On safe post-selection for Bell tests with ideal detectors: Causal diagram approach

Pawel Blasiak1, Ewa Borsuk1, and Marcin Markiewicz2

1Institute of Nuclear Physics Polish Academy of Sciences, PL-31342 Kraków, Poland
2International Centre for Theory of Quantum Technologies, University of Gdańsk, PL-80308 Gdańsk, Poland

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Reasoning about Bell nonlocality from the correlations observed in post-selected data is always a matter of concern. This is because conditioning on the outcomes is a source of non-causal correlations, known as a $\textit{selection bias}$, rising doubts whether the conclusion concerns the actual causal process or maybe it is just an effect of processing the data. Yet, even in the idealised case without detection inefficiencies, post-selection is an integral part of experimental designs, not least because it is a part of the entanglement generation process itself. In this paper we discuss a broad class of scenarios with post-selection on multiple spatially distributed outcomes. A simple criterion is worked out, called the $\textit{all-but-one}$ principle, showing when the conclusions about nonlocality from breaking Bell inequalities with post-selected data remain in force. Generality of this result, attained by adopting the high-level diagrammatic tools of causal inference, provides safe grounds for systematic reasoning based on the standard form of multipartite Bell inequalities in a wide array of entanglement generation schemes, without worrying about the dangers of selection bias. In particular, it can be applied to post-selection defined by single-particle events in each detection chanel when the number of particles in the system is conserved.

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[5] Seungbeom Chin, Yong-Su Kim, and Sangmin Lee, "Graph Picture of Linear Quantum Networks and Entanglement", Quantum 5, 611 (2021).

[6] Valentin Gebhart, Luca Pezzè, and Augusto Smerzi, "Genuine Multipartite Nonlocality with Causal-Diagram Postselection", Physical Review Letters 127 14, 140401 (2021).

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