49,655 research outputs found
Bell's theorem as a signature of nonlocality: a classical counterexample
For a system composed of two particles Bell's theorem asserts that averages
of physical quantities determined from local variables must conform to a family
of inequalities. In this work we show that a classical model containing a local
probabilistic interaction in the measurement process can lead to a violation of
the Bell inequalities. We first introduce two-particle phase-space
distributions in classical mechanics constructed to be the analogs of quantum
mechanical angular momentum eigenstates. These distributions are then employed
in four schemes characterized by different types of detectors measuring the
angular momenta. When the model includes an interaction between the detector
and the measured particle leading to ensemble dependencies, the relevant Bell
inequalities are violated if total angular momentum is required to be
conserved. The violation is explained by identifying assumptions made in the
derivation of Bell's theorem that are not fulfilled by the model. These
assumptions will be argued to be too restrictive to see in the violation of the
Bell inequalities a faithful signature of nonlocality.Comment: Extended manuscript. Significant change
Two destructive effects of decoherence on Bell inequality violation
We consider a system of two spin-1/2 particles, initially in an entangled
Bell state. If one of the particles is interacting with an environment (e.g. a
collection of N independent spins), the two-particle system undergoes
decoherence. Using a simple model of decoherence, we show that this process has
two consequences. First, the maximal amount by which the CHSH inequality is
violated decays to zero. Second, the set of directions of measurement for which
the inequality is violated is reduced in the course of decoherence. The volume
of that set is bounded above by C|r|^2, where r is the decoherence factor. We
obtain similar results for the case when each of the two particles is in
interaction with a separate environment.Comment: v2: added results for decoherence due to interactions of both
particles + minor changes; v3: minor change
Generation of GHZ and W states for stationary qubits in spin network via resonance scattering
We propose a simple scheme to establish entanglement among stationary qubits
based on the mechanism of resonance scattering between them and a
single-spin-flip wave packet in designed spin network. It is found that through
the natural dynamical evolution of an incident single-spin-flip wave packet in
a spin network and the subsequent measurement of the output single-spin-flip
wave packet,multipartite entangled states among n stationary qubits,
Greenberger-Horne-Zeilinger (GHZ) and W states can be generated.Comment: 8 pages, 6 figure
A generalized structure of Bell inequalities for bipartite arbitrary dimensional systems
We propose a generalized structure of Bell inequalities for arbitrary
d-dimensional bipartite systems, which includes the existing two types of Bell
inequalities introduced by Collins-Gisin-Linden-Massar-Popescu [Phys. Rev.
Lett. 88, 040404 (2002)] and Son-Lee-Kim [Phys. Rev. Lett. 96, 060406 (2006)].
We analyze Bell inequalities in terms of correlation functions and joint
probabilities, and show that the coefficients of correlation functions and
those of joint probabilities are in Fourier transform relations. We finally
show that the coefficients in the generalized structure determine the
characteristics of quantum violation and tightness.Comment: 6 pages, 1 figur
An upper limit on CP violation in the system
In a previous publication we noted that the time dependence of an incoherent
mixture undergoes a qualitative change when the magnitude of CP
violation exceeds a critical value. Requiring, on physical grounds,
that the system evolve from an initial incoherent state to a final pure state
in a monotonic way, yields a new upper limit for . The recent
measurement of the wrong charge semileptonic asymmetry of mesons
presented by the D0 collaboration is outside this bound by one standard
deviation. If this result is confirmed it implies the existence of a new
quantum mechanical oscillation phenomenon.Comment: 7 pages, 2 figures, version submitted for publication (Physical
Review
A simultaneous generalization of independence and disjointness in boolean algebras
We give a definition of some classes of boolean algebras generalizing free
boolean algebras; they satisfy a universal property that certain functions
extend to homomorphisms. We give a combinatorial property of generating sets of
these algebras, which we call n-independent. The properties of these classes
(n-free and omega-free boolean algebras) are investigated. These include
connections to hypergraph theory and cardinal invariants on these algebras.
Related cardinal functions, Ind, which is the supremum of the cardinalities
of n-independent subsets; i_n, the minimum size of a maximal n-independent
subset; and i_omega, the minimum size of an omega-independent subset, are
introduced and investigated. The values of i_n and i_omega on P(omega)/fin are
shown to be independent of ZFC.Comment: Sumbitted to Orde
Generating optimal states for a homodyne Bell test
Published versio
How much larger quantum correlations are than classical ones
Considering as distance between two two-party correlations the minimum number
of half local results one party must toggle in order to turn one correlation
into the other, we show that the volume of the set of physically obtainable
correlations in the Einstein-Podolsky-Rosen-Bell scenario is (3 pi/8)^2 = 1.388
larger than the volume of the set of correlations obtainable in local
deterministic or probabilistic hidden-variable theories, but is only 3 pi^2/32
= 0.925 of the volume allowed by arbitrary causal (i.e., no-signaling)
theories.Comment: REVTeX4, 6 page
Creation and localization of entanglement in a simple configuration of coupled harmonic oscillators
We investigate a simple arrangement of coupled harmonic oscillators which
brings out some interesting effects concerning creation of entanglement. It is
well known that if each member in a linear chain of coupled harmonic
oscillators is prepared in a ``classical state'', such as a pure coherent state
or a mixed thermal state, no entanglement is created in the rotating wave
approximation. On the other hand, if one of the oscillators is prepared in a
nonclassical state (pure squeezed state, for instance), entanglement may be
created between members of the chain. In the setup considered here, we found
that a great family of nonclassical (squeezed) states can localize entanglement
in such a way that distant oscillators never become entangled. We present a
detailed study of this particular localization phenomenon. Our results may find
application in future solid state implementations of quantum computers, and we
suggest an electromechanical system consisting of an array of coupled
micromechanical oscillators as a possible implementation.Comment: 7 pages, 8 figures, minor typos fixe
Dynamical Reduction Models: present status and future developments
We review the major achievements of the dynamical reduction program, showing
why and how it provides a unified, consistent description of physical
phenomena, from the microscopic quantum domain to the macroscopic classical
one. We discuss the difficulties in generalizing the existing models in order
to comprise also relativistic quantum field theories. We point out possible
future lines of research, ranging from mathematical physics to phenomenology.Comment: 12 pages. Contribution to the Proceedings of the "Third International
Workshop DICE2006", Castello di Piombino (Tuscany), September 11-15, 2006.
Minor changes mad
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