Coherence Definition and 124 Threads

In physics, two wave sources are coherent if their frequency and waveform are identical. Coherence is an ideal property of waves that enables stationary (i.e. temporally and spatially constant) interference. It contains several distinct concepts, which are limiting cases that never quite occur in reality but allow an understanding of the physics of waves, and has become a very important concept in quantum physics. More generally, coherence describes all properties of the correlation between physical quantities of a single wave, or between several waves or wave packets.
Interference is the addition, in the mathematical sense, of wave functions. A single wave can interfere with itself, but this is still an addition of two waves (see Young's slits experiment). Constructive or destructive interferences are limit cases, and two waves always interfere, even if the result of the addition is complicated or not remarkable. When interfering, two waves can add together to create a wave of greater amplitude than either one (constructive interference) or subtract from each other to create a wave of lesser amplitude than either one (destructive interference), depending on their relative phase. Two waves are said to be coherent if they have a constant relative phase. The amount of coherence can readily be measured by the interference visibility, which looks at the size of the interference fringes relative to the input waves (as the phase offset is varied); a precise mathematical definition of the degree of coherence is given by means of correlation functions.
Spatial coherence describes the correlation (or predictable relationship) between waves at different points in space, either lateral or longitudinal. Temporal coherence describes the correlation between waves observed at different moments in time. Both are observed in the Michelson–Morley experiment and Young's interference experiment. Once the fringes are obtained in the Michelson interferometer, when one of the mirrors is moved away gradually from the beam-splitter, the time for the beam to travel increases and the fringes become dull and finally disappear, showing temporal coherence. Similarly, in a double-slit experiment, if the space between the two slits is increased, the coherence dies gradually and finally the fringes disappear, showing spatial coherence. In both cases, the fringe amplitude slowly disappears, as the path difference increases past the coherence length.

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  1. S

    I Original HBT experiment with mercury isotope lamp

    From "Boffin : a personal story of the early days of radar, radio astronomy, and quantum optics", by R Hanbury Brown... If it had been truly coherent (e.g. laser light), wouldn't the detection events have been uncorrelated? That is, two independent Poisson processes?
  2. red65

    I Why do we use low-coherence light in Optical Coherence Tomography?

    Hello , I study the principles of optical coherence tomography, where we emit light and by the refraction that we detect we reconstruct and image, but I don't understand why we use low coherence light , if i want to measure the refracted light i would prefer to have coherent light so that the...
  3. M

    I Interference by reflection on a CD

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  4. A

    B Is quantum coherence a random event?

    How we should understand the randomness of quantum events in the context of the significant role that they apparently play in our macroscopic world. Using processes as superconductivity, super-fluidity, and in Bose-Einstein Condensates researchers have been able to produce macroscopic quantum...
  5. Erik Ayer

    I Understanding Coherence vs. Entanglement in Quantum Eraser

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  6. DarkMattrHole

    B Regarding the coherence of a photon pair decaying from Higgs

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  7. entropy1

    B What causes loss of quantum coherence?

    Generally, what physical process causes loss of quantum coherence? Is it known?
  8. W

    B Are all quantum fields coherent?

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  9. N

    Coherence of Planar Wavefronts - Spatial & Temporal

    Hi, are planar wavefronts both spatially and temporally coherent? Or, they are only spatially coherent and need not be temopral?
  10. I

    B Do photons bunch up at an interface? Can they lase (laze?)

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  11. F

    Question about wave interference and coherence

    I have encountered the following definition of interference: Interference is a wave phenomenon in which two or more waves from coherent sources meet and superpose to form a resultant wave such that the amplitude of the resultant wave at any point is the vector sum of the amplitudes of the...
  12. P

    A Understanding Spectral Coherence w/ Radiation Sources

    What is the relationship between spectral coherence with respect to a radiation source's mean wavelength and bandwidth? I have no idea how to estimate, given a particular radiation spectrum from a source, what it's spectral coherence is going to be.
  13. A

    I Coherence length of velocity fields

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  14. T

    A What is a 'Quantum Coherence'?

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  15. Erik Ayer

    I Entanglement vs. Coherence in SPDC

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  16. A

    I Coherence, Young's double slit experiments.

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  17. CDL

    Mean Frequency and Frequency Spread of a Laser Pulse

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  18. DariusP

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  19. Marisa5

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  20. J

    B Pauli Exclusion Principle: Quantum Coherence & Range

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  21. D

    Creating Coherent Light from UV LEDs and Pinhole Aperture

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  22. M

    Wave interference - coherence and wavelength

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  23. B

    I Bose-Einstein condensates coherence

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  24. M

    Conditions for waves to be coherent?

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  25. S

    I Understanding Coherence in Quantum Mechanics

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  26. P

    I What is Bipartite Coherence? Exploring 2-Electron Examples

    Hello, i read about "Bipartite Coherence". In different Papers e.g. http://quantumcorrelations.weebly.com/uploads/6/6/5/5/6655648/2015_badhonnef_activations_cde.pdf or https://arxiv.org/abs/1611.00413 Can someone explain it a little bit? Maybe with an example with 2 Electrons that are described...
  27. Oganesson

    I What happens to Quantum decoherence at Absolute Zero?

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  28. D

    A Single qubit coherence time record

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  29. durant35

    I Quantum Coherence: Macroscopic Superpositions Explained

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  30. sciencejournalist00

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  31. Anton Alice

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  32. Anton Alice

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  33. Anton Alice

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    I don't like the common interpretation of coherence of two waves. Please tell me if something is wrong in my argumentation: 1. It is often said, that, if two waves are coherent, then the interference pattern is stationary, which means, that the amplitudes are the same. And often, this...
  34. C

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  35. DaTario

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    Hi All, Is there any experimental evidence that photons may have attached to itself a coherence length? Best Regards, DaTario
  36. S

    What is the coherence of laser beam?

    what is coherence of light beam
  37. E

    How Long Do Quantum Effects Last in Biological Processes?

    Not long ago some biological phenomena were found that have quantum background. These are photosyntesis, quantum smell (Luca Turin), sensation of Earth magnetic field by European Robin etc. What are coherence times of these phenomena?
  38. G

    How/why decoherence rather than how/why coherence

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  39. elif

    Coherence length of monochromatic laser

    Homework Statement Hi.I need an experiment to study and present it.the subject is how to measure coherence length of a monochromatic laser ? lm also search on google.However,is there any advice from you ? Homework EquationsThe Attempt at a Solution
  40. A

    Coherence in spontaneous radiation processes

    Hi, I am trying to understand what keeps getting referred to as "super radiance". So this has led me to R. H. Dickes 1954 paper "Coherence in spontaneous radiation processes". http://dx.doi.org/10.1103/PhysRev.93.99 . On the first page Dicke presents a "simple example", that is not simple at...
  41. B

    Quantum Coherence - Decoherence Question

    So I understand that as the number of entangled particles increases, observable quantum mechanical properties decrease to the extent that the mass of particles collectively loses its wave-particle character and behaves classically. In other words, the particles' collective position-space...
  42. H

    Coherence in density matrix formalism

    Hello, In the density matrix formalism I have read in numerous places that coherence is identified with the off-diagonal components of the density matrix. The motivation for this that is usually given is that if a state interacts with the environment in such a way that the basis state...
  43. vtahmoorian

    Spontaneous generated coherence

    Hi everyone Is there anyone who can explain when does Spontaneously generated coherence(SGC) happen and how? Thank you in advance Vahideh
  44. DrChinese

    Can single photons be made coherent through entanglement and manipulation?

    I thought that it would be possible to get a single photon to a double slit which would be sufficiently coherent to create an interference pattern. You would use entangled photon pairs, which I realize is not coherent light to start with. And use the Alice stream to herald the Bob stream. But...
  45. M

    Aharonov Bohm Effect - Impurity Scattering and Phase Coherence

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  46. N

    Coherence time of parametrically downconverted entangled photons

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  47. N

    Temporal coherence in Computational ghost imaging

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  48. P

    Coherence - A Must for Interference?

    I can't get my head around the fact that for the interference of two waves to occur, the two sources producing the waves must be coherent (Well, that's what my A level Physics book says). Wouldn't it perhaps be more appropriate to say that coherence is only a necessary condition if an...
  49. L

    Superconductor coherence length and penetration depth

    Homework Statement I have a lot of information about 2 different superconductor materials; indium and lead. The indium is pretty much 100% indium with no impurities, and the lead is unknown purity. I have the temperatures and magnetic fields at which they are superconducting and the...
  50. O

    Calculating Coherence Time: Understanding the Relationship Between g(t) and t_c

    Homework Statement I have the complex term g(t) = e^{\frac{-|t|}{t_c}} which is the degree of the coherence. Homework Equations Now I want to verify that: t_c = \int_{-\infty}^\infty \! |g(t)|^2 \, dt The Attempt at a Solution \int_{-\infty}^\infty \! |g(t)|^2 \, dt =...
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