What is Coherent: Definition and 166 Discussions

Coherent is a clone of the Unix operating system for IBM PC compatibles and other microcomputers, developed and sold by the now-defunct Mark Williams Company (MWC). Historically, the operating system was a proprietary product, but it became open source in 2015, released under the BSD-3-Clause license.

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  1. M

    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...
  2. ATY

    Question about Lagrangian Coherent Structures (LCS)

    Has somebody a paper, text, (whatever), which explains the Lagrangian Coherent Structures ? I want an explanation by words and not all the mathematical stuff. All I can find about LCS is 1. They organize flow 2. you can detect them with the Lyapunov exponents. But I want to understand what LCS...
  3. AdrianMachin

    Laser Pointer Troubleshooting: Solving Incoherence in a Red 650nm <5mW Beam

    I have a red 650nm and <5mW LASER pointer, but the beam is not very coherent. Actually, when I point to a wall at an approximately 300 meters away, the red point gets very large (about a meter in diameter). I wonder how a LASER could act like this?
  4. CricK0es

    Interference from coherent light sources

    Homework Statement Four equally spaced coherent light sources (n=4) with a wavelength of 500nm are separated by a distance d = 0.1mm. The interference pattern is viewed on a screen at a distance of 1.4m. Find the positions of the principle interference maxima and compare their width with that...
  5. G

    Difference between elastic and coherent scattering

    Hi everyone! What is the difference between coherent and elastic scattering. Maybe the elastic scattering implies that there is not loss of energy, whereas the coherent scattering implies that the wavelength of a ray (or rays) is the same before and after the scattering? Thank you in advance.
  6. T

    A Importance of optical period in coherent radiation

    I am reading a text on coherent radiation and not quite understanding a particular statement. To provide some background, the authors state that coherent radiation can arise from light-matter interactions even when considering lengths, ##L##, much smaller than the wavelength (i.e. ##V \sim L^3...
  7. S

    Expectation value in coherent state

    Homework Statement In a coherent state ##|\alpha\rangle##, letting ##P(n)## denote the probability of finding ##n^{\text{th}}## harmomic oscillator state. Show that $$\displaystyle{\langle\hat{n}\rangle \equiv \sum\limits_{n}n\ P(n)=|\alpha|^{2}}$$ Homework Equations The Attempt at a...
  8. hackhard

    Rotating coherent laser beam

    if a coherent laser beam rotates about an axis at very high angular frequency can a light dot with radiant glow (of short radius) be obtained due to persistence of vision ?
  9. V

    I Confusion about initial states and coherent states

    I've found online that the coherent state of the harmonic oscillator is |\alpha \rangle = c \sum_{n=0}^\infty \frac{\alpha^n}{\sqrt{n!}} | n\rangle where |n\rangle = \frac{(a^\dagger)^n}{\sqrt{n!}} |0\rangle and |0> is called the initial state. I've some code where I need to have this...
  10. B

    I The Lagrangian of a Coherent State

    How does one write a Lagrangian of a coherent state of vector fields (of differing energy levels) in terms of the the individual Lagrangians? I desperately need to know how to know to do this, for a theory of mine to make any progress. Please stick with me, if I didn't make sense just ask...
  11. D

    I What is the Eigenvalue of Coherent States?

    Hi. I don't understand what is meant by the eigenvalue α of a coherent state where a | α > = α | α >. The eigenket |α > is an infinite superposition of the number states , ie | α > = ∑ cn | n > and for each number state a | n > = √n | n-1 >. So for each number state the eigenvalue of the...
  12. L

    A What kind of coherence do we have in the ground state of a harmonic oscillator?

    I am confused. In harmonic oscillator problem in quantum mechanics eigenstates of operator ##\hat{a}## are called coherent states. So we practically get Gaussian. In problem of quantum linear harmonic oscillator we have phonons ##|n \rangle##. In case of laser we have photons ##|n \rangle##...
  13. E

    I Direction of magnetic field measured by 2 coherent electrons

    According to the article: http://physicsworld.com/cws/article/news/2016/apr/07/birds-measure-magnetic-fields-using-long-lived-quantum-coherence : Is it possible simply to explain mechanism, how two coherent spins of electrons measure direction of magnetic field. I think that this explanation...
  14. I

    Position representation of coherent state and time evolution

    Homework Statement I ended up solving the problem as I was typing it up, I am posting what I did anyway as it took so long to type and might be useful to someone else. I am trying to figure out the position representation of a coherent state and it's time evolution. I should be getting a...
  15. Y

    How can coherent wave not have constant phase difference

    One condition for wave interference is that the sources of the waves must be coherent, which means they emit identical waves with a constant phase difference. I can understand that identical waves means they have the same wavelength. However, I don't understand what is a constant phase...
  16. Y

    Help with Coherent Wave Questions

    Hi guys. Was studying waves and got a bit confused. Pl help me out. Coherent sources are those which produce waves of same frequency, wavelength and velocity. Amplitude need not necessarily be same. Am I right? There's another doubt in my mind... If two waves produce beats then which of the...
  17. ShayanJ

    Ehrenfest theorem and coherent states

    From the Ehrenfest theorem, we know that the equation below is correct for any state ## \psi ##. ##m\frac{d^2}{dt^2}\langle x \rangle_{\psi} =-\langle \frac{\partial V(x)}{\partial x} \rangle_{\psi} ## But then one of the definitions of coherent states is states for which the expected value of...
  18. K

    Phase of coherent states and single photons

    Hello I am confused and I can't figure out what is meant by phase when it comes to quantum mechanics for single photons and coherent states as I am a new to this field. Best regards
  19. S

    Prime Implicants of a Non-Coherent Fault Tree

    I am stuck on some non-coherent fault tree analysis. I have a non-coherent fault tree for which the TOP event breaks down to TOP = AD' + DA' + A'E. These are (I think) some of the prime implicants of the fault tree. There is also another prime implicant ED'. I've been trying to work through it...
  20. Q

    Harmonic oscillator coherent state wavefunction

    Hi, I am trying to find the wavefunction of a coherent state of the harmonic oscillator ( potential mw2x2/2 ) with eigenvalue of the lowering operator: b. I know you can do this is many ways, but I cannot figure out why this particular method does not work. It can be shown (and you can find...
  21. S

    Understanding Coherent Large Scale Modes in Power Spectra

    Hi everyone, I am reading a paper, which discusses about power spectrum. There is a plot which is power versus k(wave number). It says something about "coherent large scale modes". Would you please tell me what the meaning of this is?
  22. S

    Can Lasers Used for Building Molecules Initiate Nuclear Fusion?

    http://www.iflscience.com/space/molecules-have-been-built-using-laser-beam-first-time Is there a way that fusion could be achieved with coherent control like this? If it can build molecules couldn't deuterium-deuterium be achieved?
  23. A

    Proving Coherent States are Eigenfunctions of Annihilation Operators

    Look at the following attached picture, where they prove the coherent states are eigenfunctions of the annihiliation operators by simply proving aexp(φa†)l0> = φexp(φa†)l0>. I understand the proof but does that also prove that: aiexp(Σφiai†)l0> = φiexp(Σφiai†)l0> ? I can see that it would if you...
  24. J

    Norm of a vector related to coherent states

    Hi, For the past couple of days I've been attempting to derive the equality (for any normalised ##\varphi##) ||(a+za^\dagger+ \lambda)\varphi ||^2 = ||(a^\dagger+\bar{z}a+ \bar{\lambda} )\varphi ||^2 + (|z|^2-1)||\varphi ||^2 First of the summation seemed like a typo. So I first tried to prove...
  25. O

    What restricts two separte sources of light to act coherent?

    Hi all To observe interference of light waves in Young's Double Slit Experiment one needs to have coherent sources.Some,doubts are coming in my mind regarding coherent sources. Q1. Will it make sense to talk about a single source as coherent source(not sources) Also two(or more) sources are...
  26. S

    Probability and Energy of a Particle in Coherent State

    Homework Statement A particle in harmonic potential $$H=\hbar \omega (a^\dagger a+1/2)$$ is at ##t=0## in coherent state $$a|\psi>=2|\psi>$$ a) Calculate expected value of energy and it's uncertainty at ##t=0## b) With what probability do we at ##t=0## measure the energy less than ##3\hbar...
  27. D

    Coherent States of Harmonic Oscillator

    Homework Statement Given the coherent state of the harmonic oscillator |z>=e^{-\frac{|z|^2}{2}}\sum_{n=0}^\infty\frac{z^{n}}{\sqrt{n!}}|n> compute the probability for finding n quanta in the sate |z> and the average excitation number <z|n|z>Homework Equations...
  28. vtahmoorian

    What makes a superposition of states a coherent superposition?

    Hi everyone I am investigating spontaneously generated coherence(SGC), I found that it happens when an excited atomic state decays to one or more closed atomic levels so that atom goes to a coherent superposition of states , Effect of State Superpositions Created by Spontaneous Emission on...
  29. R

    A calculation about coherent state

    Hi, there. The following is copied from a book " atom-photon interaction " by Prof. Claude Cohen-Tannoudji, Page 415. I do not understand it at all. I do know some thing about the coherent state. such as ##a |\alpha\rangle = \alpha |\alpha\rangle## and ##|\alpha\rangle =...
  30. S

    Polariton Beams Provide Coherent Energy for 250x Less Power

    Scientists have developed a new type of laser, called a polariton laser, which can be generated using 250x less power: http://www.eurekalert.org/pub_releases/2014-06/uom-anw060514.php Polariton beams are said to have properties of both matter and light, since polaritons are a combination...
  31. X

    Exploring Coherent States in Quantum Mechanics: From Lasers to Photons

    So a coherent state in quantum mechanics is "the most classical" quantum state (A Gaussian wave packet), which satisfies the Heisenberg uncertainty relation with an equality. This allows the wave packet to travel in space in a more localized fashion (like a classical particle) because its...
  32. lfqm

    Understanding Coherent Interaction in Dicke Model

    Hi! I would really appreciate you to help me answer this question: What is the physical meaning of coherent interaction (in the Dicke model for example)? I mean, coherence is well defined in classical optics, but what does it means when you ask for a system of N atomic dipoles to interact...
  33. snoopies622

    How does particle detection affect a coherent state?

    ". . . a coherent state remains unchanged by the detection (or annihilation) of field excitation or, say, a particle." — Wikipedia's entry on "Coherent States" I do not understand this. Is it saying that if a system in a coherent state, like a quantum harmonic oscillator, emits a...
  34. R

    Coherent States (Quantum Fields)

    \HugeHomework Statement Consider a state of the EM field which satisfies \left\langle \textbf{E}_x(\vec{r})\right\rangle =f(\vec{r}) Find a coherent state which satises these expectation values.Homework Equations \textbf{E}(\textbf{r})=\frac{i}{\sqrt{2 V}}\sum _{\textbf{k},\lambda }...
  35. P

    Coherent waves for interference

    Hi, Can someone give me an explanation as to why coherent waves are needed for interference. I know coherent waves are when the phases difference between the waves is a constant and the frequency of the waves are constant. Thanks :)
  36. H

    Coherent States: Ʃ(|α|2)n = 1?

    Hi guys, Just a quick question, is the following statement true (it seems to be implied in the article I'm looking at); Ʃ(|α|2)n = 1 (The sum over n=0 to infinity) Thanks to anyone who takes a look.
  37. D

    Checking that a coherent state is an eigenfunction of an operator

    Homework Statement Hey guys, I'll type this thing up in Word. http://imageshack.com/a/img716/8219/wycz.jpg
  38. ShayanJ

    Fermions and Coherent States: What's the Connection?

    I know coherent states are minimal uncertainty states and can provide a link from quantum to classical physics.But when I hear fermions can't have coherent states,or at least are limited in this sense,I can't see any relationship! What's the point? And...another thing...is there sth called...
  39. S

    Coherent state between a fermion and a photon

    Can we have a coherent state between a fermion and a photon! I mean can there ever be a fermionic polariton?
  40. S

    Explaining Fermionic Fields & Classical Coherent States

    hi i know what is coherent state, but i read this text in an article and i don't understand this " if we wish to describe long range macroscopic forces, only bosonic fi elds will do, since fermionic fi elds cannot build up classical coherent states. " can you explain it for me, how...
  41. S

    Young's double slit and coherent light

    Hello all, I was looking through my textbook on the double slit and it talks about constructive interference occurring at path differences of m \lambda and destructive interference occurring at path differences of (m + 1/2) \lambda where m is an integer. This is obvious if the waves leave the 2...
  42. J

    Destructive Interference and coherent light in anti-phase

    I have a question about coherent light in anti-phase, I'm probably thinking about this wrong but I just can't understand what happens to the energy in the wave when two waves of the same wavelength, frequency and amplitude are in anti-phase as the resultant wave is 0.
  43. MathematicalPhysicist

    A coherent survey of the problems in LQG and M theory.

    Is there any survey article that accounts for all the problems that are as of yet to be solved in M theory and LQG? I mean if I'll have time to dive into this field once. (I don't have a girlfriend and I don't plan to, so maths and physics will do). :biggrin:
  44. A

    A basic question: jinc function in coherent and incoherent system

    I am learning Fourier optics recently and I have a problem of the jinc function. In optical systems, digital image is blurred with the kernel of jinc function h(x,y)=jinc(r)=\frac{J_1(2\pi r / \lambda)}{ 2\pi r / \lambda} in the coherent system, the blurred image g(x,y) = |h(x,y) \star...
  45. B

    QM I - Decomposition of countable basic states into coherent states

    Homework Statement Consider a quantum system with a countable number of basic states \left|n\right\rangle. Calculate the decomposition into a basis of coherent states \left|λ \right\rangle all obeying \hat{a} \left|λ \right\rangle = λ \left|λ \right\rangle Homework Equations \hat{a}...
  46. E

    What is the Difference Between Coherent and Incoherent Light?

    what is "coherent" light? In high school, I learned that coherent light is light which has same frequency,phase and so on. In college , I learn that there is temporal coherent and spatial coherent. why we say laser is coherent source? I am confused. what is coherent? Now the new light source...
  47. J

    What is the significance of having coherent light in a laser?

    I understand that stimulated emissions produce coherent photons. What happens as a result of their coherence? My guess is that the laser beam does not spread out as much allowing it to travel further. Also, what is the property of a laser that allows the beam to travel great distances?
  48. Z

    Coherent state of photons in a laser

    Do photons used to produce the Sagnac effect (interference) have to have their phase in a coherent state or will a collection of random photons have the same property. Along the same lines, if you purchase a simple laser pointer are the photon produced here in a coherent state? Finally, is...
  49. W

    Proving Completeness of SHO's Coherent States

    Homework Statement I must prove that the set of coherent states \left\{ \left| \lambda \right\rangle \right\} of the quantum simple harmonic oscillator (SHO) is a complete set, i.e. it forms a basis for the Hilbert space of the SHO. Homework Equations The coherent states are defined as...
  50. L

    A classical wave as sum of many coherent quantum wavelets?

    Merzbacher - Quantum mechanics, second edition, chapter 1 page 4,writes: "A classically observable wave will result only if the elementary wavelets representing the individual quanta add coherently. " Is this analysis correct? The context is the following: We may read (1.3) [ë =...
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