Vindication of entanglement-based witnesses of non-classicality in hybrid systems

TL;DR

This paper defends an entanglement-based non-classicality witness in hybrid systems, clarifying misconceptions and confirming its validity even against classical counterexamples, thus strengthening its foundational role in quantum gravity tests.

Contribution

The paper refutes claims that classical systems can violate the entanglement-based witness, thereby reaffirming its robustness and general applicability in hybrid quantum-classical contexts.

Findings

The proposed witness correctly identifies non-classicality in hybrid systems.

Classical counterexamples do not violate the entanglement-based witness.

The validity of the witness is confirmed in full generality.

Abstract

Recently, Vedral and one of us proposed an entanglement-based witness of non-classicality in systems that need not obey quantum theory, based on constructor-information-theoretic ideas, which offers a robust foundation for recently proposed table-top tests of non-classicality in gravity. The witness asserts that if a mediator can entangle locally two quantum systems, then it has to be non-classical. Hall and Reginatto claimed that there are classical systems that can entangle two quantum systems, thus violating our proposed witness. Here we refute that claim, explaining that the counterexample proposed by Hall and Reginatto in fact validates the witness, vindicating the witness of non-classicality in its full generality.