How to avoid (apparent) signaling in Bell tests

TL;DR

This paper highlights the importance of avoiding signaling in Bell tests, identifies common systematic errors, and demonstrates strategies to minimize signaling, achieving a high Bell-CHSH parameter with negligible errors.

Contribution

It provides experimental methods to identify and reduce signaling in Bell tests, improving the reliability of quantum nonlocality measurements.

Findings

Identified common sources of systematic errors causing signaling.

Demonstrated approaches to minimize signaling in Bell tests.

Achieved a Bell-CHSH parameter of 2.812 with negligible systematic errors.

Abstract

Bell tests have become a powerful tool for quantifying security, randomness, entanglement, and many other properties, as well as for investigating fundamental physical limits. In all these cases, the specific experimental value of the Bell parameter is important as it leads to a quantitative conclusion. However, experimental implementations can also produce experimental data with (apparent) signaling. This signaling can be attributed to systematic errors occurring due to weaknesses in the experimental designs. Here we point out the importance, for quantitative applications, to identify and address this problem. We present a set of experiments with polarization-entangled photons in which we identify common sources of systematic errors and demonstrate approaches to avoid them. In addition, we establish the highest experimental value for the Bell-CHSH parameter obtained after applying strategies to minimize signaling that we are aware of: $S = 2.812 \pm 0.003$ and negligible systematic errors. The experiments did not randomize the settings and did not close the locality loophole.