127 research outputs found
Comment on "Quantitative wave-particle duality in multibeam interferometers"
In a recent paper [Phys. Rev. {\bf A64}, 042113 (2001)] S. D\"urr proposed an
interesting multibeam generalization of the quantitative formulation of
interferometric wave-particle duality, discovered by Englert for two-beam
interferometers. The proposed generalization is an inequality that relates a
generalized measure of the fringe visibility, to certain measures of the
maximum amount of which-way knowledge that can be stored in a which-way
detector. We construct an explicit example where, with three beams in a pure
state, the scheme proposed by D\"{u}rr leads to the possibility of an ideal
which-way detector, that can achieve a better path-discrimination, at the same
time as a better fringe visibility. In our opinion, this seems to be in
contrast with the intuitive idea of complementarity, as it is implemented in
the two-beams case, where an increase in path discrimination always implies a
decrease of fringe visibility, if the beams and the detector are in pure
states.Comment: 4 pages, 1 encapsulated figure. In press on Phys. Rev.
An efficient numerical algorithm on irreducible multiparty correlations
We develop a numerical algorithm to calculate the degrees of irreducible
multiparty correlations for an arbitrary multiparty quantum state, which is
efficient for any quantum state of up to five qubits. We demonstrate the power
of the algorithm by the explicit calculations of the degrees of irreducible
multiparty correlations in the 4-qubit GHZ state, the Smolin state, and the
5-qubit W state. This development takes a crucial step towards practical
applications of irreducible multiparty correlations in real quantum many-body
physics.Comment: 4 pages. Comments are welcom
Creation of NOON states by double Fock-state/Bose-Einstein condensates
NOON states (states of the form where and
are single particle states) have been used for predicting violations of
hidden-variable theories (Greenberger-Horne-Zeilinger violations) and are
valuable in metrology for precision measurements of phase at the Heisenberg
limit. We show theoretically how the use of two Fock state/Bose-Einstein
condensates as sources in a modified Mach Zender interferometer can lead to the
creation of the NOON state in which and refer to arms of the
interferometer and is the total number of particles in the two condensates.
The modification of the interferometer involves making conditional ``side''
measurements of a few particles near the sources. These measurements put the
remaining particles in a superposition of two phase states, which are converted
into NOON states by a beam splitter. The result is equivalent to the quantum
experiment in which a large molecule passes through two slits. The NOON states
are combined in a final beam splitter and show interference. Attempts to detect
through which ``slit'' the condensates passed destroys the interference.Comment: 8 pages 5 figure
Three-particle entanglement versus three-particle nonlocality
The notions of three-particle entanglement and three-particle nonlocality are
discussed in the light of Svetlichny's inequality [Phys. Rev. D 35, 3066
(1987)]. It is shown that there exist sets of measurements which can be used to
prove three-particle entanglement, but which are nevertheless useless at
proving three-particle nonlocality. In particular, it is shown that the quantum
predictions giving a maximal violation of Mermin's three-particle Bell
inequality [Phys. Rev. Lett. 65, 1838 (1990)] can be reproduced by a hybrid
hidden variables model in which nonlocal correlations are present only between
two of the particles. It should be possible, however, to test the existence of
both three-particle entanglement and three-particle nonlocality for any given
quantum state via Svetlichny's inequality.Comment: REVTeX4, 4 pages, journal versio
On a recent proof of nonlocality without inequalities
Recently a quite stimulating paper [1] dealing with the possibility of
exploiting the nonlocal aspects of a superposition of states of a single photon
appeared. We regard as greatly relevant the results which have been obtained.
However we think that the presentation of the matter and the way to derive the
conclusion are not fully satisfactory and do not put the necessary emphasis on
some subtle basic aspects like locality and realism. In view of its interest we
consider it useful to reconsider the line of reasoning of ref.[1] and to derive
once more its results by following a procedure which seems to us more lucid and
which makes fully clear the role of the various conceptual aspects of the
treatment. We hope that our analysis will contribute to clarify and to deepen
the interesting results of ref.[1]
Bohm's interpretation and maximally entangled states
Several no-go theorems showed the incompatibility between the locality
assumption and quantum correlations obtained from maximally entangled spin
states. We analyze these no-go theorems in the framework of Bohm's
interpretation. The mechanism by which non-local correlations appear during the
results of measurements performed on distant parts of entangled systems is
explicitly put into evidence in terms of Bohmian trajectories. It is shown that
a GHZ like contradiction of the type+1=-1 occurs for well-chosen initial
positions of the Bohmian trajectories and that it is this essential
non-classical feature that makes it possible to violate the locality condition.Comment: 18 page
The Free Will Theorem
On the basis of three physical axioms, we prove that if the choice of a
particular type of spin 1 experiment is not a function of the information
accessible to the experimenters, then its outcome is equally not a function of
the information accessible to the particles. We show that this result is
robust, and deduce that neither hidden variable theories nor mechanisms of the
GRW type for wave function collapse can be made relativistic. We also establish
the consistency of our axioms and discuss the philosophical implications.Comment: 31 pages, 6figure
Rotationally invariant proof of Bell's theorem without inequalities
The singlet state of two spin-3/2 particles allows a proof of Bell's theorem
without inequalities with two distinguishing features: any local observable can
be regarded as an Einstein-Podolsky-Rosen element of reality, and the
contradiction with local realism occurs not only for some specific local
observables but for any rotation whereof.Comment: REVTeX4, 3 page
Nonlocal appearance of a macroscopic angular momentum
We discuss a type of measurement in which a macroscopically large angular
momentum (spin) is "created" nonlocally by the measurement of just a few atoms
from a double Fock state. This procedure apparently leads to a blatant
nonconservation of a macroscopic variable - the local angular momentum. We
argue that while this gedankenexperiment provides a striking illustration of
several counter-intuitive features of quantum mechanics, it does not imply a
non-local violation of the conservation of angular momentum.Comment: 10 pages, 1 figur
Bell inequalities as constraints on unmeasurable correlations
The interpretation of the violation of Bell-Clauser-Horne inequalities is
revisited, in relation with the notion of extension of QM predictions to
unmeasurable correlations. Such extensions are compatible with QM predictions
in many cases, in particular for observables with compatibility relations
described by tree graphs. This implies classical representability of any set of
correlations , , , and the equivalence of the
Bell-Clauser-Horne inequalities to a non void intersection between the ranges
of values for the unmeasurable correlation associated to different
choices for B. The same analysis applies to the Hardy model and to the "perfect
correlations" discussed by Greenberger, Horne, Shimony and Zeilinger. In all
the cases, the dependence of an unmeasurable correlation on a set of variables
allowing for a classical representation is the only basis for arguments about
violations of locality and causality.Comment: Some modifications have been done in order to improve clarity of
presentation and comparison with other approache
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