Single-copy activation of Bell nonlocality via broadcasting of quantum states

TL;DR

This paper demonstrates that broadcasting a single bipartite quantum state to multiple parties can activate Bell nonlocality, allowing for device-independent entanglement certification at lower thresholds than previously known.

Contribution

It introduces a novel broadcasting scenario that activates Bell nonlocality in single-copy states, lowering the bounds for nonlocality detection under general correlations.

Findings

Bell nonlocality activated for ta>0.559 with no-signalling correlations

Device-independent certification achieved for ta>0.5 with quantum correlations

Lower bounds improve previous thresholds from 0.697 to 0.559 and 0.5

Abstract

Activation of Bell nonlocality refers to the phenomenon that some entangled mixed states that admit a local hidden variable model in the standard Bell scenario nevertheless reveal their nonlocal nature in more exotic measurement scenarios. We present such a scenario that involves broadcasting the local subsystems of a single-copy of a bipartite quantum state to multiple parties, and use the scenario to study the nonlocal properties of the two-qubit isotropic state: \begin{align} \nonumber \rho_\alpha = \alpha\,|\Phi^+ \rangle\langle \Phi^+|+(1-\alpha)\frac{\mathbb{1}}{4}. \end{align} We present two main results, considering that Nature allows for (i) the most general no-signalling correlations, and (ii) the most general quantum correlations at the level of any hidden variable theory. We show that the state does not admit a local hidden variable description for $\alpha>0.559$ and $\alpha>\frac{1}{2}$, in cases (i) and (ii) respectively, which in both cases provides a device-independent certification of the entanglement of the state. These bounds are significantly lower than the previously best-known bound of $0.697$ for both Bell nonlocality and device-independent entanglement certification using a single copy of the state. Our results show that strong examples of non-classicality are possible with a small number of resources.