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MON: Monday Contributed Sessions

MON 9: Quantum Entanglement

MON 9.4: Talk

Monday, September 8, 2025, 15:00–15:15, ZHG101

Exact steering bound for two-qubit Werner states — •Martin J. Renner — ICFO - The Institute of Photonic Sciences, 08860 Castelldefels, Barcelona, Spain

Many quantum technologies rely on nonlocality, correlations between distant particles that defy classical explanation. To harness this, it's essential to know which quantum states can or cannot display nonlocal behavior. A seminal 1989 result by Reinhard Werner showed that some entangled states can be fully explained by local models, but only under the restricted class of projective measurements. We extend this result for two-qubit Werner states to the most general class of measurements, known as positive operator-valued measures (POVMs). Our model identifies exactly which of these states can demonstrate quantum steering, the effect Einstein famously called "spooky action at a distance." Surprisingly, we find that POVMs offer no advantage over projective measurements for revealing steering in these states, resolving a long-standing open question in quantum foundations. Beyond this, our results have implications for measurement incompatibility: we determine the critical visibility under white noise at which all qubit measurements become jointly measurable.

Reference: MJ Renner, Compatibility of Generalized Noisy Qubit Measurements, Phys. Rev. Lett. 132, 250202

Keywords: Bell nonlocality; Quantum steering; Einstein-Podolsky-Rosen paradox; Werner states