Quantum steering beyond instrumental causal networks

Understand

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.