How to avoid (apparent) signaling in Bell tests

Quantum 9, 1760 (2025).

https://doi.org/10.22331/q-2025-06-04-1760

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.