Quantum steering beyond instrumental causal networks

TL;DR

This paper demonstrates that quantum steering can be detected beyond classical causal models with outcome communication, introducing new inequalities and quantifiers that reveal stronger forms of steering in quantum systems.

Contribution

It introduces 1S-DI instrumental inequalities and the robustness of non-instrumentality, revealing that steering alone can surpass classical causal explanations.

Findings

Violations of instrumental inequalities with entangled photons

Steering can be demonstrated without outcome communication

Robustness of non-instrumentality quantifies non-classicality

Abstract

We theoretically predict, and experimentally verify with entangled photons, that outcome communication is not enough for hidden-state models to reproduce quantum steering. Hidden-state models with outcome communication correspond, in turn, to the well-known instrumental processes of causal inference but in the 1-sided device-independent (1S DI) scenario of one black-box measurement device and one well-characterised quantum apparatus. We introduce 1S-DI instrumental inequalities to test against these models, with the appealing feature of detecting entanglement even when communication of the black box's measurement outcome is allowed. We find that, remarkably, these inequalities can also be violated solely with steering, i.e. without outcome communication. In fact, an efficiently-computable formal quantifier -- the robustness of non-instrumentality -- naturally arises; and we prove that steering alone is enough to maximize it. Our findings imply that quantum theory admits a stronger form of steering than known until now, with fundamental as well as practical potential implications.