| Photon | |
|---|---|
| Name | Photon |
| Classification | Gauge boson |
| Composition | Elementary particle |
| Statistics | Bose–Einstein statistics |
| Interaction | Electromagnetic interaction |
| Mass | 0 (rest mass) |
| Discovered | 1905 (conceptual), 1923 (name) |
| Discoverer | Albert Einstein (concept), Gilbert N. Lewis (name) |
Photon
A photon is the quantum of the electromagnetic radiation field: an elementary, massless boson that mediates the electromagnetic interaction in quantum electrodynamics. Photons underpin phenomena from visible light to radio waves and are central to experiments and technologies that test and exploit quantum mechanics, such as quantum optics, quantum information science, and the study of fundamental symmetries.
In quantum field theory the photon is the gauge boson associated with the unbroken U(1) gauge symmetry of electromagnetism and appears as excitations of the quantized electromagnetic field. Photons have zero rest mass, travel at the speed of light c in vacuum, carry energy E = hν and momentum p = E/c, and possess two physical helicity states corresponding to circular polarizations. Their integer spin (spin‑1) and bosonic statistics allow multiple photons to occupy the same quantum state, enabling coherent states used in laser operation. The photon is electrically neutral and does not directly couple to the strong interaction or weak interaction except through higher‑order processes in the Standard Model.
The concept evolved from classical electromagnetism and early quantum ideas. In 1905 Albert Einstein explained the photoelectric effect by proposing quantized light packets, building on Planck's work on blackbody radiation. The term "photon" was introduced by Gilbert N. Lewis in 1926, though the quantum field theoretic formulation emerged later with the development of quantum electrodynamics (QED). Key contributors include Max Planck, Niels Bohr, Arthur Compton (Compton scattering experiments), and theoreticians such as Paul Dirac and Richard Feynman who formalized particle–field duality and Feynman diagram methods. Experimental advances at institutions like Bell Labs, CERN, and MIT refined understanding and enabled precision tests of photon properties.
Photons are described in QED by creation and annihilation operators acting on the Fock space; single‑photon states are eigenstates of particle number with a well‑defined frequency or wavepacket profile. Common state bases include plane waves, polarization (horizontal/vertical, circular), and time–frequency modes; nonclassical states include Fock states, squeezed states, and entangled photon pairs produced by spontaneous parametric down-conversion (SPDC) or four-wave mixing. Photon polarization encodes quantum information in qubits for protocols developed by groups at IBM, University of Oxford, and University of Vienna. Coherent states describe classical‑like light from lasers and are central to the semiclassical limit where quantum fluctuations are minimal.
Photons interact with charged particles through absorption, emission, and scattering processes governed by QED. Key mechanisms include the photoelectric effect, Compton scattering, Rayleigh scattering, Raman scattering, stimulated and spontaneous emission, and absorption by atoms and molecules described by transition matrix elements and selection rules from quantum mechanics. Photons exert radiation pressure and can transfer angular momentum (spin and orbital), exploited in optical tweezers and studies of optical angular momentum. In media, photons acquire effective dispersion and group velocity changes described by refractive index and lead to phenomena like Cherenkov radiation and slow light. Nonlinear optics mediates photon–photon interactions via matter degrees of freedom, enabling photon entanglement and quantum gates.
Evidence for quantization includes the photoelectric effect, discrete energy transfer in atomic spectra, and particle‑like scattering (Compton effect). Modern experiments detect single photons using photomultiplier tubes, avalanche photodiodes, and superconducting nanowire single-photon detectors (SNSPDs). Correlation measurements using the Hanbury Brown and Twiss experiment revealed photon bunching and antibunching signatures of bosonic statistics and single‑photon sources. Interference experiments such as the double‑slit with single photons and Bell tests (e.g., experiments by Alain Aspect) probe wave–particle duality and nonlocal correlations. Precision measurements of QED processes at facilities like SLAC and DESY test higher‑order corrections and the photon's role in radiative processes.
Photons are primary information carriers in quantum communication (quantum key distribution by companies like ID Quantique and protocols such as BB84), in optical implementations of quantum computing (linear optics quantum computing and photonic cluster states), and in quantum metrology (atomic clocks, interferometric sensors such as LIGO). Single‑photon sources and detectors enable quantum imaging, lidar, and sensing. Integrated photonics platforms developed at Caltech, Bell Labs, and university foundries combine waveguides, modulators, and nonlinear crystals for scalable quantum photonic circuits. Photons also mediate hybrid quantum systems coupling to superconducting qubits and trapped ions for distributed quantum networks.
While the photon is well described within QED, open questions remain at interfaces with gravity and beyond‑Standard‑Model physics. The role of the photon in quantum gravity, possible tiny effective photon mass bounds (tested by galactic magnetic field observations and laboratory experiments), and searches for parity‑violating or Lorentz‑violating effects are active areas. Phenomena like photon mixing with hypothetical axion‑like particles are probed by experiments such as CAST and light‑shining‑through‑walls setups. Foundations of measurement, photon localization, and descriptions of time in single‑photon wavepackets continue to motivate theoretical and experimental work in quantum foundations and at research centers including Perimeter Institute and Max Planck Institute for Quantum Optics.
Category:Quantum optics Category:Elementary particles