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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.

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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Entanglement and coherence

I have come across a wonderful review of entanglement by Chris Drost in his answer to this post. One part that left me puzzled was: (This post is merely an attempt to understand a portion of Chris' answer, unfortunately I do not have enough…
user098876
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Why is the photon emitted in the same direction as incoming radiation in Laser?

When an atom “lases” it always gives up its energy in the same direction and phase as the incoming light. Why does this happen? How can this be explained? How does the photon generated because of stimulated emission, know which direction to…
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How are coherent states in the real world made?

Coherent states are quantum states that are said to act "as classically as possible". You can define coherent states for the harmonic oscillator, or more generally for any collection of harmonic oscillators, such as a free quantum field. It is often…
knzhou
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Where does a fermionic coherent state live (which Hilbert space)?

There have been a couple of questions on fermionic coherent states, but I didn't find any that covered the following question: If I define a coherent fermionic state in the 2-level-system spanned by $|0\rangle$ and $|1\rangle$, I will write it…
LFH
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Why Does Passing Light Through a Slit Make it Coherent?

Related: Why incoherent source of waves have to be passed though a single slit for double slit interference pattern to occur? If you have a an incoherent light source, why does passing it through a single slit make it coherent? Surely it makes a…
user24082
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How are the definitions of a coherent state equivalent?

I am trying to understand coherent states. As far as I could find there are three equivalent definitions and in general many sources start from a different one, still I fail to see their equivalence. I restate the definitions and their equivalences…
ckrk
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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 parts of van Enk and Fuchs referenced therein. I have…
The Vee
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Spatial coherence of SLED light (width of active region?)

As far as I know, super-luminescent diodes (a.k.a. SLEDs) have a very similar structure to laser diodes, except that the front faces of the active region are tilted to prevent the sort of reflections that would benefit lasing. However, I know that a…
Wylde
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What is coherence in quantum mechanics?

What are coherence and quantum entanglement? Does it mean that two particles are the same? I read this in a book called Physics of the Impossible by Michio Kaku. He says that two particles behave in the same way even if they are separated. He also…
user29660
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Quantum interpretation of light coherence

When I studied interferences, I saw that only coherent sources could interfere. In physics, two wave sources are perfectly coherent if they have a constant phase difference and the same frequency, and the same waveform. Coherence is an ideal…
Cabirto
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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 light source that I have access to is a tunable…
slithy_tove
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What is superconducting coherence length?

I'm an electronics student, and don't know much about some physics concepts. I was studying superconductivity and came to the London equation, Meissner effect and BCS theory. I kind of understood these things, but am still wondering: what is…
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Non coherence of Fermions and Bosons through $U(1)$

I "know" the textbook answer why we cannot write, $$ |\psi\rangle = a|j=\tfrac{1}{2}\rangle + b|j=1\rangle $$ as "each term in the quantum superposition transforms differently under $U(1)$", $$ U(2\pi)\ |\psi\rangle = -a|j=\tfrac{1}{2}\rangle +…
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How does such strange microscopic behavior at the atomic level (quantum mechanics) lead to the macroscopic behavior at our level?

So, I'm only a high school student researching quantum physics, and I find it very interesting. However, there's one question that keeps nagging at me in the back of my head. How exactly do odd behaviors like quantum parallelism that occur on the…
Brandon
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What exactly is the "coherence" between waves?

I know, by definition, that coherence means that a pair of waves have constant phase difference. What does this mean? Does it mean they always have a 360 degrees, or 0 degrees phase difference? Or could they have 40 degrees or any constant phase…
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