From Joint to Single-System Psi-Onticity Without Preparation Independence

TL;DR

This paper demonstrates that the PBR theorem's conclusion of $-onticity for individual quantum systems can be derived without the Preparation Independence Postulate, strengthening the theorem's foundational implications.

Contribution

It shows $$-onticity of individual systems follows from the tensor-product structure, removing the need for the PIP assumption in the PBR theorem.

Findings

$$-onticity of subsystems derived without PIP

Strengthens the conceptual foundations of $$-ontology

Closes loopholes for $$-epistemic models

Abstract

The Pusey-Barrett-Rudolph (PBR) theorem establishes $\psi$-onticity for individual quantum systems, but its standard formulation relies on the Preparation Independence Postulate (PIP). This has led to a prevalent view that rejecting PIP leaves open the possibility of $\psi$-epistemic models for individual systems. In this work, we show that this understanding is incomplete: once the PBR theorem establishes $\psi$-onticity for composite systems prepared in product states, the $\psi$-onticity of the individual subsystems follows directly from the tensor-product structure of quantum mechanics, without invoking PIP or any further auxiliary assumptions. This result removes a key auxiliary assumption from the PBR theorem, closes a persistent loophole for preserving $\psi$-epistemic models, and strengthens the conceptual foundations of $\psi$-ontology.