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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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Coupling a single mode laser to multimode fibre - does this reduce coherance?

If I have a single mode laser coupled to a multimode fibre with imperfect overlap between the laser mode and the fibre core, the light will propagate in several LP modes. In this case, does the light ...
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Loss of coherence in the double-slit experiment

Let's suppose we perform the double-slit experiment, using two detectors to detect which slit the particle has passed through. I describe the detectors using quantum mechanics, so they are represented ...
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Are temporal coherence (for classical EM waves) and quantum coherence related concepts?

For my wave optics class, I was taught that the temporal coherence of a light source is the average time interval for which the phase of that light is predictable, after which the source will undergo ...
Hervé Schmit-Veiler's user avatar
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Why is a monochromatic wave temporally coherent for all time delays $\tau$?

I've been learning about coherence of waves and the Wikipedia page on says a monochromatic electromagnetic plane wave is coherent because: A wave containing only a single frequency (monochromatic) is ...
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Significance of the phase of the condensate as compared to that of the regular Fermi sea in the Anderson-Higgs mechanism

I do not fully understand how the phase of the charged Cooper pair condensate is different from the phase of e.g. the Fermi liquid in a regular metal. The state of the metal (any quantum state really) ...
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When to add signals coherently and incoherently?

I just learned in transmission electron microscopy about coherent and incoherent summation. I can't seem to wrap my head around the fact that intensities can be added incoherently. Why can you do that?...
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Spontaneous emission and coherence

Assume I prepare a linear superposition $\frac{1}{\sqrt{2}}(|g \rangle+|e\rangle)$ between a ground and excited level for a large number of "atoms" (it can by any multilevel system, not ...
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Relation between a diagonal element of a density matrix and the off-diagonal elements in the same row/column [duplicate]

In a density matrix $\rho=(\rho_{ij})$, the diagonal elements $\rho_{jj}$ contains the information about the probability $|\rho_{jj}|^2$ that the $j$-th eigenstate is occupied by the system. Meanwhile,...
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Quantum Coherence [closed]

Coherence is a term frequently used in quantum mechanics. However, I struggle to get the bigger picture. In fact, it seems like some definitions contradict each other. Could someone provide a basic ...
QuantumQuasar's user avatar
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Necessity of Coherence for Interference in the Context of Indistinguishable Photon Paths

I've been studying the interference phenomena observed in the double-slit experiment and came across a concept that piqued my interest. It is well understood that for interference patterns to form, ...
Hakan Akgün's user avatar
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Transition from Coherent to Incoherent Light / Partially coherent Light for Imaging

I want to know how to model partially coherent/incoherent light for imaging applications. Usually, you find mathematical treatments of imaging just for the extreme cases. For the coherent light, there ...
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Is it possible to model unidirectional evolution of an atom-photon system using the Schrodinger equation?

The physical system I'm imagining is pretty simple: suppose there's an atom in free space with three states $|g_0\rangle, |g_1\rangle, |e\rangle$ initialized to some superposition of $|g_0\rangle$ and ...
whooie's user avatar
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Why do light bulbs don't produce coherent waves? [duplicate]

My textbook says that light ways produced by light bulbs are not coherent but it doesn't describe the reason. I was wondering how could two waves not be coherent regardless of the source they are ...
Muhammad Ali's user avatar
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How to understand the spatial coherence of light?

Can I understand spatial coherence in the following way (I'm not sure if it's correct)? As shown in the diagram: Each wavefront or wave surface on the left is irregular, so it is spatially incoherent. ...
Sean Chu's user avatar
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How monochromatic waves can be incoherent?

Is it possible to have a monochromatic wave which is incoherent? On the one hand, it could be obtained by irradiating a laser beam on a strongly scattering medium like white paper. But on the other ...
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Can you have diffraction without a slit, simply by reducing the size of light source?

Since the diffraction pattern only depends on the width of the slit and the wavelength of light, could we see a diffraction pattern if we use an extremely small (to the order of micrometers) light ...
Dumber Everyday's user avatar
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Modelling incoherent sources

Suppose some (coherent) light propagates onto a one-dimensional barrier with two open slits, each of length $\delta$ and separated by a distance $s$ (we can assume the centroid is known and placed at $...
Balter 90s's user avatar
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Clarifications on interference of waves

Here is my understanding: Superposition describes the effect of two waves, of the same type, coinciding at a point, stating that the resultant displacement is equivalent to the vector sum of the ...
Quin Gardiner Bax's user avatar
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Derivation of the coherence length of the light coming from a star

Consider a star perceived under an angle $\alpha$ from the earth. In the Hanbury Brown Twiss experiment, they say that the coherence length $L_{coh}$ of this light is given by $$ L_{coh}= \frac{\...
Nicolas Schmid's user avatar
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How does mirror reflection affect laser beam coherence?

I am an engineer that uses 10.6 micon laser beams to damage material. I am not a physicist so be gentle with your answers please. Two adjacent photons with their wave phases locked strike a 45 degree ...
 Russ  John's user avatar
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Is a ray of light, produced by an ordinary light bulb, filtered through a perfect monochromatic filter and 2 (in-line) pinholes, coherent?

A physicist once told me it is not, because even if it is perfectly monochromatic and perfectly collimated (by the two in-line pinholes), each photon has a completely random phase. For a while I ...
Felix Tritschler's user avatar
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How can valley coherence be defined if the crystal is initially in a mixed state?

In the field of valleytronics, they refer to valley coherence as: "the phase relationship between a particle in a superposition of two different valleys" [S. Vitale et al., Small 1801483 (...
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Incoherence and the bandwidth of a source

It is generally accepted in the literature that the emission bandwidth $\Delta\nu$ of a light source is inversely proportional to its coherence time $\tau_\mathrm{coh}$, i.e.: $$ \Delta\nu \propto 1/\...
moryev's user avatar
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Are hydrogen lamps coherent?

Those hydrogen or mercury tubes should not be coherent, but when looking at them through a grating without using a small aperture in front of the tube, you still see the discrete lines. Or just any ...
Cosmo's user avatar
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Coherence, Correlation and causality in Quantum Field Theory

I've started to study in details quantum optics and I find difficulties in linking the concepts of coherence and correlation among fields, especially because I'm building right now a background on ...
Luca Bianchi's user avatar
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How to imagine the superconductive coherence length

the Ginzburg-Landau theory predicts two parameters: the penetration depth of a magnetic field into the superconductor $\lambda$ (which from agrees to London's penetration depth) and the coherence ...
choc1709's user avatar
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What is the reason two rays originally polarized orthogonally but were rotated to parallel polarization does not visibly interfere even slightly?

The Fresnel–Arago laws by Augustin-Jean Fresnel and François Arago summarise some of the more important properties of interference between light of different states of polarization. The laws are as ...
Duke William's user avatar
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In a plain language, what does it mean when one says LASER has a high temporal and spacial coherence?

Does spacial coherence mean that if I took a cross section of the laser beam beam, then I can find a relation between the phase of the electric field between any two points across the cross section ...
Jack's user avatar
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Measuring coherence length with the Michelson Interferometer

In lab, I've been asked to measure the coherence length of a green-light laser source using a Michelson Interferometer. To do so, I configure the interferometer such that a circular fringe pattern ...
PerplexedDimension's user avatar
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What defines "minimal coherence" as a condition for the emergence of stationary interference in a chaotic wave field?

Consider the following observations: A superposition of two electromagnetic waves with different frequencies will never produce visible interference patterns. Such waveforms will produce ...
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On the field leakage problem in quantum experiments

In a quantum experiment, does the leakage of the electromagnetic fields lead to the suppression of quantum effects? For example if I have an electron in a box, in some quantum superposition. Can I use ...
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Quantifying coherence with density matrices

I'm aware that the coherence of a statistical ensemble can be measured by looking at the off-diagonal elements of its density matrix, which is computed as the weighted sum of the density matrices of ...
slithy_tove's user avatar
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Chaotic monochromatic light

I am currently reading through the book "The Quantum Theory of Light", by Rodney Loudon, in order to prepare for a science fair project which will involve measuring plant response to ...
slithy_tove's user avatar
2 votes
1 answer
479 views

What is a 'good' coherence/correlation function for multimode spectra?

Following up on the question here and this answer in particular, I would like to pose the following question: How to derive a formula for the coherence length of a multispectral source, such as a ...
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9 votes
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"Anti-collimators" for laser beams

I'm a student currently working on a science fair project which will measure the response of plants to coherent vs incoherent light of varying wavelengths. However, the only tunable and narrowband ...
slithy_tove's user avatar
8 votes
2 answers
579 views

Magically reappearing fringes in Michelson interferometer for large path length differences?

I've built a simple Michelson interferometer from two mirrors and a beam splitter according to the following schematic: Image source My setup differs from the one in the publication in that mirror M1 ...
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1 answer
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Unsustained Interference Visuals

How does the interference (of coherent EM waves) that is not sustained look like? Kindly provide me with pictures or visual description. I tried searching the web but no avail. People say fluctuations ...
Gaurav Kumar's user avatar
4 votes
2 answers
335 views

Does light from incoherent source interfere?

We say that for us to observe interference we use coherent source but even if we use non coherent source light should still interfere, right?
Kutubkhan Bhatiya's user avatar
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1 answer
35 views

Interferences between coherent sources facing each other

I don't understand how 2 coherent sources of longitudinal waves (i.e.: sound) facing each other have destructive interferences at certain fixed points between the 2 sources. How come those points aren'...
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Phase shift between coherent longitudinal waves facing each other [closed]

In the following problem: 2 speakers are 6 feets away from each other and facing each other, assuming that they are 2 coherent sources of sound waves (so longitudinal waves), what is the phase shift ...
c.leblanc's user avatar
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1 answer
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Molecules are made up of numerous subatomic particles, so don't they constantly interact? How can we achieve quantum coherence? ($\rm C_{60}$)

Since $\rm C_{60}$ is a molecule made up of numerous subatomic particles, no matter how separated from the environment, the interaction between $\rm C_{60}$'s subatomic particles inevitably causes ...
Nunes's user avatar
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1 answer
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What is the difference between coherence and entanglement? [duplicate]

I am confused to the two terms, my question is what is the difference between coherence and entanglement?
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1 answer
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Squeezed vacuum state

From: Loudon, Rodney. The quantum theory of light. OUP Oxford, 2000. Consider the single-mode quadrature-squeezed vacuum state defined by $ | \zeta \rangle = \hat{S} (\zeta) | 0 \rangle $ where the ...
MementoMori's user avatar
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1 answer
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Is this an example of a spatially coherent wave?

My ultimate goal is to understand why the spatial coherence of a wave increases with distance from the wave source. From what I have read though, there seems to be conflicting definitions of spatial ...
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How does lasers emit photons of constant phase and frequency

In my alevel physics book it says that the electrond in the atoms of the laser get excited and de-excited releasing photons of constant frequency. How is this possible if the photons have energy ...
Richie's user avatar
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How does coherence of a light affect how we perceive a sample in a microscope?

First of all, does coherence of a light, which is used for illumination in a microscope, affect the sample, or the way we see/perceive it? I want to know what optical elements do I need to use if I ...
Meera's user avatar
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2 votes
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What does it imply when we say that the waves are perfectly correlated?

I am having a hard time understanding difference between correlation and coherence. I got a nice answer for coherence here- Are two waves being in phase is the same as saying that the two waves are ...
Aniruddha Trivedi's user avatar
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3 answers
159 views

Will quantum events ever occur on a macro-scale rather than a vacuum? Michio kaku says there's a chance we'll wake up on Mars tomorrow

https://www.theatlantic.com/science/archive/2018/10/beyond-weird-decoherence-quantum-weirdness-schrodingers-cat/573448/ In this post, it is shown that quantum decoherence in the macro world occurs ...
Nunes's user avatar
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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 ...
K. Nielson's user avatar
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1 answer
390 views

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 ...
Nunes's user avatar
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