Measurement-induced remote activation of nonclassicality

This paper explores whether nonclassicality—defined by the nonpositivity of quasiprobability distributions like the Wigner or Kirkwood-Dirac (KD) distributions—can be generated on demand through local measurements on a correlated partner. Rather than treating nonclassicality as a static property of a state, the author frames it as an operational resource that can be activated via quantum steering.

The study utilizes a geometric approach to quantum steering, focusing on the set of conditional states reachable at one location (Bob) through measurements performed at another (Alice). By leveraging the convexity of the set of classical states, the author demonstrates that while a classical average cannot be made nonclassical, specific conditional branches of a mixture can exhibit genuine nonclassicality. The author provides closed-form expressions for the maximal and average steered nonclassicality in two-qubit systems, as well as an exact reduction for Wigner negativity in a hybrid qubit-oscillator model.

• Exact Quantification: For two-qubit systems, the author derives closed-form expressions for both the maximal single-branch KD nonclassicality and the probability-weighted average, proving that the latter is optimal for two-outcome projective measurements.
• Noise Robustness: The study reveals that local noise on Bob's side does not always degrade the effect. While unital channels (like depolarizing noise) can only reduce the induced nonclassicality, specific non-unital channels (like amplitude damping) can enhance it if they are sufficiently misaligned with the system's polarization axis.
• Anomalous Weak Values: The paper establishes that steering allows Bob to access anomalous weak values that are strictly unavailable to him if he holds only the unsteered reduced state, providing a concrete metrological application for the activated nonclassicality.