Fake violations of the quantum Bell-parameter bound

TL;DR

This paper reveals that experimental limitations in photodetection can artificially inflate Bell parameter measurements, potentially leading to false violations of quantum bounds like the Cirel'son limit.

Contribution

It demonstrates theoretically that unresolved photon numbers in detection can cause pseudo-violations of Bell bounds, challenging assumptions about experimental nonclassicality.

Findings

Unresolved photon detection can pseudo-increase Bell parameters.

Proper detection models prevent false violations of quantum bounds.

The paradox is resolved by squash models ensuring physical validity.

Abstract

Shortcomings of experimental techniques are usually assumed to diminish nonclassical properties of quantum systems. Here it is demonstrated that this standard assumption is not true in general. It is theoretically shown that the inability to resolve different photon numbers in photodetection may pseudo-increase a measured Bell parameter. Under proper conditions one even pseudo-violates the quantum Cirel'son bound of the Bell parameter, the corresponding density operator fails to be positive semi-definite. This paradox can be resolved by appropriate squash models.