# What is Coherence: Definition and 124 Discussions

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. ### 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. ### 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. ### I Interference by reflection on a CD

A fairly focused beam of light incident on a CD projects a clearly visible interference pattern onto a screen. May I infer that it has good (or at least enough) spatial coherence? This property is evident, for example, in a lens-focused beam of light produced by a LED or by a ray of sunlight...
4. ### 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. ### I Understanding Coherence vs. Entanglement in Quantum Eraser

As I understand it, either coherence or entanglement can be in an experiment or some of each, varying continuously between the two. Because of this, coincidence detection is needed to pick out interference patterns among all the data. Coherence would refer to quantum waves of photons taking...
6. ### B Regarding the coherence of a photon pair decaying from Higgs

Hi all. I have been reading the following article and have a couple of basic questions about the decay of the Higgs into photons process - https://www.physicsforums.com/insights/lhc-part-4-searching-new-particles-decays/ As i understand this decay - the photons will have the same frequency and...
7. ### B What causes loss of quantum coherence?

Generally, what physical process causes loss of quantum coherence? Is it known?
8. ### B Are all quantum fields coherent?

Is coherence the medium in which quantum waves propagate? Does coherence also exist in the future to flag decoherence events?
9. ### 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. ### B Do photons bunch up at an interface? Can they lase (laze?)

If the light slows at an interface, what happens to the photons coming in after the slowed ones? Can these cohere with the leading (slowed) ones and create what amounts to a FEL?
11. ### 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. ### 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. ### I Coherence length of velocity fields

I have came across a term "coherence length of velocity field ##\lambda## ",tried to search in the internet but doesn't find any result...will anyone please give a brief summary of this in this context..
14. ### A What is a 'Quantum Coherence'?

I have recently been reading some stuff on quantum information in the physics literature which refers to 'a mechanism by which a measurement in A determines quantum coherences in B', where A and B are subsystems of a larger system. I am aware of the meaning of the terms 'decoherence' and...
15. ### I Entanglement vs. Coherence in SPDC

Hello all, I have read that these two properties are complimentary, similar, at least in a sense, to uncertainty between momentum and position. John Cramer used what he referred to as a Sagnac Source to vary the balance between entanglement and coherence. What, physically, has to be done to...
16. ### I Coherence, Young's double slit experiments.

In most textbook/internet explanation of light coherence, it gives two conditions 1) monochromatic light, same frequency. 2) light in phase or constant phase difference. Only with the two conditions can light interference pattern be observed in the double slit experiment. But this cannot be...
17. ### Mean Frequency and Frequency Spread of a Laser Pulse

Homework Statement Laser probes are being used to examine the states of atoms and molecules at high temporal resolution. A laser operating at a wavelength of 400 nm produces a 1 femtosecond pulse. Compute the mean frequency and frequency spread, ∆ν, of this laser pulse. Homework Equations c =...
18. ### How Does Optical Coherence Tomography Work to Image Different Tissue Layers?

Anyone familiar with how this device works? I'm having trouble understanding how different layers of tissue can be imaged using interference. I am sort of familiar with michaelson interferometer and what is low coherence light if that helps.
19. ### Coherence pattern of HeNe laser in interferometer

Hello, i hope i posted this in the correct forum! When measuring coherence of a HeNe laser using a Michelson interferometer, there are key points along the moving interferometer arm where coherence is highest. The distance between each of these peaks corresponds with the length of the laser...
20. ### B Pauli Exclusion Principle: Quantum Coherence & Range

Over what range is the Pauli Exclusion Principle important? As a chemist I take it as an atom, but that seems an arbitrary stipulation. For instance, in metals how far apart must 2 electrons be to have the same 4 quantum numbers? I have seen that it has to do more with quantum coherence field...
21. ### Creating Coherent Light from UV LEDs and Pinhole Aperture

I've read that using a pinhole aperture and a wavelength filter can turn a "white" incoherent light source like a light bulb into a temporally and spatially coherent light source (albeit at low efficiency). Can a temporally and spatially coherent light source be made with a monoenergetic (or...
22. M

### Wave interference - coherence and wavelength

I know to produce an observable inteference pattern two waves must be coherent(same frequency/phase differnce). Do coherent waves therefore always have the same wavelength? Is it possible to have interference between two waves with the same frequency and different wavelengths, and if so what...
23. ### I Bose-Einstein condensates coherence

Hello forum! I'm studying classical and quantum coherence and there's some bug in particular I can't solve on my own, nor I could find anywhere. I've read that a Bose-Einstein condensate is a coherent state but it seems to me inconsistent with the definition of coherence given by Glauber. In...
24. ### Conditions for waves to be coherent?

Homework Statement Hello, For two waves to be coherent, they must have the same frequency right? Does this on its own implies a constant phase different between any point on one wave and any on the other. So, for example, if we had two waves with different wavelengths and velocities but equal...
25. ### I Understanding Coherence in Quantum Mechanics

Hi, Can anyone please explain the physical meaning of coherence(Quantum Mechanics). Thanks is advance
26. ### 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...

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28. ### A Single qubit coherence time record

Is it possible yet to store a single qubit for an extended period of time? If I only care about storing a BB84 state, is it possible to take (say) 128 of these single qubit storages? They're not entangled and I don't need to send them through any general quantum circuitry.
29. ### I Quantum Coherence: Macroscopic Superpositions Explained

Hi guys, as it has been said many times on the forum and outside of it the ability to achieve and maintain coherence is the biggest reason why macroscopic superpositions cannot be measured. The typical examples of macroscopic coherence are superconductors where atoms all behave in a similar...
30. ### What is the difference between entanglement and coherence

It is said that a coherent light beam is described by a single wave because all the individual waves add up in phase to produce a single big wave. It is also said that entangled light is described by a single wavefunction...
31. ### How does a Resonator enhance coherence? (LASER)

Hello, the resonator of a laser system is said to have 3 functions: Amplification in one direction, selection of one or few frequencies, and enhancement of coherence. I have a question about the coherence part. I see that the stimulated emission process produces phase matched light. Initially...
32. ### Time dependent optical path length vs coherence

Hello, I have a simple question. Suppose a perfect point source in front of a mirror. The virtual image of the point source acts like a second point source. Now let's look at the interference of the direct point source light and the virtual point source, at some position. Since the path...
33. ### Common interpretation of coherence

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. ### 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...
35. ### Coherence length of a single photon

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. ### 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. ### How/why decoherence rather than how/why coherence

Many discussions get around the subject of how / why does decoherence happen. Interaction with the environment, getting to macroscopic scale, irreversible change of information... Somehow they seem to imply that the natural state of things in the Universe is quantum coherence and then they ask...
39. ### 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. ### 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. ### 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. ### 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. ### 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. ### 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. ### Aharonov Bohm Effect - Impurity Scattering and Phase Coherence

I'm having difficulties understanding the role of scattering on phase coherence in the Aharonov Bohm Effect. In particular I am trying to reconcile the following points: Inelastic scattering destroys phase coherence and prevents us to see the Aharonov Bohm Effect. Elastic scattering does...
46. ### Coherence time of parametrically downconverted entangled photons

What is the coherence time for a pair of entangled photons produced in a nonlinear crystal? Is it related to coherence time of the pump photon? Also, if we say that the coherence time of the two photons is T, then does it mean that the two photons can interfere with each other even if their...
47. ### Temporal coherence in Computational ghost imaging

In pseudo-thermal ghost imaging using a laser beam passing through a rotating ground glass plate, the temporal coherence is important since the intensities in both scanning and bucket detectors have to be measured within the coherence time of the beam. However, in computational ghost imaging...
48. ### 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. ### 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. ### 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 =...