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Questions tagged [quantum-entanglement]

Quantum entanglement is the mechanism by which quantum correlations between two sub-systems survive even after being physically separated from an interaction region. The correlations could in principle survive without neither time nor space constraint.

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Polarization analyzer [closed]

My understanding is that the entanglement between two photons is not broken by a polarization analyzer. One will emerge from the Y projector and one will emerge from the X projector. Is this correct?
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Entangled Photons passing different polarizers

Consider two entangled photons leaving a source. Both are polarized at angle 0. One travels to the left and the other to the right. The one on the left encounters a polarizer at 30 degrees and the one ...
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Vacuum flucutuations = local entanglement between quantum fields?

I'm puzzled by this statement by Dieter Zeh: "Various types of quantum fluctuations (in particular vacuum fluctuations, often visualized in terms of 'virtual particles') are used to describe ...
Husserliana's user avatar
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Are states partially ordered in the same way via entanglement and Bell violations?

Recall what a partially ordered set is. Let $E(\rho)$ be an entanglement measure. Let $B(\rho) \leq 0$ be a Bell inequality. Define the Bell violation measure with respect to $B$ as $$\tilde{B}(\rho) =...
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Why can we not experimentally evaluate the expectation value of the CHSH operator after simplifying its expression to check entanglement?

We have the observable corresponding to the CHSH inequality given by, \begin{align} \mathcal{\hat{S}} &= \hat{Z}_1 \left(\frac{1}{\sqrt{2}} (-\hat{Z}_2 - \hat{X}_2)\right) + \hat{X}_1 \left(\frac{...
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Where to find the Expectation Values of Aspect's and Clauser Experiments [duplicate]

Alain Aspect and John Clauser won the Nobel Prize in Physics for their epxeriments on quantum entanglement. Does anyone know where to find the raw data or at least published calculated expectation ...
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Bell's inequality in Claude Cohen-Tannoudji's Quantum Mechanics Volume III

Claude Cohen-Tannoudji et al, Quantum Mechanics Volume III, 2020 edition, Chapter XXI.F-3-b.$\alpha$ (F-7) on p.2210 states $$\big\langle\hat A(a)\hat B(b) \big\rangle={\cal P_{++}+P_{--}-P_{+-}-P_{-+}...
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Example of a classical correlation and a quantum correlation

I'm trying to understand the fundamental differences between classical and quantum correlations through examples of a quantum entangled state and a classically correlated state. I know that this is an ...
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6 votes
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When is a state entangled?

I have read from What's the difference between an entangled state, a superposed state and a cat state? that an entangled state is one that cannot be expressed as product state. Suppose we have the ...
Daniel Janjani's user avatar
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What is a physical example of entanglement occurring naturally?

What is a physical example of entanglement occurring naturally? I am looking for an instance that I would see in nature outside the lab. I am looking for this example to be concrete and physical. I ...
Relativisticcucumber's user avatar
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Is it possible that a macroscopic object tends to a separable state without the need for objective collapse?

For a multi-particle system, superposition is in some sense equivalent to entanglement; with the Dirac field being treated as classical under second quantization, for example, we could at least argue ...
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Understanding entanglement [duplicate]

I’m probably missing something obvious. I'm new to physics, so please bear with me. Imagine flipping a perfect coin in a vacuum. The moment we through it up it splits exactly in half, with both halves ...
Curious's user avatar
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Whats wrong with concept of "faster than light" information transfer device? (not about technical implementation, just about theory) [closed]

Yesterday, while lying in bed, an idea came to me, and I don’t understand why it won’t work? Setup: 1.1) Two infinite stores of entangled pairs of photons, each connected to a double-slit device. 1.2)...
Nikolay Prays's user avatar
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Importance of Environment in System-Observer Entanglement and Decoherence

It has been stated in places such as Sean Carroll's talks that including the environment in our description of a quantum measurement process is important in understanding why we do not "feel like&...
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Transmitting Data faster than the speed of light using entangled particles [duplicate]

If we have a pair of super-asymmetrical entangled particles, and move them a light year away so that they retain their quantum entanglement, and we set a clockwise spin (or vertical polarization) as 0 ...
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Why can quantum entanglement not use temporal encoding to transmit information? [closed]

It's common knowledge that quantum entanglement can't be used to transmit information; however, I'm struggling to understand why an encoding based on time intervals in which measurements are made ...
Max's user avatar
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Simple examples of computing the quantum discord

I'm looking for a relatively simple, nontrivial example of computing quantum discord (if such an example exists). Could anyone provide such an example?
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How can I construct a trivial product state in the continuum?

When working on the lattice it is easy to define a trivial product state. A state $|\psi\rangle$ is a trivial product state if it admits the following tensor decomposition, \begin{equation} |\psi\...
Truth and Beauty and Hatred's user avatar
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Time evolution keeps a certain product state always a product state. Is there a time-independent factorizable evolution for this state?

I am typically thinking of quantum spin chains in the following of some length $L$. I am OK without any locality in the assumptions on $H$. I have a product state $|\psi\rangle$ and a potentially very ...
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Quantum entanglement from different frames of reference and the conservation laws

From my understand, if the wave function of a particle in entanglement collapses by being observed, the other particle's wave function also collapse immediately. My questions are: Is it possible to ...
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Operational meaning of mode dependence of mode-entanglement

In this paper, Vedral shows that the entanglement in second quantized states can depend on the choice of modes. As a simple example, he points out that the ground state of two non-interacting harmonic ...
Varun Immanuel's user avatar
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Rotation of Entangled States $(R \otimes R)|\phi\rangle = e^{i\delta}|\phi\rangle$

Maximally entangled states are said to be rotational invariant should a rotation be applied to both particles. That is (for some rotation operator $R$): $$(R \otimes R)|\phi\rangle = e^{i\delta}|\phi\...
Dr. Ernesto Chinchilla's user avatar
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What is the relationship between long range order and long range entanglement?

I'm wondering whether there's a deep connection between long-range entanglement and long-range order. Here's my understanding of these two concepts. Entanglement. Short-range entanglement basically ...
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Monogamy of entanglement and Quantum Cryptography

I would like to understand thoroughly the monogamy of entanglement and its use in Quantum Cryptography. The literal phrasing is that : "Whenever two systems A and B are maximally entangled, a ...
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Is there any restriction for locally mapping a given 2-qubit density matrix into a desired 2-qubit density matrix with lower entanglement?

Suppose we're given a 2-qubit density matrix($\rho_{4\times4}$). we can apply two local maps on each of these qubits seperatly. So the output is density matrix($\rho^{\prime}_{4\times4}$). I'm ...
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Seperable Quantum States

Some similar questions have been ask before, but I still don't really get the definition of seperable states in quantum mechanics. Consider a bell state of a two qubit system. \begin{align} \left|\Psi\...
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What's the relationship between wavefunction (anti-)symmetrization and entanglement? [duplicate]

Wavefunction symmetrization for bosons, or antisymmetrization for fermions, renders the wavefunction no longer a simple tensor product, i.e. it is no longer separable. This is the same thing that ...
Adam Herbst's user avatar
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Does quantum entanglement arise from perpendicular time vectors? [closed]

From what I understand, "quantum entanglement" is a phenomenon where certain information travels instantly between entangled particles, regardless of distance in space. When thinking of ...
Quantum Wonder's user avatar
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1 answer
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Is two degree quantum entanglement possible? And by two degree of entanglement I mean one in space and other in time? [closed]

Whenever we try to entangle two particles, the entanglement lasts for a very short period. If we observe our present universe we can see that all the matters present in our universe are finely ...
PARADOXIAN PARADOX's user avatar
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States of entangled particles after no/partial/full measurement

I'm getting contradictory information from the internet concerning entangled particles, measurement, and state knowledge that I'm hoping can be cleared up with a simple setup. Say we have a source of ...
user401228's user avatar
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What is the appropriate 'Page Value' for entanglement entropy in a symmetry sector, say for example with a $U(1)$ symmetry?

Don Page derived the formula for the average entropy of a subsystem of a quantum system (assumed to be in a pure state), if the system is partitioned into two subsystems of dimensions $m$ and $n$, ...
Tanmay Bhore 's user avatar
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Why are PEPS more frequently used when simulatind 2D systems rather than branching MERA?

From what I've read it seems that PEPS is a go-to method while simulating 2D quantum systems. Why is it the preferred method rather than branching MERA? The contraction of PEPS is a #P-complete ...
brzepkowski's user avatar
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2 answers
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How to measure the direction of circular polarization of single photons?

I just watched this video about the 2022 Nobel prize in physics which mentions the experiments that have been conducted to test Bell's inequality. In the video, they also described the experiment by ...
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If there is nothing deeper than quantum mechanics, does that mean correlations in entanglement are coincidences? [closed]

Again, I am a layman, so please forgive me if I use incorrect terminology. So according to Bell’s theorem, we cannot have a local theory that determines, say, the spin measurement of either particle ...
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4 answers
196 views

Does entanglement rule out oscillating properties?

This will be a very noob question and I am a novice but I just wanted to understand entanglement better. This is just a toy analogy that I thought of and again, I don’t know the intricate details of ...
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2 answers
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How can the quantum state of one particle depend on the other in entanglement without non-locality? [duplicate]

There are many different interpretations of quantum mechanics. Some posit determinism and non-local influences between particles. Others do not, such as in the Copenhagen interpretation. My question ...
user avatar
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1 answer
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If particles do not have definite properties before measurement in entanglement, what are they? [closed]

It is said that in quantum entanglement, a photon does not have definite properties before measurement. I have two questions. Is this true for all interpretations of quantum mechanics? what does it ...
inquisitive 's user avatar
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3 answers
219 views

Was Einstein "spooky action at a distance" about entanglement or about wave function collapse?

I've been watching Sabine's videos and this is my understanding: There is no "spooky action at a distance" based simply on entanglement. Entanglement is a correlation. There is no ...
Ray Wood's user avatar
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How exactly do we go from an indefinite to a definite property at measurement in quantum mechanics? [duplicate]

I’m terribly confused on how this process happens. In entanglement, suppose we are making a measurement on the spin of an electron. From Alice’s perspective, the spin could either be up or down. Alice ...
inquisitive 's user avatar
4 votes
3 answers
292 views

Is maximal entanglement basis independent?

Is there a basis such that if we measure the bell state $\dfrac{|00\rangle+|11\rangle}{\sqrt2}$ the results might not be correlated at all (or at least not maximally)? For example $\dfrac{|00\rangle+|...
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What states/properties of a pair of particles can be entangled?

From my understanding two particles' quantum states being entangled is what leads to the capacity to determine a property when the other entangled element is measured. I'm looking into exactly which ...
Thomas Mulier's user avatar
1 vote
4 answers
147 views

Can a single particle state be entangled? [duplicate]

When I do think of entangled states I usually do think of 2 or more particle states, which can not be written as products of 1 particle states. But what, if we have just 1 particle being described by ...
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Can a non-local theory be consistent with special relativity?

If there was a non-local theory that explained quantum entanglement correlations, does it follow that it would violate special relativity?
Hume's user avatar
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4 answers
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All the logically possible ways to get around Bell-type inequalities

I'm a high school student, so my (mis)understanding here is not very rigorous or precise, but I will write it below so as to concretely frame my question: Bell's theorem takes three assumptions ...
user110391's user avatar
5 votes
2 answers
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Understanding the measurement of "evil" using the GHZ effect

I was reading this interesting article originally published by Quanta Magazine, which tries to explain the GHZ effect in simple terms. The GHZ effect is, in the physicist Sidney Coleman’s words, “...
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Limiting factors in traditional microscopy

I have been researching Entanglement-Enhanced Microscopy, and the overall breakthrough seems to be obtaining a resolution of $1/N$ as opposed to $1/\sqrt N$. This led me to ask what the limiting ...
murtadee hughes's user avatar
2 votes
4 answers
189 views

Does Bell’s theorem rule out changing hidden variables?

In the EPR experiment, it is said that there are predictions that are made if there are hidden variables determined by something locally in each particle that results in a correlation in entanglement. ...
Alejandro's user avatar
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3 answers
148 views

How do physicists talk about spin of individual particles when the universe is massively entangled?

The following two things seem to be true: (1) The universe is massively entangled because the wave function that describes the entire universe has measure 1 of being entangled. Further, given how ...
zzz's user avatar
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What is the percentage of entangled content (matter and radiation) in the universe after big bang? And what are the implications?

I searched for entangled matter in cosmological context but didn't find anything that actually answers my question. Big bang nucleosynthesis is actually a series of chain reactions that produces a ...
jim andr's user avatar
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0 answers
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Computing Fubini-Study expectation values over $\mathbb{C}P^n$

In finite-dimensional textbook quantum mechanics, we postulate that states of our system are rays in a Hilbert space $\mathcal{H}$ with dimension $\dim{\mathcal{H}} = n+1$ where $n \in \mathbb{N}$, ...
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