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arXiv:1907.08933 · Physical Review A

Structure of quantum and classical implementations of the Popescu-Rohrlich box

Anna Jenčová, Martin Plávala

Published Oct 9, 2020• 2 citations
arXivHTMLPDF

Abstract

We construct implementations of the PR-box using quantum and classical channels as state spaces. In both cases our constructions are very similar and they share the same idea taken from general probabilistic theories and the square state space model. We construct all quantum qubit channels that maximally violate a given CHSH inequality, we show that they all are entanglement-breaking channels, that they have certain block-diagonal structure, and we present some examples of such channels.

DOI
10.1103/physreva.102.042208

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We construct implementations of the PR-box using quantum and classical channels as state spaces. In both cases our constructions are very similar and they share the same idea taken from general probabilistic theories and the square state space model. We construct all quantum qubit channels that maximally violate a given CHSH inequality, we show that they all are entanglement-breaking channels, that they have certain block-diagonal structure, and we present some examples of such channels.
We consider quantum ensembles which are determined by pre and post selection. Unlike the case of only pre selected ensembles, it is shown that in this case the probabilities for measurements outcomes at intermediate times respect causality only rarely: disconnected regions can generally use such ensembles to signal each other. We show that under restrictive conditions, there are certain non-trivial bi-partite ensembles which do respect causality. Some of these ensembles give rise to a violation of the CHSH inequality which exceeds the quantum maximal violation given by Cirel'son's bound (BrmCHSHle2sqrt2B_{\\rm CHSH}\\le 2\\sqrt2BrmCHSH​le2sqrt2). We find that one of these causality respecting cases saturates the Popescu-Rohrlich bound for the maximal violation BrmCHSHle4B_{\\rm CHSH}\\le 4BrmCHSH​le4. This case can be regarded as an a posteriori realization of ``super-correlations'' which has been recently termed as ``Popescu-Rohrlich box''.
We study the consequences of superquantum nonlocal correlations as represented by the PR-box model of Popescu and Rohrlich, and show that PR boxes can enhance the capacity of noisy interference channels between two senders and two receivers. PR-box correlations violate Bell and CHSH inequalities and are thus stronger---more nonlocal---than quantum mechanics, yet weak enough to respect special relativity in prohibiting faster-than-light communication. Understanding their power will yield insight into the nonlocality of quantum mechanics. We exhibit two proof-of-concept channels: First, we show a channel between two sender-receiver pairs where the senders are not allowed to communicate, for which a shared superquantum bit (a PR box) allows perfect communication. This feat is not achievable with the best classical (senders share no resources) or quantum-entanglement-assisted (senders share entanglement) strategies. Second, we demonstrate a class of channels for which a tunable parameter $\ensuremath{\epsilon}$ achieves a double separation of capacities; for some range of $\ensuremath{\epsilon}$, the superquantum-assisted strategy does better than the entanglement-assisted strategy, which in turn does better than the classical one.
Abstract Chapter 8 introduces Popescu and Rohrlich’s no-signalling PR-box of superquantum joint probabilities violating the CHSH Bell inequality more strongly than QM. We present Mermin’s derivation of the CHSH Bell inequality. Using Tanya and Jeff Bub’s quantum guessing game, we show how the PR-box violates conservation per NPRF whence Bell state entanglement. This is shown to be the reason the PR-box violates the CHSH-Bell inequality more strongly than QM. Therefore, we conclude that the PR-box is akin to a perpetual motion machine.
Entanglement generation is a fundamental dynamical capability in quantum information science and underpins many quantum advantages. While quantum theory enables it through unitary dynamics, boxworld, a generalized probabilistic theory admitting Popescu--Rohrlich boxes with supraquantum correlations, has no reversible transformation capable of generating entanglement. We show that this no-go picture changes fundamentally once reversibility is relaxed to pure-state preservation. We construct a pure-state-preserving transformation that maps every uncorrelated pure state to a Popescu--Rohrlich box and completely classify all pure-state-preserving entangling transformations in the simplest bipartite boxworld. Our results provide the first explicit mechanism for generating beyond-quantum entanglement without introducing mixing and demonstrate a physical distinction between reversibility and pure-state preservation that is obscured by the structure of quantum theory.
The propensity for certain quantum system to consistently violate Bell inequalities—dubbed quantum nonlocality—has recently been found to be an information theoretic resource in quantum communication that is distinct from entanglement. Accordingly, a useful measurement of “how much” of this resource is present in a given system is needed. One candidate, the PR‐box (sometimes called a nonlocal machine), has been suggested as a possible benchmark, but one of the difficulties in using it has been to determine exactly what kinds of quantum systems are simulable by it. The present work is a first step towards a graph‐theoretic understanding of PR‐boxes that may prove useful in this endeavor.
A set of nonlocal correlations that have come to be known as a Popescu-Rohrlich (PR) box suggest themselves as a natural unit of nonlocality, much as a singlet is a natural unit of entanglement. We present two results relevant to this idea. One is that a wide class of multipartite correlations can be simulated using local operations on PR boxes only. We show this with an explicit scheme, which has the interesting feature that the number of PR boxes required is related to the computational resources necessary to represent a function defining the multipartite box. The second result is that there are quantum multipartite correlations, arising from measurements on a cluster state, that cannot be simulated with n PR boxes, for any n.