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Entanglement and coherence

Okay, this is getting even more into depth, which is great stuff! I heartily recommend anyone who is this dedicated take a few courses on the subject, if you haven't already. The state matrix ...
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12 votes

Entanglement and coherence

I am posting these notes following a request for further information regarding this question. Should not affect the OP's choice of answer. Notes added in proof: On the meaning of quantum coherence: ...
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11 votes

How light from a thermal source gets coherent behind a small hole?

Spatial coherence can be seen as a Classic effect of EM wavefronts, with no necessity to model light as photons.[*] We can use a wave-tank to produce incoherent water waves, or a stretched rubber ...
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10 votes
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How far apart can the slits be in a double-slit experiment using direct sunlight?

The visibility of interference fringes from a double slit depends on the how correlated the fields from the source at the two slits are. The typical slit-distance for which the fringes are manifestly ...
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Why are coherent states of harmonic oscillators called "coherent"?

Coherent states are eigenvectors for the (bosonic) annihilator,$$\hat a ~|\alpha\rangle = \alpha~|\alpha\rangle,$$and if we define the position and momentum quadratures as $\hat x = \hat a^\dagger + \...
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What is superconducting coherence length?

Superconductivity is about the appearance of an energy gap $\Delta$ in the excitation spectrum of the electron quasi-particles, which become paired in the form of a Cooper pair below the critical ...
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What is meant by phase coherence of electrons?

Strictly speaking, a phase coherent electron device is an electronic device whose dimensions is smaller than the phase coherence length of the electrons. This definition is the one adopted in ...
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9 votes

How are coherent states in the real world made?

Coherent states appear in nature because they're what you get when you drive a harmonic oscillator through a dipole interaction. Suppose we push on a harmonic oscillator with a time dependent force $F(...
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8 votes

How are coherent states in the real world made?

Coherent states arise naturally in many quantum systems besides the free electromagnetic field. Many of them are mentioned in the following review by WM Zhang, which is actually dedicated to the same ...
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7 votes

Do stars produce spatially coherent light? Why?

The star acts as an "effective point source" since it is so far away and its angular extent is so small - in other words, the optical signal arriving at earth is "very nearly" a plane wave with the ...
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What exactly is the "coherence" between waves?

Waves can be coherent and yet not have the same wavelength. It is sufficient that they have the same frequency - because that is sufficient to imply a constant phase difference. If you make a ...
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Non coherence of Fermions and Bosons through $U(1)$

This is the prototypical example of a superselection rule. The operator $U(2\pi)$ commutes with all observables (because it represents a full rotation, and is hence physically a "do nothing" operator)...
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Why is laser light described by a coherent state?

A true but misleading identity Consider a single mode, and let $|n\rangle$ be the state with $n$ photons in that mode. A coherent state has the form $$ \newcommand{\la}{\langle} \newcommand{\ra}{\...
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6 votes

Why is a filament lamp an incoherent light source?

There exists two types of coherences: temporal coherence, which describes how the field amplitude $u$ at time $t$ is correlated with the field amplitude at time $t+\tau$, $$\langle u(x,t)\cdot u(x,...
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Why is a laser beam monochromatic and coherent?

This is actually an excellent question because I feel that the actual operation of lasers is usually dumbed down to a simple narrative of "in-step photons" that come out of a hole. Even by ...
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What is a wavefront?

A wavefront is the crest of the wave. When you go down to the beach, and see those things called waves, the front is the whole line that is at the same height. In electromagnetics, it's the same ...
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How to find Intensity from field of train of pulses?

You're conflating two different views on the description of the attosecond pulse train; in particular, you're flitting back and forth between the time-domain and frequency-domain descriptions, and it'...
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Validity of the 'photon wavetrain' model of coherence

A comment about coherence in general I find the definition of coherence as some sort of "unrelated phase" problematic for a couple of reasons: This formulation somewhat implies that coherence is ...
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5 votes

Why are coherent states of harmonic oscillators called "coherent"?

Coherent States of Harmonic Oscillator are Quantum Mechanical states which have definite phase and Minimum Uncertainty. The Quantum Mechanical states of Harmonic Oscillator do not have definite phase ...
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5 votes

Why are coherent states of harmonic oscillators called "coherent"?

In my opinion, the most intuitive way of explaining the meaning of harmonic oscillator coherent state is the following: An Harmonic Oscillator Coherent State (AOCS) is a solution of the time-...
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What is meant by quantum coherence in the context of a two-level atomic system?

When you prepare a pure quantum state of a two-level system, $| \psi \rangle = a |0 \rangle + b |1 \rangle$, the associated density matrix will be $$\rho = \begin{pmatrix} |a|^2 & a b^* \\ a^* b ...
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Difference between pure quantum states and coherent quantum states

The confusion arises because the word “coherent” evolved to have different meanings in different contexts where it is not fully qualified. Going back to the 2-slit experiment, one shows that the ...
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Quantum interpretation of light coherence

That's a very good question, but is actually very difficult to answer. The problem is that to understand the quantum mechanics of light, you really have to understand quantum field theory, not just ...
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5 votes

Hong-Ou-Mandel Effect for incoherent photons

The Hong–Ou–Mandel effect can be described like this: Two photons enter a particular type of beamsplitter, one from each side. If the two photons are "identical" in the sense defined below, then the ...
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Optical coherence versus quantum coherence

Coherence is the ability to track the phase of a wave. In optical coherence, a light source is coherent in time/space if you can determine the phase difference between two points in time/space. For an ...
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Relationships between elements in the density matrix

Consider a finite dimensional complex Hilbert space $H$ of dimension $d$ equipped with an inner product denoted by $(\cdot,\cdot)$ and let $\rho$ be a generic density operator, i.e. a positive semi-...
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5 votes

In real life, for a tennis ball to go through a wall, does it have to completely prevent the particles from interacting? (decoherence)

The problem is the tension here between "real life" and "infinitely". In "real life" we cannot do an experiment infinitely many times, and if we tried other factors like ...
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4 votes

Simple explanation of Coherent integration radar

Yes, that is about it: coherent integration = voltages (currents) are added, noncoherent integration = powers are added. For example, say the transmitted radar pulse is $$p(t)cos(\omega t) \text{ ...
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What exactly is the "coherence" between waves?

Coherence means a constant phase relationship; the phase difference could be anything, such as $\pi$ or $7 \pi / 4$. Naively, that means that two waves are coherent if and only if they have the same ...
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How does the coherence of a spin state relate to the physical concept of "coherence"?

Suppose I have two different spins, spin $A$ is in state $\left|\uparrow \right\rangle$ and sping $B$ is in state $\left|\downarrow \right\rangle$. I give you one of these spins randomly, so you have ...
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