482 research outputs found
Classification and monogamy of three-qubit biseparable Bell correlations
We strengthen the set of Bell-type inequalities presented by Sun & Fei [Phys.
Rev. A 74, 032335 (2006)] that give a classification for biseparable
correlations and entanglement in tripartite quantum systems. We will
furthermore consider the restriction to local orthogonal spin observables and
show that this strengthens all previously known such tripartite inequalities.
The quadratic inequalities we find indicate a type of monogamy of maximal
biseparable three-particle quantum correlations, although the nonmaximal ones
can be shared. This is contrasted to recently found monogamy inequalities for
bipartite Bell correlations in tripartite systems.Comment: Accepted final version for PRA. 6 page
No-signaling, perfect bipartite dichotomic correlations and local randomness
The no-signaling constraint on bi-partite correlations is reviewed. It is
shown that in order to obtain non-trivial Bell-type inequalities that discern
no-signaling correlations from more general ones, one must go beyond
considering expectation values of products of observables only. A new set of
nontrivial no-signaling inequalities is derived which have a remarkably close
resemblance to the CHSH inequality, yet are fundamentally different. A set of
inequalities by Roy and Singh and Avis et al., which is claimed to be useful
for discerning no-signaling correlations, is shown to be trivially satisfied by
any correlation whatsoever. Finally, using the set of newly derived
no-signaling inequalities a result with potential cryptographic consequences is
proven: if different parties use identical devices, then, once they have
perfect correlations at spacelike separation between dichotomic observables,
they know that because of no-signaling the local marginals cannot but be
completely random.Comment: Published in 'Proceedings of the International Conference Advances in
Quantum Theory', AIP Conference Proceedings, vol. 1327, 2011. pp. 36-5
Can quantum theory and special relativity peacefully coexist?
This white paper aims to identify an open problem in 'Quantum Physics and the
Nature of Reality' --namely whether quantum theory and special relativity are
formally compatible--, to indicate what the underlying issues are, and put
forward ideas about how the problem might be addressed.Comment: Invited white paper for Quantum Physics and the Nature of Reality,
John Polkinghorne 80th Birthday Conference. St Anne's College, Oxford. 26-29
September 201
Strengthened Bell inequalities for orthogonal spin directions
We strengthen the bound on the correlations of two spin-1/2 particles
(qubits) in separable (non-entangled) states for locally orthogonal spin
directions by much tighter bounds than the well-known Bell inequality. This
provides a sharper criterion for the experimental distinction between entangled
and separable states, and even one which is a necessary and sufficient
condition for separability. However, these improved bounds do not apply to
local hidden-variable theories, and hence they provide a criterion to test the
correlations allowed by local hidden-variable theories against those allowed by
separable quantum states. Furthermore, these bounds are stronger than some
recent alternative experimentally accessible entanglement criteria. We also
address the issue of finding a finite subset of these inequalities that would
already form a necessary and sufficient condition for non-entanglement. For
mixed state we have not been able to resolve this, but for pure states a set of
six inequalities using only three sets of orthogonal observables is shown to be
already necessary and sufficient for separability.Comment: v2: Considerably changed, many new and stronger results v3: Published
version; To appear in Physics Letters A. Online available from publishers
websit
Discerning Elementary Particles
We extend the quantum-mechanical results of Muller & Saunders (2008)
establishing the weak discernibility of an arbitrary number of similar fermions
in finite-dimensional Hilbert-spaces in two ways: (a) from fermions to bosons
for all finite-dimensional Hilbert-spaces; and (b) from finite-dimensional to
infinite-dimensional Hilbert-spaces for all elementary particles. In both cases
this is performed using operators whose physical significance is beyond
doubt.This confutes the currently dominant view that (A) the quantum-mechanical
description of similar particles conflicts with Leibniz's Principle of the
Identity of Indiscernibles (PII); and that (B) the only way to save PII is by
adopting some pre-Kantian metaphysical notion such as Scotusian haecceittas or
Adamsian primitive thisness. We take sides with Muller & Saunders (2008)
against this currently dominant view, which has been expounded and defended by,
among others, Schr\"odinger, Margenau, Cortes, Dalla Chiara, Di Francia,
Redhead, French, Teller, Butterfield, Mittelstaedt, Giuntini, Castellani,
Krause and Huggett.Comment: Final Version. To appear in Philosophy of Science, July 200
Automatic tuning of quality factors for VHF CMOS filters
A VHF transconductance-C filter technique with automatic tuning of the quality factors and the cutoff frequency, based on a voltage controlled oscillator (VCO), is presented. The Q-tuning circuit is very simple and contains no signal carrying nodes, resulting in a well-controlled Q up to very high frequencies (100 MHz). A breadboard realization of the Q-tuning circuit gave good result
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