Genuine multipartite nonlocality with causal-diagram postselection

TL;DR

This paper explores conditions under which genuine multipartite nonlocality can be demonstrated using collective postselection strategies, even when such strategies involve communication among parties, with implications for quantum experiments.

Contribution

It establishes criteria for demonstrating GMN with collective postselection and shows that certain communication-involving strategies can still reveal nonlocality.

Findings

GMN can be demonstrated with collective postselection under specific conditions.

Certain postselection strategies involving communication still reveal GMN.

Genuine three-partite nonlocality can be generated with independent sources.

Abstract

The generation and verification of genuine multipartite nonlocality (GMN) is of central interest for both fundamental research and quantum technological applications, such as quantum privacy. To demonstrate GMN in measurement data, the statistics are commonly postselected by neglecting undesired data. Until now, valid postselection strategies have been restricted to local postselection. A general postselection that is decided after communication between parties can mimic nonlocality, even though the complete data are local. Here, we establish conditions under which GMN is demonstrable even if observations are postselected collectively. Intriguingly, certain postselection strategies that require communication among several parties still offer a demonstration of GMN shared between all parties. The results are derived using the causal structure of the experiment and the no-signalling condition imposed by relativity. Finally, we apply our results to show that genuine three-partite nonlocality can be created with independent particle sources.