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Questions tagged [bells-inequality]

Bell's inequality is a no-go theorem contrasting crucial correlations of physical observables in QM to those of the world as described by classical mechanics, essentially local hidden variables theories serving as a viable "explanation" of QM: QM inevitably violates these inequalities, as observed. Further use for the CHSH inequality generalization.

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Is there a systematic method to derive Bell's inequalities?

BACKGROUND The purpose of Bell's inequalities is to put bounds on certain combinations of classical measurement probabilities. One can then show that quantum entanglement can yield probabilities that ...
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Can we exlain Bell's Theorem without dwelving into Quantum Mechanics?

I have recently started learning Quantum Mechanics. We learnt some things on light polarization, so I referred to this site. Luckily I stumbled upon this answer which links to this site which sated my ...
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Does reality express non-local realism, local non-realism or non-local non-realism? [closed]

So I read that Bell's tests prove that reality cannot follow the requirements of both realism and locality. My understanding is that: Realism: Objects have definite properties before they are ...
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How are contextuality and locality related?

I know how quantum non-locality is defined in Wikipedia quantum nonlocality is a characteristic of some measurements made at a microscopic level that contradict the assumptions of local realism ...
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Does Bell's theorem imply nonlocality using a false assumption?

In https://arxiv.org/abs/1409.5158, the author concludes that Bell tests cannot refute local realism, because they employ a wrong analysis. He says: "The quantum joint prediction cannot be ...
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Bell inequality proof

I am trying to understand how the correlation function in John Bell's paper on EPR is derived for a spin singlet state $|{\Psi}\rangle$. This is defined to be $$ \langle{\Psi}|\left(\bf{\sigma}\cdot\...
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In a Bell scenario, why can correlations be nonlocal only if there are at least two measurement settings to choose from?

In (Brunner et al. 2013), the authors mention (end of pag. 6) that a set of correlations $p(ab|xy)$ can be nonlocal only if $\Delta\ge2$ and $m\ge2$, where $\Delta$ is the number of measurement ...
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Is there a relation between the factorisation of the joint conditional probability distribution and Bell inequality?

I'm approaching the study of Bell's inequalities and I understood the reasoning under the Bell theorem (pdf : https://cds.cern.ch/record/111654/files/vol1p195-200_001.pdf) and how the postulate of ...
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What is the variance of |S| in Bell's inequality (CHSH inequality)

Sorry that this isn't a quick question but I didn't know how to make it shorter. I am struggling with this for quite a long time and I would appreciate every help that I can get. I could not find a ...
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Shouldn't local realism imply the superdeterminism?

I see that Bell has ruled out Einstein's local realism but not supredeterminism. I see that Bell has confessed that superdeterminism kills his inequality. You cannot apply Bell's inequality if ...
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Can quantum randomness be somehow explained by classical uncertainty? [closed]

In quantum mechanics, the outcome of each measurement is random, distributed according to the squared amplitude of the wave function obtained from the Schrodinger's equation. Now, can someone suggest ...
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What is the reason for the reported opposite-of-QM handedness of photons in a Bell test with circularly polarized photons?

In https://file.scirp.org/pdf/OPJ_2016111414355792.pdf, a Bell test using circularly-polarized photons is reported. The data shows that coincidences happen in the reverse handedness predicted by ...
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Are all Bell tests valid only for linearly-polarized photons? [duplicate]

In https://arxiv.org/abs/1407.2605, it is argued that all photons are necessarily circularly polarized, and linearly-polarized photons must be a superposition of such circularly-polarized photons. ...
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Can Bell-CHSH inequality be used to prove quantumness of an physical effect?

I would like to apologize if this is the wrong StackExchange site but I couldn't think of anything better. I'm writing my thesis about random number generators. In a section about quantum random ...
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What experimental results lead us to conclude that the universe is nondeterministic?

I've recently been learning about Bell's Theorem and test experiments (e.g. as described here) and feel I understand why they imply that we cannot have local hidden variables. However, I have heard ...
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What is an example of a hidden variable model that meets the bound of Bell's inequality?

Following https://en.wikipedia.org/wiki/Bell%27s_theorem: The best possible local realist imitation (red) for the quantum correlation of two spins in the singlet state (blue), insisting on perfect ...
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Detection loophole in CHSH-inequality

For the CHSH inequality, one can derive a bound on the detector efficiency required to avoid the fair sampling assumption, and thereby close the detection loophole. This bound is: $$\eta > \frac{2}...
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What are non-local hidden variables?

It is said that Bell's Inequality basically denies all possible local hidden variables theories as solutions to entanglement but what does a non-local hidden variable theory mean and how does it get ...
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Tracking down the locality assumption in a CHSH inequality derivation

CHSH inequality requires both locality and realism. I will equate here realism with counterfactual definiteness. Now counterfactual definiteness tells us that given two different measurements on the ...
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Are there Bell test experiments that don't require coincidence monitor?

CHSH (2-channel) for instance doesn't count for a case where all 4 channels didn't detect anything (simultaneously) at some point in time. As I understand, such all-absence shouldn't happen/matter at ...
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Was the phase randomized in this particular CHSH test (M. A. Rowe and others)?

Measuring phase of photon should always be random while checking CHSH inequality, but i can't explain this: http://qudev.ethz.ch/content/courses/QSIT08/pdfs/Rowe01.pdf (the most clear experiment i ...
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Should S always be more than 2 to violate CHSH inequality?

Should the measured value always be about $2\sqrt{2}$ or is it just a maximum value and it is possible to measure something like $S = \{1.90, 1.8, 2.0, 2.4, 2.6, 1.8\}$ (with average equal 2) to ...
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Does the order of detection settings matter in CHSH-inequality check experiment?

Does the order of detection settings matters when checking CHSH inequality? For example, i'm trying to build a simple (and very approximate) computer simulation of experiment. Now i'm testing only ab ...
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How do we know that entanglement allows measurement to instantly change the other particle's state? [duplicate]

I have never found experimental evidence that measuring one entangled particle causes the state of the other entangled particle to change, rather than just being revealed. Using the spin up spin down ...
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Where is locality used in CHSH/Bell's inequality?

A very similar question is asked here, but I'm still confused :( From Bell, in a hidden variable model, $A = A(\lambda, a)=\pm 1$ is the observed spin of the first particle around axis $a$, and $B = ...
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Bell inequality as a game: Why is it impossible to always win?

Another Bell's Theorem Question I am trying to follow the simple model of Bell's Theorem outlined in this paper: https://people.eecs.berkeley.edu/~vazirani/s07quantum/notes/lecture1.pdf. Please read ...
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Why does E91 protocol use the CHSH inequality instead of QBER?

I am now studying about E91 protocol. In the process, I saw that E91 protocol utilizes the CHSH inequality instead of QBER. Other protocol like BB84 protocol uses QBER for the rate of the existence of ...
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Is the spin in quantum entanglement set at the moment the particles are separated, instead of when measured? [closed]

Is the spin in quantum entanglement set the moment the particles are separated, instead of transmitted instantaneously to its twin particle at the time of measurement? Wouldn't this make more sense ...
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Does Bell's theorem imply a causal connection between the measurement outcomes?

It is a standard result that quantum mechanics does not allow for superluminal communication, which would seem to directly imply that the answer to this question is no. After all, Bell's result tells ...
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How exactly does the proof of Bell's theorem fail if you remove the locality assumption?

In this paper Bell derives his famous inequality using the assumtions of locality and realism. In order to understand how the locality assumption affects the derivation of the inequality, and why it ...
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How would a “loop-hole free” Bell test appear to an observer moving at relativistic speeds compared to Alice and Bob?

How would a “loop-hole free” Bell test appear to an observer moving at relativistic speeds compared to Alice and Bob? In most such tests, Alice and Bob are 1km+ away from each other to prevent ...
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Do Bell inequality violations appear instantly when the source is turned on, or do they increase over time?

This experimental Question is a result of reading a particular article on Bell violations. I addressed the e-mail below to the corresponding authors —because who knows, they might reply— but it is not ...
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Single photon polarization measurement setup

I am studying experiments testing local realism through the violation of Bell's inequality. In particular, I am trying to understand the setup of the so called loophole-free experiment carried out in ...
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Questions on Bell's theorem and Disproving the hidden variable theory

I'm not a physicist (computer science background), but always was fascinated with the concept of entanglement. As many, the first intuition I had was - well, the two particles share some knowledge ...
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How can Bell's inequalities be used to rule out hidden variable theories when they do not take quantum effects into account?

EPR paradox shows that quantum physics is not complete. And Bell's inequality demonstrates by assuming that there is a hidden variable due to incompleteness, and it shows that does not match with the ...
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Why is the quantum Venn diagram paradox considered a paradox?

I've just watched this video on YouTube called Bell's Theorem: The Quantum Venn Diagram Paradox I don't quite understand why it is considered a paradox At 0:30, he says that as you rotate 2 ...
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Why do local hidden variable theories predict a triangular pattern for the graph?

My friends and I got into an argument about determinism, and I brought up that quantum events are random. But I couldn't prove it. I found the Wikipedia page on Bell's theorem, which seems to imply ...
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Local determinism vs. Local realism vs. Local causality

I found the following definitions for the three different terms. Do you find these definitions exhaustive, or can you suggest more precise formulations? Quoting the wikipedia page on Local realism: ...
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The $I_{3322}$ Inequality

I am trying to understand the $I_{3322}$ inequality which is an another example of Bell inequalities and which is different from the famous CHSH inequality. I haven't got hold of any standard ...
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Why would classical correlation in Bell's experiment be a linear function of angle?

Sorry if it's a newbie question, but I have trouble understanding the following part in the Wikipedia's explanation for the Bell's theorem: With the measurements oriented at intermediate angles ...
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Can local hidden variables models be ruled out without invoking expectation values?

Bell's inequalities, in their standard form, are a statement about the limitations faced by a probability distribution that can be written as $$p(a,b|x,y)=\sum_\lambda p(\lambda) p(a,b|x,y,\lambda)=\...
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Wigner Function for an Entangled Composite System

How is it possible to compute the Wigner function for a composite system that is prepared in an entangled state? In particular, consider the state $|ψ_{AB}\rangle=\frac{1}{\sqrt{2}}(|0_A\rangle|1_B\...
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How does Bell's test validate the Uncertainty principle?

I feel like I have done quite a bit of reading about Bell's test experiments and quantum entanglement. I see very well how Bell's test experiments provide different outcomes for a local realism ...
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Does Bell's Theorem suggest both Nonlocality and “Temporal Nonlocality”? [closed]

Bell's Theorem pretty much rules out any local hidden variable theories, and any hidden variable theory that's nonlocal is just regular quantum mechanics with an extra variable thrown in. But, since ...
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Does this Bell's experiment actually disprove Local Hidden Variable Theories (LHVT)

I'm watching some archived video lectures on QM in Coursera given by Umesh Vazirani from UC Berkeley and I have a question regarding a Bell's experiment (I guess something close to this) described in ...
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How does using a Bell state lead to a $\cos \left(\frac{1}{8}\pi \right)$ probability of winning in the CHSH game?

I have trouble understanding how the CHSH (which stands for John Clauser, Michael Horne, Abner Shimony, and Richard Holt) game, as described in this paper (and shortly explained in this post), works. ...
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Can we measure two entangled qubits in two different bases simultaneously?

Suppose we have the pair of entangled qubits: $$ |\psi\rangle = \frac{1}{ \sqrt{2} } ( |00\rangle + |11\rangle ),$$ where $|\psi\rangle$ is one of the $4$ Bell states. Suppose now that we give one ...
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Why is $\frac{\pi}{8}~\left({22.5}^{\circ}\right)$ often selected as a Bell test angle for the CHSH inequality?

I understand the mechanism of the CHSH inequality, however one thing that bugs me is why $b=\frac{\pi}{8}={22.5}^{\circ}$ is selected as a Bell test angle: Four separate subexperiments are ...
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Finding all the entangled states and the measurements for which they violate Bell's CHSH inequality

I have heard that finding all the entangled states that violate Bell's CHSH inequality, and finding the measurements for which they do so is an NP hard problem. Is that true?
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Is the electric charge associate with quantum systems counterfactual definite?

Is the electric charge associated with quantum systems counterfactual definite? Counterfactual definiteness refers to the inability to ''speak'' meaningfully (in quantum systems) of the definiteness ...