Nonlocality and hidden variables

Stefan Teufel, Karin Berndl, Detlef Dürr +2 more

Motivated by Popescu's example of hidden nonlocality [Phys. Rev. Lett. 74, 2619 (1995)], we elaborate on the conjecture that quantum states that are intuitively nonlocal, i.e., entangled, do not admit a local causal hidden-variables model. We exhibit quantum states which either (i) are nontrivial counterexamples to this conjecture or (ii) possess a new kind of more deeply hidden irreducible nonlocality. Moreover, we propose a nonlocality complexity classification scheme suggested by the latter possibility. Furthermore, we show that Werner's [Phys. Rev. A 40, 4277 (1989)] (and similar) hidden-variables models can be extended to an important class of generalized observables. Finally, a result of Fine [Phys. Rev. Lett. 48, 291 (1982)] on the equivalence of stochastic and deterministic hidden variables is generalized to causal models.

Leon Bankston

The Pauli measurements (the measurements that can be performed with Clifford operators followed by measurement in the computational basis) are a fundamental object in quantum information. It is well-known that there is no assignment of outcomes to all Pauli measurements that is both complete and consistent. We define two classes of hidden variable assignments based on relaxing either condition. Partial hidden variable assignments retain the consistency condition, but forfeit completeness. Contextual hidden variable assignments retain completeness but forfeit consistency. We use techniques from spectral graph theory to characterize the incompleteness and inconsistency of the respective hidden variable assignments. As an application, we interpret our incompleteness result as a statement of contextuality and our inconsistency result as a statement of nonlocality. Our results show that we can obtain large amounts of contextuality and nonlocality using Clifford gates and measurements.

Hrvoje Nikolić

A simple relativistic quantum hidden-variable theory of particle trajectories, similar to the Bohm theory but without nonlocal forces between the particles, is proposed. To provide compatibility with statistical predictions of quantum mechanics one needs to assume the initial probability density |psi|^2 of particle positions in spacetime, which is the only source of nonlocality in the theory. This demonstrates that the usual Bohm hidden-variable theory contains much more nonlocality than required by the Bell theorem.

Henry Pierce Stapp