Quantum Computing Stack Exchange
Quantum Computing Stack Exchange

Questions tagged [non-locality]

For questions about the phenomena related to correlations among distant observers. For example, this tag should be used for questions about Bell or CHSH inequalities, or for questions about quantum steering.

The phenomenon by which measurements made at a microscopic level can contradict a collection of notions known as "local realism" that are regarded as intuitively true in classical mechanics. Some observations about the correlations among multiple parties, while correctly predicted by quantum mechanics, can be proved to be impossible to achieve via local hidden variable theories.

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Is it true to say that one qubit in an entangled state can instantaneously affect all others?

When a qubit is measured, there is a ‘collapse of the wave-function’ as a result is randomly chosen. If the qubit is entangled with others, this collapse will also effect them. And the way it affects them depends on the way we chose to measure our…
James Wootton
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Are correlations stronger than those allowed by quantum mechanics possible?

We know how a quantum correlation setup can help us with a better probability of winning games like the CHSH. But what is the upper bound that physics can allow? Is it the quantum correlation setup? Or can we exceed them in general sense to get much…
Siddhant Singh
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What is the relationship between the Toffoli gate and the Popescu-Rohrlich box?

Background The Toffoli gate is a 3-input, 3-output classical logic gate. It sends $(x, y, a)$ to $(x, y, a \oplus (x \cdot y))$. It is significant in that it is universal for reversible (classical) computation. The Popescu-Rohrlich box is the…
Evan Jenkins
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Are there any experimental groups currently measuring quantum contextuality?

I work as a theorist with my current research interests in "quantum contextuality". For those perusing the question, this is essentially a generalization of non-locality where we can show a quantum system does not admit a hidden-variable model. So…
Emily Tyhurst
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Is quantum complexity basis-invariant?

Quantum computing refers (occasionally implicitly) to a "computational basis". Some texts posit that such a basis may arise from a physically "natural" choice. Both mathematics and physics require meaningful notions to be invariant under a change of…
Martin Ziegler
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How would a Quantum Computer (network) perform loophole-free Bell tests?

In a simple form, Bell's theorem states that: No physical theory of local hidden variables can ever reproduce all of the predictions of quantum mechanics. Bell developed a series of inequalities to provide specific experimental examples to…
agaitaarino
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Are there Bell-like violations that can be observed without collecting statistics?

Observing the violation of Bell inequalities, be it in their original formulation, or in the nowadays more commonly used CHSH formulation, involves computing averages of specific experimentally measurable quantities. In the CHSH formulation, these…
glS
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Definition of locality in Bell experiments

Continuing from my previous question on Brunner et al.'s paper (arXiv); so given a standard Bell experimental setup: where independent inputs $x,y \in \{0, 1\}$ decide the measurement performed by Alice & Bob on quantum state $S$ with outcomes $a,b…
ahelwer
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Is there a relation between the factorisation of the joint conditional probability distribution and Bell inequality?

[I'm sorry, I've already posted the same question in the physics community, but I haven't received an answer yet.] I'm approaching the study of Bell's inequalities and I understood the reasoning under the Bell theorem (ON THE EINSTEIN PODOLSKY…
LadyOfShalott
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Has anyone analyzed multi-round nonlocal games?

The traditional definition of a nonlocal game is restricted to having two players and one round (e.g., here), but it is natural to consider a more general class of games that may have more than two players and more than one round of questions. While…
Evan Jenkins
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What are the vertices of the no-signalling set $\mathcal{NS}$?

Consider the standard 2-2 Bell scenario, with two parties each one choosing between two measurement settings, with each measurement setting leading to one of two possible measurement outcomes. Consider the space of possible corresponding behaviours,…
glS
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What is the no-signaling set and how can it be related to other types of correlations?

The paper Bell nonlocality by Brunner et. al includes a striking diagram on page 7: This is fascinating to me because it suggests a framework of categorizing correlations that encompasses classical, quantum, "no-signaling", and maybe full…
ahelwer
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Understanding Hardy's proof of "nonlocality without inequalities"

I'm reading the proof of "nonlocality without inequality" presented in (Hardy 1992). In this protocol, we consider two particles (say, an electron and a positron) evolving almost independently: they both pass through one of two beamsplitters…
glS
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Are inseparable states with positive partial transpose nonlocal?

In Horodecki, Horodecki and Horodecki (1998), Mixed-state entanglement and distillation: is there a ``bound'' entanglement in nature?, the authors remark in the conclusions (beginning of pag. 4, emphasis mine): So, one is now faced with the problem…
glS
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Non maximally entangled states for QKD

Why aren't non maximally entangled states produced and used in quantum key distribution schemes? What would be the advantage/disadvantage to use such states rather than maximally entangled ones?
Sujan Vijayaraj
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