Locality vs Non-Locality; a way to test?

The point of quantum entanglement as a paradigm breaking experiment is that the phenomenon indicates that reality is, in a fundamental way, not properly described in classical terms. Entanglement is a way of creating correlated states of being in simple systems that allow (in principle) instant communication. You change one part of the system, the other part instantly responds. The key-word is “instantly”. Classical physics (which includes relativity) says that instant communication is disallowed because no information can travel faster than the speed of light. (I’m simplifying here, but hopefully you get the basic idea.)

It turns out from some work done a couple of decades or two ago that there is a limit to which the size of the correlations that underlie entanglement can grow. These were called “local” correlations because they couldn’t grow larger.

And now then some work done showed there did exist some special sorts of correlations which might grow larger than that limit, but only in special circumstances. Such correlations were called non-local and even post-quantum by the people who wrote out the theory behind them.

There’s news today of a way to possibly test these theories:

“To demonstrate that post-quantum correlations cannot exist in nature, Brunner and Skrzypczyk developed a protocol for deterministically distilling nonlocality in post-quantum states. That is, the technique refines weakly nonlocal states into states with greater nonlocality. In this context, ‘distillation’ can also be thought of as ‘purifying,’ ‘amplifying,’ or ‘maximizing’ the nonlocality of post-quantum correlations. Since nonlocal correlations are more useful if they are stronger, maximizing nonlocality has significant implications for quantum information protocols. The physicists’ protocol works specifically with ‘correlated nonlocal boxes,’ which are a particular class of post-quantum boxes.

Brunner and Skrzypczyk’s distillation protocol builds on a recent breakthrough by another team (Forster et al.), who presented the first nonlocality distillation protocol just a few months ago. However, the Forster protocol can distill correlated nonlocal boxes only up to a certain point, violating a Bell inequality called the Clauser-Horne-Shimony-Holt (CHSH) inequality only up to CHSH = 3. While this value is greater than Tsirelson’s bound of 2.82, it does not reach the bound of 3.26, which marks the point at which communication complexity becomes trivial.

Taking a step forward, Brunner and Skrzypczyk’s protocol can distill nonlocality all the way up to the maximum nonlocality of the Popescu-Rohrlich box, which is 4. In passing the 3.26 bound of triviality, they show that these post-quantum correlated nonlocal boxes do indeed collapse communication complexity.”

Read the full article here.

The article will explain more efficiently than I could about what the implications of non-locality of 3-4 would indicate.

The upshot is that this would give us a robust handle on the scales in which quantum phenomenon dominate (and why) and where classical physics dominates. It doesn’t unify the two, but giving a limit would actually lead to a deeper insight on just what the problem is that occurs when we try to do that. It’s also of critical importance in the area of quantum computing but I’m not nearly as interested in that so I’ll leave that as an exercise for the reader.. grin.

So read the article and enjoy.

(I was discussing something of this nature in a recent talk I gave. I’ve got to get around to posting the audio.)

Author: Nick Knisely

Episcopal bishop, dad, astronomer, erstwhile dancer...