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:ρα=α|Φ+⟩⟨Φ+|+(1−α)14.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 α>0.559 and α>12, 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.
Highlights
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 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: ρα = α |Φ+
One subjects the Accepted in Quantum 2021-07-05, click title to verify standard scenario single copy sequential scenairo multiple copy scenario quantum state to a more complex measurement scenario, which is able to prove a lack of local hidden variables (LHVs) description for some states that are local in the standard scenario
Summary
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. Measurement outcomes arising from local measurements made on entangled quantum systems exhibit correlations which provably forbid any explanation in terms of LHVs [1, 2] This phenomenon—termed Bell nonlocality—can be experimentally witnessed via the violation of Bell inequalities: functions of the measurement statistics that are bounded for LHV theories. One subjects the Accepted in Quantum 2021-07-05, click title to verify standard scenario single copy sequential scenairo multiple copy scenario quantum state to a more complex measurement scenario, which is able to prove a lack of LHV description for some ( still not all) states that are local in the standard scenario. Such scenarios typically come in two flavours (see figure 1): 1. single-copy sequential scenarios [17, 21, 23, 24], where one makes a sequence of time-ordered local measurements on a single copy of the state; 2. multiple-copy scenarios [16, 18,19,20, 22], where one prepares multiple copies of the state in some network structure and makes joint measurements on the local subsystems
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