2,216 research outputs found
Nonlocality-induced front interaction enhancement
We demonstrate that nonlocal coupling strongly influences the dynamics of
fronts connecting two equivalent states. In two prototype models we observe a
large amplification in the interaction strength between two opposite fronts
increasing front velocities several orders of magnitude. By analyzing the
spatial dynamics we prove that way beyond quantitative effects, nonlocal terms
can also change the overall qualitative picture by inducing oscillations in the
front profile. This leads to a mechanism for the formation of localized
structures not present for local interactions. Finally, nonlocal coupling can
induce a steep broadening of localized structures, eventually annihilating
them.Comment: 4 pages, 6 figure
Polynomial Bell inequalities
It is a recent realization that many of the concepts and tools of causal
discovery in machine learning are highly relevant to problems in quantum
information, in particular quantum nonlocality. The crucial ingredient in the
connection between both fields is the tool of Bayesian networks, a graphical
model used to reason about probabilistic causation. Indeed, Bell's theorem
concerns a particular kind of a Bayesian network and Bell inequalities are a
special case of linear constraints following from such models. It is thus
natural to look for generalized Bell scenarios involving more complex Bayesian
networks. The problem, however, relies on the fact that such generalized
scenarios are characterized by polynomial Bell inequalities and no current
method is available to derive them beyond very simple cases. In this work, we
make a significant step in that direction, providing a general and practical
method for the derivation of polynomial Bell inequalities in a wide class of
scenarios, applying it to a few cases of interest. We also show how our
construction naturally gives rise to a notion of non-signalling in generalized
networks.Comment: 9 pages (including appendix
Causal hierarchy of multipartite Bell nonlocality
As with entanglement, different forms of Bell nonlocality arise in the
multipartite scenario. These can be defined in terms of relaxations of the
causal assumptions in local hidden-variable theories. However, a
characterisation of all the forms of multipartite nonlocality has until now
been out of reach, mainly due to the complexity of generic multipartite causal
models. Here, we employ the formalism of Bayesian networks to reveal
connections among different causal structures that make a both practical and
physically meaningful classification possible. Our framework holds for
arbitrarily many parties. We apply it to study the tripartite scenario in
detail, where we fully characterize all the nonlocality classes. Remarkably, we
identify new highly nonlocal causal structures that cannot reproduce all
quantum correlations. This shows, to our knowledge, the strongest form of
quantum multipartite nonlocality known to date. Finally, as a by-product
result, we derive a non-trivial Bell-type inequality with no quantum violation.
Our findings constitute a significant step forward in the understanding of
multipartite Bell nonlocality and open several venues for future research.Comment: 6 pages + appendix, 3 figures, 3 tables. Minor errors corrected,
discovery of strongest form of quantum multipartite non-locality known so far
added. v3: text improved. v4: Accepted by Quantu
Classical wave-optics analogy of quantum information processing
An analogous model system for quantum information processing is discussed,
based on classical wave optics. The model system is applied to three examples
that involve three qubits: ({\em i}) three-particle Greenberger-Horne-Zeilinger
entanglement, ({\em ii}) quantum teleportation, and ({\em iii}) a simple
quantum error correction network. It is found that the model system can
successfully simulate most features of entanglement, but fails to simulate
quantum nonlocality. Investigations of how far the classical simulation can be
pushed show that {\em quantum nonlocality} is the essential ingredient of a
quantum computer, even more so than entanglement. The well known problem of
exponential resources required for a classical simulation of a quantum
computer, is also linked to the nonlocal nature of entanglement, rather than to
the nonfactorizability of the state vector.Comment: 9 pages, 6 figure
Experimental nonlocality-based network diagnostics of mutipartite entangled states
Quantum networks of growing complexity play a key role as resources for
quantum computation; the ability to identify the quality of their internal
correlations will play a crucial role in addressing the buiding stage of such
states. We introduce a novel diagnostic scheme for multipartite networks of
entangled particles, aimed at assessing the quality of the gates used for the
engineering of their state. Using the information gathered from a set of
suitably chosen multiparticle Bell tests, we identify conditions bounding the
quality of the entangled bonds among the elements of a register. We demonstrate
the effectiveness, flexibility, and diagnostic power of the proposed
methodology by characterizing a quantum resource engineered combining
two-photon hyperentanglement and photonic-chip technology. Our approach is
feasible for medium-sized networks due to the intrinsically modular nature of
cluster states, and paves the way to section-by-section analysis of large
photonics resources.Comment: 5 pages, 3 figures, RevTex4-
Entanglement without hidden nonlocality
We consider Bell tests in which the distant observers can perform local
filtering before testing a Bell inequality. Notably, in this setup, certain
entangled states admitting a local hidden variable model in the standard Bell
scenario can nevertheless violate a Bell inequality after filtering, displaying
so-called hidden nonlocality. Here we ask whether all entangled states can
violate a Bell inequality after well-chosen local filtering. We answer this
question in the negative by showing that there exist entangled states without
hidden nonlocality. Specifically, we prove that some two-qubit Werner states
still admit a local hidden variable model after any possible local filtering on
a single copy of the state.Comment: 16 pages, 2 figure
Nonlinear Bell inequalities tailored for quantum networks
In a quantum network, distant observers sharing physical resources emitted by
independent sources can establish strong correlations, which defy any classical
explanation in terms of local variables. We discuss the characterization of
nonlocal correlations in such a situation, when compared to those that can be
generated in networks distributing independent local variables. We present an
iterative procedure for constructing Bell inequalities tailored for networks:
starting from a given network, and a corresponding Bell inequality, our
technique provides new Bell inequalities for a more complex network, involving
one additional source and one additional observer. The relevance of our method
is illustrated on a variety of networks, for which we demonstrate significant
quantum violations.Comment: 8 pages, 2 figures. Comments welcom
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