Abstract
As first shown by Popescu (1995 Phys. Rev. Lett. 74 2619), some quantum states only reveal their nonlocality when subjected to a sequence of measurements while giving rise to local correlations in standard Bell tests. Motivated by this manifestation of ‘hidden nonlocality’ we set out to develop a general framework for the study of nonlocality when sequences of measurements are performed. Similar to Gallego et al (2013 Phys. Rev. Lett. 109 070401) our approach is operational, i.e. the task is to identify the set of allowed operations in sequential correlation scenarios and define nonlocality as the resource that cannot be created by these operations. This leads to a characterization of sequential nonlocality that contains as particular cases standard nonlocality and hidden nonlocality.
Highlights
One of the major problems in physics is the characterization of the different correlations that arise between distant observers performing measurements on physical systems
Popescus example already showed that the standard notion of locality is not sufficient to capture the behavior of correlations that can arise in a sequential correlation scenario
Sequential correlations give rise to inequivalent notions of nonlocality that we summarize in the following
Summary
One of the major problems in physics is the characterization of the different correlations that arise between distant observers performing measurements on physical systems. Since the work of Bell [1], it has been known and widely studied that some correlations obtained from measurements on quantum systems cannot be simulated by any local and classical theory (local hidden-variable models) This phenomenon is referred to as nonlocality. This is already implied by the results of [7] where it was shown that some quantum states display only local correlations in scenarios with single projective measurements per round, but give rise to nonlocal correlations when a sequence of measurements is performed instead We further investigate a type of hidden-variable models specially suited for scenarios with sequential measurements, in the spirit of time-ordered local models considered in [8, 9] These different notions of classicality for the scenario with sequential measurements are generalized to the quantum case. This analysis leaves numerous open problems that we enunciate and motivate for further study
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