Questions tagged [coherence]

Coherence describes properties of correlations among physical quantities of waves, with special emphasis on constant phase differences and frequencies in common. Such collective linkages enable stationary (temporally and spatially constant) interference.

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What is the eponym associated with the relationship between phase variance and expected magnitude?

I'm looking for the eponym (e.g., Stigler's law) associated with the following relationship: \begin{equation} \left|\langle e^{-i\phi} \rangle\right| = e^{-\langle\phi^2\rangle/2} \end{equation} which ...
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In real life, for a tennis ball to go through a wall, does it have to completely prevent the particles from interacting? (decoherence)

In real life, for a tennis ball to go through a wall, does it have to completely prevent the particles from interacting? What would happen if we tried infinitely in the real world, where the ...
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Do things and people in the macro world maintain a quantum decoherence? Or do you repeat quantum coherence and quantum decoherence every moment? [duplicate]

Does quantum decoherence remain uninterrupted for things and people in the macro world? Does the interaction of particles occur permanently without interruption in the macro world? Is that why our ...
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Why aren't macroscopic objects—or really the entire universe—coherent?

From what I understand, when quantum particles interact with the environment, they get entangled with it and the entanglement rapidly spreads until it's dispersed into the environment. So wouldn't ...
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Coherence and superposition principle - equivalent properties?

I'd like to revisit a question I posted earlier. While I now know what coherence means (and how it can be defined) I still struggle to give an intuitive explanation that goes along with the word "...
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Coherent and incoherent electron diffraction

I am learning about electron diffraction in a transmission electron microscope and was wondering about following topic: A single crystalline material will exhibit a diffraction pattern with discrete ...
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What is the meaning of this statement in Optics, Born-&-Wolf regarding coherence length?

In their treatise "Principles of Optics", Max Born and Emil Wolf state in the section on coherence length (page 317, para 1) that: ...... as the difference of optical path is increased, the ...
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Calculating Coherence Length and spectral width given an interferogram

I'm currently doing a lab in which we use a Michelson-Morley interferometer to analyse different light source. One of the mirrors in the interferometer is moved by a stepper motor. One of the light ...
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Long temporal coherence sunlight

What is the reason that sunlight have short temporal coherence? I know everybody says that coherence length is inversely proportional to bandwidth. Does it mean that if sunlight passes through a very ...
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How to scale Poissonian light?

In quantum optics, coherent light with constant frequency, phase, and amplitude shows poissonian photon number statistics: $$P(n) = \frac{\bar{n}^{n}}{n!}e^{-\bar{n}}.$$ A well-known result for ...
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Measuring unit of Spatial Coherence

Usually, the temporal coherence of light is described with the coherence length. This quantity can be measured conveniently with an interferometer and is directly related to the coherence time and the ...
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What does "coherent evolution" of an $N$-body quantum system mean?

In classical physics we know of coherence of waves and in quantum physics we identify coherent states. While those are clearly defined concepts/terms, in literature we regularly encouter also that a $...
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Reduce subjective speckle contrast while increasing objective contrast

I have been trying to learn more about laser speckle phenomena, and I’m wondering if it’s possible to increase the contrast of objective laser speckles while decreasing subjective speckles without ...
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Any experiment preserving coherence up to the Heisenberg time?

The Heisenberg time is inversely proportional to the level spacing. In a many body system, it is huge. For a single- or few-body system, it might be not that large. So, so far, is there any experiment ...
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Gaining Quantum information without destroying the coherence

I've read about new optical interference among several telescopes all around the globe. It is supposed to work by combining the quantum states of a photon A arriving at once in every telescope of the ...
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What does the coherence time signifies?

I was reading a book named "Laser Physics And Applications by LV Tarasov". In page 19, It tells that The degree of non-monochromaticity and the time interval $\tau$ consistent with the ...
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Relation between first-order correlation function and coherence time

In quantum optics books, it is often shown the following relation between coherence time and first-order correlation function for chaotic light with Lorentzian or Gaussian spectra: $\tau_{c}=\int_{-\...
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Degree of first-order coherence doesn't imply always 100% visibility?

In classic books of quantum optics visibility of interference fringes (for example in Michelson interferometer) is equal to the modulus of the degree of first order coherence $g^{(1)}$, when the ...
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Electric field at detector in double-slit experiment

I am currently working through Knight & Gerry "Introduction to Quantum Optics" and struggle with the following. The setting is an ordinary double-slit type of experiment, i.e. we have a ...
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How does the double slit experiment ensure phase coherence?

I am quite unsure of how phase coherence is ensured in the double slit experiment. The most typical answer that I have found is that the path length difference between the lights going in either ...
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Relationship between Purity and Coherence of a quantum state

I know that there is an important relationship between purity and coherence. Loss of purity is often used as a basis-independent sign of decoherence. I'm not completely sold on this. I would have ...
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First-order coherence of thermal light

so I have the following question: I want to calculate the first order coherence function for the light coming from a thermal source. I should note that this is a question I have and not a HW problem. ...
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Does a low-gain free electron laser (FEL) emit coherent radiation?

For the low-gain free electron laser (FEL), an external field is injected into an undulator alongside an electron bunch. Due to phase slippage (because the light is faster than the electrons) the ...
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Do coherent wave sources need to produce waves of the same wavelength?

I've been taught that in order for two sources to be coherent, they must produce waves with identical wave speed, wavelength and frequency. However, when I think about this more, I question whether ...
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Is there a fundamental time limit to how long a superposition of two states can maintain coherence?

If a system composed of two states is coherently split, for example, into a coherent superposition of two momentum states or two hyperfine levels of the ground state. Is there a fundamental time limit ...
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Negative coherence

I am constructing the interference of two Gaussian wave packets onto which I apply a complex phase, attempting to obtain the coherence measure C (how well they overlap). The $\Delta x(T)$ term in the ...
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Does colliminating light with optics require monochromacity? Or coherency? Or both?

I've had it in my head for a while that collimation requires both monochromacity and coherency. However, lately I've started thinking about it and I can't really see why coherency is required for ...
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Quantum and non-equilibrium properties of the radiation, emitted by "thermal light sources"

Free photon gas Let us consider different thermal light sources, such as the Sun, an incandescent lamp, or a fluorescent bulb. In elementary quantum mechanics and statistical physics one describes the ...
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Relationships between elements in the density matrix

Say we have a density matrix for a spin-1 system given by: $$ \hat \rho = \left[\begin{array} A \hat \rho_{00}& \hat \rho_{01} & \hat \rho_{02} \\ \hat \rho_{10} & \hat \rho_{11} & \...
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Why ordinary sources of light such as bulb, flash light,.. etc cannot behave as coherent sources? [duplicate]

A coherent source is a source which produces waves of constant frequency which does not change with time. In other words, it produces waves with constant phase difference which does not fluctuate. But ...
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Why does coherent enhancement cannot lead to a power improvement better than the square of interaction length?

I'm studying for an advanced course on photonics. The professor gave us a bunch of reference questions for the exam, like the following: Why does coherent enhancement cannot lead to a power ...
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Wiener-Khinchin theorem for multimode coherent light

The Wiener-Khinchin theorem says that the Fourier transform of the degree of first order coherence is equal to the power spectral density normalized (Loudon, Quantum theory of light, pag. 102): $F(\...
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How can sub- and super-Poissonian statistics be distinguished from bunching and antibunching?

This is my first post here, so I apologize if there's something wrong. I am studying quantum optics and I found myself in trouble with the difference between bunching/antibunching and super poissonian/...
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Is coherent source needed to observe Bragg diffraction?

In undergraduate solid state physics, derivations of Bragg scattering are often short and scarce in detail, often resorting to virtual "lattice planes" (which I do not find convincing) or ...
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How do you derive the Rayleigh Roughness Criterion

In many textbooks and papers discussing electromagnetic scattering from rough surfaces, the Rayleigh Roughness Criterion is defined as: $h < \frac{\lambda}{8\cos\theta}$ (1) In (1), $h$ is the ...
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Can we achieve coherence/high directivity by polarisation?

I can understand little bit what coherence means and little bit what is polarisation means. The question is there a way to achieve coherence by polarisation? Specifically consider a source of dipole. ...
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Coherence and $g^{(2)}$

We talk about how coherent light has a $g^{(2)}(\tau)=1$ and thermal light has $g^{(2)}(0)=2$. However, we can talk about the coherence length of a thermal source if we put it through a very narrow ...
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Must the distance between the reflectors of a coherent laser be a multitude of its intended wavelength?

Must the distance (aka cavity length $L$) between the two reflective mirrors of a spatial and temporal coherent laser be a multitude of the lasers intended wavelength? If no, why not? If yes, how does ...
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Why is laser light described by a coherent state?

This is a follow-up to this recent answer by Wouter to this related question from 2015, and a comment by Emilio Pisanty underneath. I have read the papers by Mølmer, Bartlett et al., Wiseman, and ...
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Coherence time of beams from broadband FT instruments?

To determine whether incident and reflected waves interfere within a film, we have to consider whether the coherence time of the beam is less than the travel time of the beam in the film. (This is the ...
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What does it mean for a light to have low-coherence?

I've been searching about OCT imaging and understood that it is based on low-coherence light source. The problem is, I don't understand what is a low-coherence light source. Based on what points that ...
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What is 'coherence' of a quantum system (with examples)?

The term 'coherence' is frequently used in relation to certain quantum systems such as LASER, superconductors, etc. Please help me understand what we mean by the coherence of a quantum system. How do ...
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What is the coherence length of Hermite-Gaussian Modes?

Can one derive the coherence length of a Hermite-Gaussian Mode just from the field \begin{equation} \small E_{m,n}(\vec{r})=E_0\frac{w_0}{w(z)}H_m\left(\sqrt{2}\frac{x}{w(z)}\right)H_n\left(\sqrt{...
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Coherent = Incoherent

A coherent source looses it’s coherence as it propagates, but an incoherent source gains coherence as it propagates. This 28 sec video illustrates this.This 28 sec video shows incoherent to coherent. ...
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Should two laser pointers always shine a point on the screen with same intensity? [duplicate]

Lasers are coherent beams so they can be totaly cancelled out if applied a source out of phase of 180 degrees. So the question is simple... do two different coherent lasers of same luminosity when ...
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Is the universal wavefunction globally coherent?

In Wikipedia's article on quantum decoherence, it states that despite decoherence creating the appearance of wavefunction collapse, A total superposition of the global or universal wavefunction still ...
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What is superradiance?

What is superradiance? I came across this term recently (used widely to explain fast radio bursts). After googling a bit, what I understand is: When several atoms are confined in a very small area and ...
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Schrödinger's Cat: Are the different quantum states inside the box coherent before it's opened?

Assume that the walls of the box prevent any interaction between the inside and the outside. Are the different quantum states within the box (living cat, dead cat, and all permutations thereof) ...
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Are coherent states destroyed by measurements?

For a quantum harmonic oscillator in a coherent superposition, what happens if the energy is measured? Would it collapse to an energy eigenstate (a single excitation) corresponding to the result of ...
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Why is a laser beam monochromatic and coherent? [closed]

Why is laser beam monochromatic and coherent? My reasoning: Laser is monochromatic because there is equal energy difference between metastable and ground states. Laser is coherent because there is no ...
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