# 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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### Looking for Experimental Result Data for Bell's Inequality, especially at 60 degrees

I've been looking for an experiment (for Bell's Inequality, using Bell's 60 degree test angle) which shows counts and resulting correlation percentage calculated. I want to see if a) for 99 runs 66 ...
1 vote
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### Inclusion-exclusion principle in modeling interactions

When I was in school, I couldn't accept the concept of binary pairwise additive interactions, aka "Force" in classical physics. For me, the emergence of magnetism was a clue that for a 3-...
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### Specifications of Bell's inequality experiments conducted untill now

Has any Bell's inequality experiment, conducted until now, which has satisfied the following criteria? Having more than three measuring orientations. In the typical experimental setup I have seen ...
1 vote
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### What is local about the hidden variable in Bell's theorem?

A pion decays to a singlet electron/positron state. We will measure the component of the electron's spin in the $\vec a$ direction and positron's spin in the $\vec b$ direction. If there exists a ...
58 views

### How to test if a bipartite density matrix violates Bell's inequality?

For a given density matrix $$\rho = \sum_{ijkl=0}^1 r_{ijkl} |i,j\rangle \langle k,l|$$ describing a bipartite two-qubit system, how can I prove for what values $r_{ij}$ the density matrix violates ...
138 views

### On the Bell's Theorem / Bell-type Inequalities and the Kochen-Specker Theorem

It appears to me that the Kochen-Specker theorem, if not Gleason’s theorem already, seals the fate of realism / value definiteness (with possibly the additional assumption of non-contextuality, ...
109 views

### How can a basis change make a state suddenly separable?

I am working through my quantum optics textbook by Grynberg, Aspect and Fabre, and this concept has tripped me up a little. (13) is an inseparable state, whereas (15) IS separable - but they are the ...
1 vote
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### Uncertainty Principle Violations?

I’m learning about Bells Inequality Experiment and have a question because the instructor in the video I am watching simply passed over something I found jarring. Ok, you have your entangled pairs of ...
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### What is required for a minimum viable Bell test experiment at home?

My objective is to create a tabletop Bell test experimental setup generating data of suitable quality for undergraduate education or satisfaction of personal curiosity; remaining free from loopholes ...
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### How to derive the theorem about CHSH inequality in the $\mathbb{C}^2\otimes\mathbb{C}^2$ space in (Horodecki, 1995)?

In the $\mathbb{C}^2\otimes\mathbb{C}^2$ space, a state can be represented as: \begin{equation} \rho=\frac{1}{4}(\mathbb{I}\otimes\mathbb{I}+\mathbf{r}\cdot\mathbf{\sigma}\otimes\mathbb{I}+\mathbb{I}\...
1 vote
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### Availability of dataset from "Loophole-free Bell inequality violation using electron spins separated by 1.3 kilometres"?

I am looking for the most-raw data from the (in)famous Loophole-free Bell inequality violation using electron spins separated by 1.3 kilometres experiment. I would like to try repeating their ...
109 views

### Do all entangled states violate some Bell inequality?

Suppose I have a multipartite pure entangled two-level system. Is it always possible to form a bell-inequality based on this state such that the observed correlations cannot be reproduced classically? ...
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### A popular explanation of Bell's inequalities

In one book on quantum mechanics, I found a very simple analogy to Bell's inequalities, which explains their essence without delving into the details of the physical experiment. I liked this analogy, ...
331 views

### In Bell's experiment, can "knowing the future" be that "hidden variable"?

(Sorry if I'm spamming your board with Popular Science speculations, but I just thought this could be an interesting thought experiment). There are quite a few videos on youtube about Bell's ...
313 views

### Measuring spin at an angle. Why is the probability $p = \cos^2(\theta/2)$?

In this video about quantum entanglement, at 1:38, it is said that "the probability [of spin being in the same direction with the measurement] depends on the square of the cosine of half the ...
80 views

### What's wrong with this extremely simple hidden variables approach?

I've seen the Veritassium video about Bell's theorem which made me think about what the hidden variables could be. I'd like somebody to explain to me what's wrong with this extremely simple hidden ...
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### Diagonalization of an operator with two Hilbert's subspaces

I was thinking how to diagonalize an operator which is composed by two Hilbert's subspaces. For example, the Bell's operator, which is defined as \begin{equation} \vec{B} = \vec{\sigma_1}\cdot \vec{a}(...
1 vote
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### Is there a source for this equation for a Bell's Theorem test?

In this, he gives an example of entangled photons spin being measured – one photon sent to a detector at $0^\circ$, and the other photon sent to a detector at $20^\circ$. He says the odds of both ...
268 views

1 vote
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### How does relaxing counterfactual definiteness allows for CHSH inequality violation?

Scenario, I want to play a game with a group of students as a teacher, I ask two teachers to help. The students are from the central classroom, going to room A and room B where the teachers there asks ...
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### A Venn diagram for: (non-)locality, (non-)realism, (non-)contextuality, (non-)signalling, (dis-)entanglement

I recently asked a (yet-unanswered) question about the relationship between state-dependence and violations of realism. The more I read on the subject, the more I find myself digging deeper in a ...