters |volume=39 |issue=11 |pages=691–695 |doi=10.1103/PhysRevLett.39.691 |bibcode=1977PhRvL..39..691}}</ref><ref name="grangier" />

=See also=
{{#invoke:PhysicsQC|tocHeadingAndList|Physics:Quantum basics/See also/Matter}}

=References=
{{reflist|3}}

{{Author|Harold Foppele}}

{{Sourceattribution|Photon|1}}

Physics:Quantum photon - Revision history

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