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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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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 ...
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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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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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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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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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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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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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What is the reason for the reported opposite-of-QM handedness of photons in a Bell test with circularly polarized photons? [closed]

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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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 ...
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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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Distinguish Definitions: Realism Scientific, Realism, and realistic

I was readying about study about bell's theorem, where I had the question about some definitions. Here's some of my summaries: Scientific Realism: (https://en.wikipedia.org/wiki/Scientific_realism#...
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What's the invariance quantity under EPR and Bell's inequality?

I was reading and writing a study report about EPR and Bell's inequality. As the hidden variable and local realism was proven to be "false", and from many posts in physics exchange websites such as ...
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Are properties such as locality and realism properties of all theories, some theories, or interpretations of theories?

I have a strong intuitive feeling, based for example on the Bothe-Geiger experiment, that quantum mechanics is nonlocal. But intelligent people who have thought about these things make claims such as ...
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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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From locality to deterministic hidden variables

According to this link here Bell said: "My own first paper on this subject ... starts with a summary of the EPR argument from locality to deterministic hidden variables. But the commentators have ...
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EPR-Bohm paradox and Robertson uncertainty relation [closed]

Consider Bohm's version of the EPR paradox, where spin-1/2 are used. The bipartite system is in the state $$ \vert \Psi\rangle = \dfrac{1}{\sqrt{2}} \left( \vert \uparrow_x \rangle_A \vert \...
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Understanding Bell's Inequality in Sakurai's text

In Sakurai's book Modern Quantum Mechanics(2nd edition) he states the following in the section on Einstein's Locality Principle and Bell's Inequality (page 241): "We derive Bell's inequality within ...
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How do loophole-free Bell’s inequality violation tests rule out conspiracy via signals that have to travel back to the experimenter?

First, I’d like to apologize for yet another question about crazy ideas on how quantum mechanics might actually work. I have no background in quantum mechanics. Recently, though, I started to study ...
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Question about Bell's Theorem

I'm working through trying to understand this, but the math of the original paper, and subsequent actual academic papers are a bit beyond me. So I've turned to simplified explanations, all of which ...
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Relativity and Bell's inequalities

I am reading a collection of interviews taken with prominent physicists as in an aftermath of famous Alain Aspect experiment. Between them John Bell states that if local realism has to go, he would be ...
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Is there any basis in stating there are different levels of 'locality' or 'reality', within physics or physical theories?

EPR’s reality criterion and quantum realism has this paragraph (the paper has 0 citations, and I'm personally happy to call this metaphysics, but still!): A popular interpretation of Bell’s theorem ...
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Why we should be disturbed by Bell's theorem [closed]

Mermin (1985) quotes an unnamed physicist as saying: Anybody who's not bothered by Bell's theorem has to have rocks in his head. This seemed like a pretty dire indictment, so I decided I had ...