38,304 research outputs found
Not throwing out the baby with the bathwater: Bell's condition of local causality mathematically 'sharp and clean'
The starting point of the present paper is Bell's notion of local causality
and his own sharpening of it so as to provide for mathematical formalisation.
Starting with Norsen's (2007, 2009) analysis of this formalisation, it is
subjected to a critique that reveals two crucial aspects that have so far not
been properly taken into account. These are (i) the correct understanding of
the notions of sufficiency, completeness and redundancy involved; and (ii) the
fact that the apparatus settings and measurement outcomes have very different
theoretical roles in the candidate theories under study. Both aspects are not
adequately incorporated in the standard formalisation, and we will therefore do
so. The upshot of our analysis is a more detailed, sharp and clean mathematical
expression of the condition of local causality. A preliminary analysis of the
repercussions of our proposal shows that it is able to locate exactly where and
how the notions of locality and causality are involved in formalising Bell's
condition of local causality.Comment: 14 pages. To be published in PSE volume "Explanation, Prediction, and
Confirmation", edited by Dieks, et a
Weak values and the Leggett-Garg inequality in solid-state qubits
An implementation of weak values is investigated in solid-state qubits. We
demonstrate that a weak value can be non-classical if and only if a
Leggett-Garg inequality can also be violated. Generalized weak values are
described, where post-selection on a range of weak measurement results.
Imposing classical weak values permits the derivation of Leggett-Garg
inequalities for bounded operators. Our analysis is presented in terms of
kicked quantum nondemolition measurements on a quantum double-dot charge qubit.Comment: 4 pages, 2 figure
Local quasi hidden variable modelling and violations of Bell-type inequalities by a multipartite quantum state
We introduce for a general correlation scenario a new simulation model, a
local quasi hidden variable (LqHV) model, where locality and the
measure-theoretic structure inherent to an LHV model are preserved but
positivity of a simulation measure is dropped. We specify a necessary and
sufficient condition for LqHV modelling and, based on this, prove that every
quantum correlation scenario admits an LqHV simulation. Via the LqHV approach,
we construct analogs of Bell-type inequalities for an N-partite quantum state
and find a new analytical upper bound on the maximal violation by an N-partite
quantum state of S_{1}x...xS_{N}-setting Bell-type inequalities - either on
correlation functions or on joint probabilities and for outcomes of an
arbitrary spectral type, discrete or continuous. This general analytical upper
bound is expressed in terms of the new state dilation characteristics
introduced in the present paper and not only traces quantum states admitting an
S_{1}x...xS_{N}-setting LHV description but also leads to the new exact
numerical upper estimates on the maximal Bell violations for concrete N-partite
quantum states used in quantum information processing and for an arbitrary
N-partite quantum state. We, in particular, prove that violation by an
N-partite quantum state of an arbitrary Bell-type inequality (either on
correlation functions or on joint probabilities) for S settings per site cannot
exceed (2S-1)^{N-1} even in case of an infinite dimensional quantum state and
infinitely many outcomes.Comment: Improved, edited versio
Characterizing the nonlocal correlations of particles that never interacted
Quantum systems that have never interacted can become nonlocally correlated
through a process called entanglement swapping. To characterize nonlocality in
this context, we introduce local models where quantum systems that are
initially uncorrelated are described by uncorrelated local variables. While a
pair of maximally entangled qubits prepared in the usual way (i.e., emitted
from a common source) requires a visibility close to 70% to violate a Bell
inequality, we show that an entangled pair generated through entanglement
swapping will already violate a Bell inequality for visibilities as low as 50%
under our assumption.Comment: 5 pages, 2 figure
Comments on the superluminal motion in Cygnus X-3
Following the recent discovery that Cyg X-3 exhibits superluminal motion, the
implications of superluminal expansion and contraction are investigated. We
propose that the effect is due to either a propagating photon pattern or to
outwardly moving shells illuminated by an intense beam of radiation.Comment: accepted for publication in MNRAS; 5 pages, 3 figure
Entanglement of two distant Bose-Einstein condensates by detection of Bragg-scattered photons
We show that it is possible to generate entanglement between two distant
Bose-Einstein condensates by detection of Hanbury Brown-Twiss type correlations
in photons Bragg-scattered by the condensates. Upon coincident detection of two
photons by two detectors, the projected joint state of two condensates is shown
to be non-Gaussian. We verify the existence of entanglement by showing that the
partially transposed state is negative. Further we use the inequality in terms
of higher order moments to confirm entanglement. Our proposed scheme can be
generalized for multiple condensates and also for spinor condensates with Bragg
scattering of polarized light with the latter capable of producing hyper
entanglement.Comment: 9 pages, 5 figure
Axial anomaly of QED in a strong magnetic field and noncommutative anomaly
The Adler-Bell-Jackiw (ABJ) anomaly of a 3+1 dimensional QED is calculated in
the presence of a strong magnetic field. It is shown that in the regime with
the lowest Landau level (LLL) dominance a dimensional reduction from D=4 to D=2
dimensions occurs in the longitudinal sector of the low energy effective field
theory. In the chiral limit, the resulting anomaly is therefore comparable with
the axial anomaly of a two dimensional massless Schwinger model. It is further
shown that the U(1) axial anomaly of QED in a strong magnetic field is closely
related to the ``nonplanar'' axial anomaly of a conventional noncommutative
QED.Comment: 18 pp, no figure. v2: The version accepted to be publidhed in PR
Multipartite Nonlocality without Entanglement in Many Dimensions
We present a generic method to construct a product basis exhibiting
Nonlocality Without Entanglement with parties each holding a system of
dimension at least . This basis is generated via a quantum circuit made of
control-Discrete Fourier Transform gates acting on the computational basis. The
simplicity of our quantum circuit allows for an intuitive understanding of this
new type of nonlocality. We also show how this circuit can be used to construct
Unextendible Product Bases and their associated Bound Entangled States. To our
knowledge, this is the first method which, given a general Hilbert space
with , makes it possible to
construct (i) a basis exhibiting Nonlocality Without Entanglement, (ii) an
Unextendible Product Basis, and (iii) a Bound Entangled state.Comment: 8 pages, 4 figure
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