91 research outputs found
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]
N-qubit entanglement via the -type collective interaction
We investigate quantum correlations of the -qubit states via a collective
pseudo-spin interaction () on arbitrary pure separable states
for a given interval of time. Based on this dynamical generation of the
-qubit maximal entangled states, a quantum secret sharing protocol with
continuous classical secrets is developed.Comment: 12 pages, 3 figure
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
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
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
Information Invariance and Quantum Probabilities
We consider probabilistic theories in which the most elementary system, a
two-dimensional system, contains one bit of information. The bit is assumed to
be contained in any complete set of mutually complementary measurements. The
requirement of invariance of the information under a continuous change of the
set of mutually complementary measurements uniquely singles out a measure of
information, which is quadratic in probabilities. The assumption which gives
the same scaling of the number of degrees of freedom with the dimension as in
quantum theory follows essentially from the assumption that all physical states
of a higher dimensional system are those and only those from which one can
post-select physical states of two-dimensional systems. The requirement that no
more than one bit of information (as quantified by the quadratic measure) is
contained in all possible post-selected two-dimensional systems is equivalent
to the positivity of density operator in quantum theory.Comment: 8 pages, 1 figure. This article is dedicated to Pekka Lahti on the
occasion of his 60th birthday. Found. Phys. (2009
EPR-Bell Nonlocality, Lorentz Invariance, and Bohmian Quantum Theory
We discuss the problem of finding a Lorentz invariant extension of Bohmian
mechanics. Due to the nonlocality of the theory there is (for systems of more
than one particle) no obvious way to achieve such an extension. We present a
model invariant under a certain limit of Lorentz transformations, a limit
retaining the characteristic feature of relativity, the non-existence of
absolute time resp. simultaneity. The analysis of this model exemplifies an
important property of any Bohmian quantum theory: the quantum equilibrium
distribution cannot simultaneously be realized in all
Lorentz frames of reference.Comment: 24 pages, LaTex, 4 figure
Hidden variables with nonlocal time
To relax the apparent tension between nonlocal hidden variables and
relativity, we propose that the observable proper time is not the same quantity
as the usual proper-time parameter appearing in local relativistic equations.
Instead, the two proper times are related by a nonlocal rescaling parameter
proportional to |psi|^2, so that they coincide in the classical limit. In this
way particle trajectories may obey local relativistic equations of motion in a
manner consistent with the appearance of nonlocal quantum correlations. To
illustrate the main idea, we first present two simple toy models of local
particle trajectories with nonlocal time, which reproduce some nonlocal quantum
phenomena. After that, we present a realistic theory with a capacity to
reproduce all predictions of quantum theory.Comment: 16 pages, accepted for publication in Found. Phys., misprints
corrected, references update
Greenberger-Horne-Zeilinger nonlocality for continuous variable systems
As a development of our previous work, this paper is concerned with the
Greenberger-Horne-Zeilinger (GHZ) nonlocality for continuous variable cases.
The discussion is based on the introduction of a pseudospin operator, which has
the same algebra as the Pauli operator, for each of the modes of a light
field. Then the Bell-CHSH (Clauser, Horne, Shimony and Holt) inequality is
presented for the modes, each of which has a continuous degree of freedom.
Following Mermin's argument, it is demonstrated that for -mode
parity-entangled GHZ states (in an infinite-dimensional Hilbert space) of the
light field, the contradictions between quantum mechanics and local realism
grow exponentially with , similarly to the usual -spin cases.Comment: RevTEX; comments are welcomed; new version with minor change
Quantum Entanglement of Excitons in Coupled Quantum Dots
Optically-controlled exciton dynamics in coupled quantum dots is studied. We
show that the maximally entangled Bell states and Greenberger-Horne-Zeilinger
(GHZ) states can be robustly generated by manipulating the system parameters to
be at the avoided crossings in the eigenenergy spectrum. The analysis of
population transfer is systematically carried out using a dressed-state
picture. In addition to the quantum dot configuration that have been discussed
by Quiroga and Johnson [Phys. Rev. Lett. \QTR{bf}{83}, 2270 (1999)], we show
that the GHZ states also may be produced in a ray of three quantum dots with a
shorter generation time.Comment: 16 pages, 7 figures, to appear in Phys. Rev.
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