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. P

    Difference between temporal and spatial coherence

    Hi, I am confused about the difference between temporal and spatial coherence. I know coherence is when the waves have the same wavelength. An explanation in simple terms would be great thanks :)
  2. S

    Questions about HUP, entanglement and coherence

    HUP --> Heisenberg Uncertainty Principle The below questions maybe a bit amateurish. 1. Can two photons be entangled on both position and momentum?...for a same time period I think not because: To entangle on momentum (or any property) we need a broad distribution of momentum for the photon...
  3. ShayanJ

    What is Coherence? Explained Clearly

    Recently I've been trying to understand coherence,about waves in general.But its just a little hazy to me.Can someone explain it clearly? Thanks
  4. J

    Does Damping Affect the Coherence Between Broadband Signals?

    The coherence I'm asking about is also known as magnitude-squared coherence. Let's say we have input A(t) and we look at the output B(t), the system L is linear but it has damping effect on the signals. In a long time, this damping will literally kill the high frequencies to zero. My...
  5. N

    Breaking the coherence of an optical signal

    Hi, I am curious to know if there any passive (or simple active) methods to convert a very coherent signal into an incoherent one? I suppose I would need to find a way to chirp the signal and a non-linear medium might be one method. Any ideas? Thanks narra
  6. N

    Calculating the coherence length of a laser

    What is the best way to calculate the coherence length of a laser?
  7. E

    Is coherence a single photon phenomenon?

    Hello folks. 2-dimensional spectroscopic techniques have been used to investigate biological systems and found that they transfer energy via coherent pathways. However, some have argued that this has no bearing on how these systems behave in nature, as the sun is not a coherent source and the...
  8. S

    Can Entropy Decrease After a Projective Measurement?

    ¿ What´s the matter if after decoherence the measured system coherences again with the bath ¿ Is measure erased
  9. O

    What is Spatial and Temporal Coherence.

    Can anyone provide a simple explanation of spatial and temporal. I can't seem to understand the Wikipedia page on Coherence.
  10. S

    What is the correlation between photon events in space and time?

    Hey people! I'm studying the article "The Quantum Theory of Optical Coherence" by Glauber (see eg. http://prola.aps.org/pdf/PR/v130/i6/p2529_1), and I've a couple of questions. 1. Why are the arguments of E(+) and E(-) in equation 3.6 two different sets of coordinates? We obtained E(-) by...
  11. E

    Temporal and Spatial Coherence

    What is the difference between the two in simple terms ?
  12. S

    Wave functions for Coherence and Entanglement

    My understanding of wave-functions is close to zero, pardon me if the questions don't sound proper. 1. Do we have wave-function usage to describe a) Coherence and b) Entanglement? 2. Has a (mathematical/conceptual) way been developed to show the complementarity between both (a & b) via...
  13. A

    In downconversion, can you get coherence between the signals from two sources?

    In the ZWM study of 1991 Mandel et al get coherence between the signal photons of two distinct downconversion sources. This is shown with an interference collection between the two possible sources of signal photons (even though they are entangled with the idler). This is achievable because...
  14. W

    How do i derive the relation between coherence time and bandwidth

    I was just curious, sorry if it is already asked, I was going through a book on photonics and came across the relation coherence time, t = λ*λ/Δλ I did not find any proof of this. Can anyone explain me how this relation is found?
  15. F

    The Mystery: Understanding Resonance vs. Coherence

    Could someone please concisely explain the difference between resonance and coherence?
  16. S

    Why is coherence and entanglement complementary?

    Why is coherence and entanglement complementary? Lets take the case of a pair of entangled photon. there is very little coherence between the entangled photons. when we try to improve the coherence; the (degree of) entanglement starts to reduce. why does that happen? Does it have...
  17. H

    Degree of coherence of light from a red semiconductor laser

    I would like to have at least a vague idea how coherent is light from an inexpensive 1 mW red semiconductor laser which I have. I tried to find the manufacturer using Google, but I haven't found it. I would like to know the coherence length and the degree of spatial coherence. Is there...
  18. Sigurdsson

    How Does First-Order Coherence Degree Relate to Signal Visibility?

    Homework Statement Hi guys, appreciate all the help I can get. This has been bugging me for 24 hours now. I'm starting to think I'm missing something in the question. We are exploring first-order coherence degree. That is, exploring the coherence of two separate signals (wave packets) by...
  19. S

    Can Coherence Be Restored in Quantum Systems?

    can coherence be restored (after de-coherence)? let's look at single particle (interference) in a double slit experiment: when we try to gain which way ... the ordering of the phase angles between the components of a photon in a quantum superposition is disturbed/changed (?) can we restore...
  20. B

    Photon coherence time as the wavepacket length and dephasing

    Quite often one can see descriptions saying that the coherence time of single photons corresponds to the length of the single photon wavepacket (for example Jelezko et al, PRA 67 041802(R) (2003) http://pra.aps.org/abstract/PRA/v67/i4/e041802). I find it hard to come to terms with this picture...
  21. S

    BEC condenstate thermal statistics vs. coherence

    Hi. I'm reading an introductory section on the Bose-Einstein condensation of a non-interacting, spinless boson gas. I'm confused by the claim that the ground state is in a coherent state with eigenvalue sqrt(N0) exp(i theta), where N0 is the expected number of particles in the ground state. The...
  22. R

    Coherence Domain in Living System

    (note: I posted this here instead of the Biology forum because the topic involves heavy quantum concepts that mere biologists can't handle) I read PF Mentor Ryan_m_b mentioned elsewhere that "Discussing quantum effects in biology is not a banned topic on this forum, however threads that...
  23. P

    Is Quantum Entanglement the Secret Behind Photosynthesis?

    http://www.physorg.com/news192726440.html researchers with the U.S. Department of Energy (DOE)'s Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California (UC), Berkeley have recorded the first observation and characterization of a critical physical phenomenon...
  24. U

    Quantum Coherence in Transmembrane Proteins?

    I am presently in a Biophysics lab, and am formulating a project wrt transmembrane protein association in lipid bilayers (based on the 4 stage model of membrane protein folding, i.e. partitioning, folding, insertion, association). I was wondering if anyone on here has heard of any decent...
  25. P

    Complex Degree of Coherence (Cittert-Zernike)

    Homework Statement A light source consists of two long thin parallel wires, separated by a distance, W. A current is passed through the wires so that they emit light thermally. A filter is placed in front of the wires to only allow a narrow spectral range, centred at λ to propagate to a...
  26. W

    How does spatial coherence affect focusability of laser?

    Hi, In studying about lasers, it was mentioned that lasers are highly focusable, and this was because of the high spatial coherence of lasers. Just want to ask: 1. Why and how does spatial coherence affect focusability? 2. How is a laser typically focused (using a lens?) 3. Why do...
  27. A

    First order coherence classical optics problem

    Homework Statement Hi, I am trying to work through exercise 2.1 on page 37 of Microcavities (by alexy kavokin, jeremy baumberg, guillaume malpuech and fabrice laussy) the problem is to prove | g^{(1)}(\tau) | = | cos( \frac{1}{2}(\omega_1 - \omega_2)\tau) ) | where...
  28. E

    Quantum coherence and neutrino detection

    It is promised that the quantum coherence will help to improve various measurements. Is it theoretically possible to improve detection of neutrinos with help of quantum coherence?
  29. P

    What Is Coherence Distance in Laser Interferometers?

    Hi people :) I want to build an interferometer. In the course of looking for a suitable light source for my instrument I have been advised to check the 'coherence distance' of the sources; it is clear to me that this distance places practical limits on the design of the machine. However...
  30. V

    Understanding Classical & Quantum Coherence in Optics

    Hello all, I'm studying classical and quantum coherence in optics and I can't understand what's the physical meaning of second order coherence. I mean, what's the fundamental difference between it and the first order one? I know they are defined differently but I can't see in which case and why...
  31. B

    How could one measure degree of coherence with Fresnel birprism?

    How could one measure degree of coherence with Fresnel biprism? Hello. I have some questions about the study of partial coherence with a Fresnel biprism. In common text degree of coherence and its relationship to visibility is introduced considering a Young Double Slit experiment. At the...
  32. S

    Is coherence restored after erasure of which-way info?

    After erasure (of which-way info) in DCQE or similar experiments, is coherence (constant relative phase) restored?
  33. S

    Explaining DCQE - via coherence in layman terms

    Does the below sound ok? 1. Interference is caused between coherent waves 2. When we try to find which-way we break the coherence a) When we try to "partially" find which way, we partially break the coherence and hence a "partial" interference pattern 3. DCQE is explained by the fact...
  34. M

    Coherence Sunlight Refraction homework

    Homework Statement On the wet road / asphalt / there is a oily stain,with refraction coeff 1.4 .Sunlight lights up the grease perpendicular .What colour will be a stain with thickness of d = 250 nm ? The water's refraction coeff is 1.33 Homework Equations Since we are asked what colour...
  35. J

    Double Slit and coherence of non-point source

    If a "line" source of visible light is not really a line but has a width of 1mm, how far must it be from a double slit which it illuminates in order for the two slits to be reasonably coherent? Assume the slit separation is .5mm. I approached this problem the same as finding the distance...
  36. R

    Decoherence, Coherence, Pure State, Mixed State

    Hi, I'm confused by subtle differences between the concept. Let's take the example of a Schrodinger Cat. Supposed you could make a box that can isolate anything inside from say gravity, microwave radiation, is in 0 kelvin, etc. or let's just accept (for sake of discussion) that a box can totally...
  37. W

    What is the coherence length and coherence time of the filtered light?

    Homework Statement A filter is used to obtain approximately monochromatic light from a white sources. If the pass band of the filter is 10 nm, what is the coherence length and coherence time of the filtered light? The mean wavelength is 600 nm. Homework Equations c\tau_{0}=l_{c}...
  38. G

    How Is Spatial Coherence Derived from Heisenberg's Uncertainty Principle?

    hi does anybody know a derivation of the formula for spatial coherence: a sin(beta) << lambda /2 where a is the length of radiating object and beta is half the opening angle.
  39. S

    How Do Coherent Waves Maintain Identity in RF Systems?

    When waves are said to be 'coherent', or in-phase, do they always act as a single wave, or do they maintain their individual identities?
  40. S

    Laser light an example of quantum coherence?

    There seems to be some debate as to whether laser light is an example of quantum coherence (as posted in this thread: link). I think it deserves its own discussion. I'm no expert in this field, so I mainly go by what I read; I found this interesting Wikipedia reference here: Comments?
  41. N

    Quantum Optics and Coherence Functions

    Hi Classically, we can interpret the coherence functions (1st order) by using the definition of visibility, i.e., when there is complete coherence, the visibily of a fringe is maximum, and when the is complete incoherence, the visibility of a fringe is minimum. Can one make the same...
  42. S

    Quantum Coherence - matter or energy?

    I'm not a physicist, and am trying to better understand quantum coherence. I understand that it's defined by the fact that the entire quantum system can be defined by a single wave function. My question is whether, due to wave-particle duality on the quantum level, it is equally valid to speak...
  43. S

    What causes quantum coherence in superconductivity and superfluidity?

    I have a question about quantum coherence phenomena. I understand (basically) how laser light results from light waves that are in sync. I also see how ferromagnetism results from orderly alignment of electron spin amongst iron atoms (though I've posted in another thread asking whether this is a...
  44. B

    Topic suggestions: Spacial coherence in Nanotechnology.

    Hi I have to give a 10 minute presentation for a course. The topic is 'Spatial Coherence' and should, ideally, be related to my research. My research field is, in broad terms, surface nanotechnology. However, in specifics I use scanning probe microscopy, which is non-optical or electron wave...
  45. StevieTNZ

    Macroscopic Quantum Coherence & Macrorealism experiments

    Has anyone performed experiments regarding macroscopic quantum coherence (superposition of and macroscopic object)? What about testing if macroscopic objects obey macrorealism, or whether QM prevails? Links to articles or experiments would be apprecited.
  46. G

    What means 'coherence' in quantum mechanics?

    I have nothing to add to the question that opens this post.
  47. J

    Can Single Photon Beams Exhibit Coherence?

    For any light beam it is normal to characterize the coherence length and time. This is done by measuring the ability light to interfere with itself. Does it make sense to talk about the coherence of a single photon beam, such as the one used in the double slit experiment? Can a single photon...
  48. S

    Calculating FRF & Coherence between Acceleration Signals

    Hello, I'm a newbie of signal processing, I would like to calculate a frf between two acceleration signals. I have several realizations of each signal. These signals are stationaries and they aren't triggered, so signal arrays doesn't have the same length. I have got these acceleration signal by...
  49. B

    Boson coherence at high temperatures?

    I was just reading up on Bose-einstein condensates and was wondering about something,hopefully you could point me in the right direction. so when you get atoms down to a low enough temperature the pauli exlusion principle gives out and they can sit ontop of each other like bosons,and they...
  50. M

    Understanding Laser Coherence Lengths and Improving Coherence

    Hi all I am hoping that somebody here will be able to tell me why that different types of lasers eg He-Ne or Nd:YAG have different coherence lengths and how the coherence of a laser can be improved. I am thinking it is to do with the different gain mediums and the emission process...
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