Approaching Tsirelson's bound in a photon pair experiment

TL;DR

This experiment measured bipartite correlations in photon pairs, surpassing a proposed informational bound and approaching the Tsirelson limit, with implications for quantum theory and information security.

Contribution

The paper reports the first experimental violation of Grinbaum's informational bound, approaching the Tsirelson limit with high precision in photon pair experiments.

Findings

Observed CHSH value of 2.82759±0.00051

Violated Grinbaum's bound by 4.3 standard deviations

Set a new lower bound for Tsirelson's bound

Abstract

Quantum theory introduces a cut between the observer and the observed system, but does not provide a definition of what is an observer. Based on an informational definition of observer, Grinbaum has recently predicted an upper bound on bipartite correlations in the Clauser-Horne-Shimony-Holt (CHSH) Bell scenario equal to 2.82537, which is slightly smaller than the Tsirelson bound of standard quantum theory, but is consistent with all the available experimental results. Not being able to exceed Grinbaum's limit would support that quantum theory is only an effective description of a more fundamental theory and would have a deep impact in physics and quantum information processing. Here we present a test of the CHSH Bell inequality on photon pairs in maximally entangled states of polarization in which a value 2.82759+-0.00051 is observed, violating Grinbaum's bound by 4.3 standard deviations and providing the smallest distance with respect to Tsirelson's bound ever reported, namely, 0.00084+-0.00051. This sets a new lower experimental bound for Tsirelson's bound, strengthening the value of principles that predict Tsirelson's bound as possible explanations of all natural limits of correlations, and has important consequences for cryptographic security, randomness certification, characterization of physical properties in device-independent scenarios, and certification of quantum computation.