Quantcast
  • Register
PhysicsOverflow is a next-generation academic platform for physicists and astronomers, including a community peer review system and a postgraduate-level discussion forum analogous to MathOverflow.

Welcome to PhysicsOverflow! PhysicsOverflow is an open platform for community peer review and graduate-level Physics discussion.

Please help promote PhysicsOverflow ads elsewhere if you like it.

News

Please welcome our new moderators!

Please do help out in categorising submissions. Submit a paper to PhysicsOverflow!

... see more

Tools for paper authors

Submit paper
Claim Paper Authorship

Tools for SE users

Search User
Reclaim SE Account
Request Account Merger
Nativise imported posts
Claim post (deleted users)
Import SE post

Users whose questions have been imported from Physics Stack Exchange, Theoretical Physics Stack Exchange, or any other Stack Exchange site are kindly requested to reclaim their account and not to register as a new user.

Public \(\beta\) tools

Report a bug with a feature
Request a new functionality
404 page design
Send feedback

Attributions

(propose a free ad)

Site Statistics

122 submissions , 103 unreviewed
3,497 questions , 1,172 unanswered
4,545 answers , 19,342 comments
1,470 users with positive rep
408 active unimported users
More ...

Which are the simplest known contextual inequalities?

+ 6 like - 0 dislike
47 views

It is well-known that quantum mechanics does not admit a noncontextual ontological model, and there are countless different proofs of it. I'm interested in the simplest proofs that can be cast as an inequality, and bonus points if there's a proof that a simpler one can't be found.

The definition of contextuality that I care about is the one by Spekkens, that is: I don't care about determinism nor require that the proof of impossibility be specifically about measurement contextuality; failure at either measurement or preparation is fine. Spekkens himself provided very simple proofs for two-dimensional Hilbert spaces, but it is not clear to me how to cast his proofs as inequalities.

Also, it's well-known that unlike nonlocality, contextuality admits state-independent proofs. It would be nice to know the simplest one in this category as well.

Of course, "simplicity" is subjective, but I hope not enough to forbid a definite answer. If you want, a criterion could be: First, dimension of the Hilbert space needed to exhibit a contradiction. Second, number of measurements needed. Or maybe the product of these numbers.

My candidates are currently Klyachko's 5-observable inequality, that is violated by 3-dimensional quantum systems, and Yu's 13-observable inequality for 3 dimensions that is violated independently of the quantum state. I have no idea if these are the best, and I find it weird that I couldn't find an inequality violated for qubits.

This post has been migrated from (A51.SE)
asked Nov 2, 2011 in Theoretical Physics by Mateus Araújo (270 points) [ no revision ]
retagged Mar 7, 2014 by dimension10

3 Answers

+ 3 like - 0 dislike

Spekkens himself and co authors developed a non-contextuality inequality for a 2-dimensional system and 2 observables here: http://arxiv.org/abs/0805.1463. Note that the inequality is derived in the context of a communication protocol with an additional assumption that no information about the parity of the message (a bit string) is transmitted. I sure it's impossible to get a "simpler" proof in terms of fewer dimensions and observables.

This post has been migrated from (A51.SE)
answered Nov 3, 2011 by Chris Ferrie (645 points) [ no revision ]
While this is an interesting result, is not the sort of inequality I am looking for, as it uses an additional assumption that's not needed in the general case.

This post has been migrated from (A51.SE)
+ 3 like - 0 dislike

In this paper: A. Cabello, S. Severini, and A. Winter, a generalisation of the notion of contextuality is presented in terms of the compatibility structure of a graph, something akin to the graph colourability arguments in Kochen Specker proofs. If you ask questions with binary outcomes, and you say that certain combinations of questions are compatible and exclusive (both answers cannot be true, or $1$) then questions are vertices of a graph, and if they are compatible and exclusive, you assign an edge. Then asking all questions and putting it into an inequality such as the Klyachko inequality, the non-contextual upper bound is for all possible graphs given by the independence number of the graph. The quantum upper bound is given the by the Lovasz theta function. So to find the smallest example of a contextual inequality which a violation by quantum mechanics, one just needs to find the smallest graph (in the number of vertices and edges) where the Lovasz theta function is larger than the independence number. This is the pentagon, or 5-Cycle graph, the one used in the Klyachko inequality.

This post has been migrated from (A51.SE)
answered Nov 3, 2011 by Matty Hoban (435 points) [ no revision ]
It seems that their method can also be used to determine the simplest state-independent inequality, but they didn't seem to have done so.

This post has been migrated from (A51.SE)
In which sense it is a generalization? It seems to me that they just represent the traditional notion of contextuality in terms of graphs. Also, they restrict themselves to the consideration of projective measurements, so they only prove that the Klyachko inequality is the simplest projective one; to claim more would contradict the fact that there can be violations of contextuality for qubits.

This post has been migrated from (A51.SE)
+ 2 like - 0 dislike

A recent paper of Adán Cabello completely characterizes state-independent contextuality, and by doing so proves that Yu and Oh's 13-observable inequality for 3-dimensional systems is in fact the simplest state-independent noncontextual inequality possible.

This post has been migrated from (A51.SE)
answered Dec 22, 2011 by Mateus Araújo (270 points) [ no revision ]

Your answer

Please use answers only to (at least partly) answer questions. To comment, discuss, or ask for clarification, leave a comment instead.
To mask links under text, please type your text, highlight it, and click the "link" button. You can then enter your link URL.
Please consult the FAQ for as to how to format your post.
This is the answer box; if you want to write a comment instead, please use the 'add comment' button.
Live preview (may slow down editor)   Preview
Your name to display (optional):
Privacy: Your email address will only be used for sending these notifications.
Anti-spam verification:
If you are a human please identify the position of the character covered by the symbol $\varnothing$ in the following word:
p$\hbar\varnothing$sicsOverflow
Then drag the red bullet below over the corresponding character of our banner. When you drop it there, the bullet changes to green (on slow internet connections after a few seconds).
To avoid this verification in future, please log in or register.




user contributions licensed under cc by-sa 3.0 with attribution required

Your rights
...