Exact Steering Bound for Two-Qubit Werner States

Abstract

We investigate the relationship between projective measurements and positive operator-valued measures (POVMs) in the task of quantum steering. A longstanding open problem in the field has been whether POVMs are more powerful than projective measurements for the steerability of noisy singlet states, which are known as Werner states. We resolve this problem for two-qubit systems and show that the two are equally powerful, thereby closing the so-called Werner gap. Using the incompatible criteria for noisy POVMs and the connection between quantum steering and measurement incompatibility, we construct a local hidden state model for Werner states with Bloch sphere radius $r\le 1/2$ under general POVMs. This construction also provides a local hidden variable model for a larger range of Werner states than previously known. {In contrast, we also show that projective measurements and POVMs can have inequivalent noise tolerances when using a fixed state ensemble to build different local hidden state models.} These results help clarify the relationship between projective measurements and POVMs for the tasks of quantum steering and nonlocal information processing.

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• Nonlocality activation in a photonic quantum network
• Thermodynamic Approach to Quantifying Incompatible Instruments
• All incompatible measurements on qubits lead to multiparticle Bell nonlocality
• Constrained Measurement Incompatibility from Generalised Contextuality of Steered Preparation
• Cost of Simulating Entanglement in Steering Scenario
• Revealing the Boundary between Quantum Mechanics and Classical Model by EPR-Steering Inequality
• Catalytic Activation of Bell Nonlocality
• Optimizing entanglement in two-qubit systems
• On compatibility of binary qubit measurements
• Threshold (Q, P) Quantum Distillation
• Witness High-Dimensional Quantum Steering via Majorization Lattice