263 research outputs found
Dimension witnesses and quantum state discrimination
Dimension witnesses allow one to test the dimension of an unknown physical
system in a device-independent manner, that is, without placing assumptions
about the functioning of the devices used in the experiment. Here we present
simple and general dimension witnesses for quantum systems of arbitrary Hilbert
space dimension. Our approach is deeply connected to the problem of quantum
state discrimination, hence establishing a strong link between these two
research topics. Finally, our dimension witnesses can distinguish between
classical and quantum systems of the same dimension, making them potentially
useful for quantum information processing.Comment: 5 page
Activation of Non-Local Quantum Resources
We find two two-qubit states such that any number of copies of one state or
the other cannot violate the CHSH Bell inequality. However, their tensor
product can produce a CHSH violation of at least 2.023. We also identify a
CHSH-local state such that two copies of it are CHSH-violating. The tools
employed here can be easily adapted to find instances of non-locality
activation in arbitrary Bell scenarios
Pure state estimation and the characterization of entanglement
A connection between the state estimation problem and the separability
problem is noticed and exploited to find efficient numerical algorithms to
solve the first one. Based on these ideas, we also derive a systematic method
to obtain upper bounds on the maximum local fidelity when the states are
distributed among several distant parties.Comment: Closer to published versio
An operational framework for nonlocality
Due to the importance of entanglement for quantum information purposes, a
framework has been developed for its characterization and quantification as a
resource based on the following operational principle: entanglement among
parties cannot be created by local operations and classical communication, even
when parties collaborate. More recently, nonlocality has been identified
as another resource, alternative to entanglement and necessary for
device-independent quantum information protocols. We introduce an operational
framework for nonlocality based on a similar principle: nonlocality among
parties cannot be created by local operations and allowed classical
communication even when parties collaborate. We then show that the
standard definition of multipartite nonlocality, due to Svetlichny, is
inconsistent with this operational approach: according to it, genuine
tripartite nonlocality could be created by two collaborating parties. We
finally discuss alternative definitions for which consistency is recovered
Almost quantum correlations
There have been a number of attempts to derive the set of quantum non-local
correlations from reasonable physical principles. Here we introduce
, a set of multipartite supra-quantum correlations that has appeared
under different names in fields as diverse as graph theory, quantum gravity and
quantum information science. We argue that may correspond to the
set of correlations of a reasonable physical theory, in which case the research
program to reconstruct quantum theory from device-independent principles is met
with strong obstacles. In support of this conjecture, we prove that
is closed under classical operations and satisfies the physical principles of
Non-Trivial Communication Complexity, No Advantage for Nonlocal Computation,
Macroscopic Locality and Local Orthogonality. We also review numerical evidence
that almost quantum correlations satisfy Information Causality.Comment: 15+2 pages, 1 figur
Quantum Steering and Space-Like Separation
In non-relativistic quantum mechanics, measurements performed by separate
observers are modeled via tensor products. In Algebraic Quantum Field Theory,
though, local observables corresponding to space-like separated parties are
just required to commute. The problem of determining whether these two
definitions of "separation" lead to the same set of bipartite correlations is
known in non-locality as Tsirelson's problem. In this article, we prove that
the analog of Tsirelson's problem in steering scenarios is false. That is,
there exists a steering inequality that can be violated or not depending on how
we define space-like separation at the operator level.Comment: Some typos corrected. Short discussion about Algebraic Quantum Field
Theory. Modified introduction and conclusio
Composing decoherence functionals
Quantum measure theory (QMT) is a generalization of quantum theory where physical predictions are computed from a matrix known as the decoherence functional (DF). Previous works have noted that, in its original formulation, QMT exhibits a problem with composability, since the composition of two decoherence functionals is, in general, not a valid decoherence functional. This does not occur when the DFs in question happen to be positive semidefinite (a condition known as strong positivity). In this paper, we study the concept of composability of DFs and its consequences for QMT. Firstly, we show that the problem of composability is much deeper than originally envisaged, since, for any n, there exists a DF that can coexist with nâ1 copies of itself, but not with n. Secondly, we prove that the set of strongly positive DFs cannot be enlarged while remaining closed under composition. Furthermore, any closed set of DFs containing all quantum DFs can only contain strongly positive DFs
Nonlocality in sequential correlation scenarios
As first shown by Popescu [S. Popescu, Phys. Rev. Lett. 74, 2619 (1995)],
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 [R. Gallego et al., Phys. Rev. Lett. 109, 070401
(2013)] 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
characterisation of sequential nonlocality that contains as particular cases
standard nonlocality and hidden nonlocality.Comment: 13 pages, 3 figure
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