Curator's Take
AI Commentary
This article shows that even the weakest‑entangled two‑qubit states can be turned into a genuine EPR‑steering resource simply by measuring two copies together, revealing a sharp “critical” point where the required measurement entanglement drops to zero. By disentangling the roles of measurement entanglement, number of settings and collective access, the work clarifies why some states appear unsteerable in single‑shot experiments yet become useful for quantum‑enhanced sensing or semi‑device‑independent protocols when modest resources are pooled. The construction of high‑dimensional states that gain steering on two copies while remaining Bell‑local over many copies also settles the long‑standing amplification question, suggesting new pathways to certify entanglement in noisy hardware without demanding full Bell violations.
— Mark Eatherly
Summary
Every PVM-critical entangled two-qubit state becomes steerable with two copies. We prove this for the full critical boundary and for two different critical inputs. The joint measurements can have arbitrarily little entanglement, while all conditional states on Bob's two systems remain separable. Criticality is exactly the point where this measurement-entanglement threshold vanishes. The number of measurements has a separate critical behavior: every fixed setting bound leaves an unsteerable neighborhood of the critical input. Moreover, for any such bound, there are critical states that remain unsteerable under every tensor power, although unrestricted measurements activate them with two copies. These results identify different roles for measurement entanglement, measurement number, and collective access in revealing steering. We also construct fixed-dimensional states whose steering activates on two copies while Bell locality persists through any prescribed finite copy range. In increasing dimension, the two-copy steering violation is unbounded, giving an affirmative solution to the amplification question of Hsieh, Liang, and Lee.