1,594 research outputs found
Experimental criteria for steering and the Einstein-Podolsky-Rosen paradox
We formally link the concept of steering (a concept created by Schrodinger
but only recently formalised by Wiseman, Jones and Doherty [Phys. Rev. Lett.
98, 140402 (2007)] and the criteria for demonstrations of
Einstein-Podolsky-Rosen (EPR) paradox introduced by Reid [Phys. Rev. A, 40, 913
(1989)]. We develop a general theory of experimental EPR-steering criteria,
derive a number of criteria applicable to discrete as well as
continuous-variables observables, and study their efficacy in detecting that
form of nonlocality in some classes of quantum states. We show that previous
versions of EPR-type criteria can be rederived within this formalism, thus
unifying these efforts from a modern quantum-information perspective and
clarifying their conceptual and formal origin. The theory follows in close
analogy with criteria for other forms of quantum nonlocality (Bell-nonlocality,
entanglement), and because it is a hybrid of those two, it may lead to insights
into the relationship between the different forms of nonlocality and the
criteria that are able to detect them.Comment: Changed title, updated references, minor corrections, added
journal-ref and DO
Bell inequalities for continuous-variable correlations
We derive a new class of correlation Bell-type inequalities. The inequalities
are valid for any number of outcomes of two observables per each of n parties,
including continuous and unbounded observables. We show that there are no
first-moment correlation Bell inequalities for that scenario, but such
inequalities can be found if one considers at least second moments. The
derivation stems from a simple variance inequality by setting local commutators
to zero. We show that above a constant detector efficiency threshold, the
continuous variable Bell violation can survive even in the macroscopic limit of
large n. This method can be used to derive other well-known Bell inequalities,
shedding new light on the importance of non-commutativity for violations of
local realism.Comment: 4 pages, 1 figure. v2: New results on detector efficiencies and
macroscopic limit, new co-author, changed title and abstract, changed figure,
added journal reference and DO
Bell inequalities for Continuous-Variable Measurements
Tests of local hidden variable theories using measurements with continuous
variable (CV) outcomes are developed, and a comparison of different methods is
presented. As examples, we focus on multipartite entangled GHZ and cluster
states. We suggest a physical process that produces the states proposed here,
and investigate experiments both with and without binning of the continuous
variable. In the former case, the Mermin-Klyshko inequalities can be used
directly. For unbinned outcomes, the moment-based CFRD inequalities are
extended to functional inequalities by considering arbitrary functions of the
measurements at each site. By optimising these functions, we obtain more robust
violations of local hidden variable theories than with either binning or
moments. Recent inequalities based on the algebra of quaternions and octonions
are compared with these methods. Since the prime advantage of CV experiments is
to provide a route to highly efficient detection via homodyne measurements, we
analyse the effect of noise and detection losses in both binned and unbinned
cases. The CV moment inequalities with an optimal function have greater
robustness to both loss and noise. This could permit a loophole-free test of
Bell inequalities.Comment: 17 pages, 6 figure
Formação de mudas enxertadas de gravioleira em substratos com fertilizantes minerais e orgânicos.
bitstream/item/33735/1/BP10004.pd
Unified criteria for multipartite quantum nonlocality
Wiseman and co-workers (Phys. Rev. Lett. 98, 140402, 2007) proposed a
distinction between the nonlocality classes of Bell's nonlocality, steering and
entanglement based on whether or not an overseer trusts each party in a
bipartite scenario where they are asked to demonstrate entanglement. Here we
extend that concept to the multipartite case and derive inequalities that
progressively test for those classes of nonlocality, with different thresholds
for each level. This framework includes the three classes of nonlocality above
in special cases and introduces a family of others.Comment: V2: corrected image display; V3: substantial changes including new
proofs, arguments, and result
Witworld: A generalised probabilistic theory featuring post-quantum steering
We introduce Witworld: a generalised probabilistic theory with strong
post-quantum features, which subsumes Boxworld. Witworld is the first theory
that features post-quantum steering, and also the first that outperforms
quantum theory at the task of remote state preparation. We further show
post-quantum steering to be the source of this advantage, and hence present the
first instance where post-quantum steering is a stronger-than-quantum resource
for information processing.Comment: 9 pages, loads of diagrams. Comments welcom
Arbitrarily loss-tolerant Einstein-Podolsky-Rosen steering allowing a demonstration over 1 km of optical fiber with no detection loophole
Demonstrating nonclassical effects over longer and longer distances is
essential for both quantum technology and fundamental science. The main
challenge is loss of photons during propagation, because considering only those
cases where photons are detected opens a "detection loophole" in security
whenever parties or devices are untrusted. Einstein-Podolsky-Rosen (EPR)
steering is equivalent to an entanglement-verification task in which one party
(device) is untrusted. We derive arbitrarily loss-tolerant tests, enabling us
to perform a detection-loophole-free demonstration of EPR-steering with parties
separated by a coiled 1 km optical fiber, with a total loss of 8.9 dB (87%).Comment: Accepted for publication in Physical Review X. This is the accepted
versio
Causarum Investigatio and the Two Bell's Theorems of John Bell
"Bell's theorem" can refer to two different theorems that John Bell proved,
the first in 1964 and the second in 1976. His 1964 theorem is the
incompatibility of quantum phenomena with the joint assumptions of Locality and
Predetermination. His 1976 theorem is their incompatibility with the single
property of Local Causality. This is contrary to Bell's own later assertions,
that his 1964 theorem began with the assumption of Local Causality, even if not
by that name. Although the two Bell's theorems are logically equivalent, their
assumptions are not. Hence, the earlier and later theorems suggest quite
different conclusions, embraced by operationalists and realists, respectively.
The key issue is whether Locality or Local Causality is the appropriate notion
emanating from Relativistic Causality, and this rests on one's basic notion of
causation. For operationalists the appropriate notion is what is here called
the Principle of Agent-Causation, while for realists it is Reichenbach's
Principle of common cause. By breaking down the latter into even more basic
Postulates, it is possible to obtain a version of Bell's theorem in which each
camp could reject one assumption, happy that the remaining assumptions reflect
its weltanschauung. Formulating Bell's theorem in terms of causation is
fruitful not just for attempting to reconcile the two camps, but also for
better describing the ontology of different quantum interpretations and for
more deeply understanding the implications of Bell's marvellous work.Comment: 24 pages. Prepared for proceedings of the "Quantum [Un]speakables II"
conference (Vienna, 2014), to be published by Springe
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