# 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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### What is the physical basis behind the independence assumption in Bell's theorem?

Bell's theorem rests on two main assumptions: locality and independence. Locality has its basis in relativity and it seem to be a property of all known physical theories accepted today (General ...
18k views

### Bell's theorem for dummies, how does it work?

I've been reading up on theoretical physics for a few years now and I feel like I am starting to get an understanding of particle physics, at least as much as you can from Wikipedia pages. One thing ...
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### Understanding Bell's inequality

I'm struggling to understand Mermin's article "Is the moon there when nobody looks?". In this article, a Bell experiment with three measurement settings is discussed. Is it possible to reduce the ...
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### Could you explain the linear correlation in Bell's theorem for a hidden variable? [duplicate]

Could you please explain why, in experiments with electron spin and light polarity, the expected correllation for a hidden variable would be linear (red line below) Answers with the least math ...
296 views

### 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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### Do Bell’s inequalities assume particles can only have one variable to consider?

I have asked the question “How do you know when pairs are entangled?” I have asked “What is the difference between entangled and correlated?” I have also asked several similar questions, but I keep ...
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### Why does the effect of measuring an entangled particle is not apparent when examining only one particle? [closed]

As far as I understand, the Bell test experiments show that observations (measurements) of entangled particles have an effect on one another. This effect is apparent when examining the results of ...
784 views

### 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 ...
67 views

### Is the Proof of Gisin's Theorem Wrong?

I have been trying to understand Gisin's paper, Bell's inequality holds for all non-product states. As I've worked through the proof he provides for his theorem, I've run into two things that simply ...
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### Can Quantum Entangled particles convey information faster than speed of light? [duplicate]

Can Quantum Entangled particles convey information faster than speed of light. If they don't what was wrong with John Bells experiment?
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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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### How is Mermin's magic square computed?

BACKGROUND The Peres-Mermin magic square consists of different combinations of tensor products of Pauli operators being applied on an arbitrary bipartite state. Suppose we're considering the entry in ...
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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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### 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 ...
261 views

### 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 ...