306 research outputs found
Quantum correlations from local amplitudes and the resolution of the Einstein-Podolsky-Rosen nonlocality puzzle
The Einstein-Podolsky-Rosen nonlocality puzzle has been recognized as one of
the most important unresolved issues in the foundational aspects of quantum
mechanics. We show that the problem is resolved if the quantum correlations are
calculated directly from local quantities which preserve the phase information
in the quantum system. We assume strict locality for the probability amplitudes
instead of local realism for the outcomes, and calculate an amplitude
correlation function.Then the experimentally observed correlation of outcomes
is calculated from the square of the amplitude correlation function. Locality
of amplitudes implies that the measurement on one particle does not collapse
the companion particle to a definite state. Apart from resolving the EPR
puzzle, this approach shows that the physical interpretation of apparently
`nonlocal' effects like quantum teleportation and entanglement swapping are
different from what is usually assumed. Bell type measurements do not change
distant states. Yet the correlations are correctly reproduced, when measured,
if complex probability amplitudes are treated as the basic local quantities. As
examples we discuss the quantum correlations of two-particle maximally
entangled states and the three-particle GHZ entangled state.Comment: Std. Latex, 11 pages, 1 table. Prepared for presentation at the
International Conference on Quantum Optics, ICQO'2000, Minsk, Belaru
Implication of the overlap representation for modelling generalized parton distributions
Based on a field theoretically inspired model of light-cone wave functions,
we derive valence-like generalized parton distributions and their double
distributions from the wave function overlap in the parton number conserved
s-channel. The parton number changing contributions in the t-channel are
restored from duality. In our construction constraints of positivity and
polynomiality are simultaneously satisfied and it also implies a model
dependent relation between generalized parton distributions and transverse
momentum dependent parton distribution functions. The model predicts that the
t-behavior of resulting hadronic amplitudes depends on the Bjorken variable
x_Bj. We also propose an improved ansatz for double distributions that embeds
this property.Comment: 15 pages, 8 eps figure
An experimental test of non-local realism
Most working scientists hold fast to the concept of 'realism' - a viewpoint
according to which an external reality exists independent of observation. But
quantum physics has shattered some of our cornerstone beliefs. According to
Bell's theorem, any theory that is based on the joint assumption of realism and
locality (meaning that local events cannot be affected by actions in space-like
separated regions) is at variance with certain quantum predictions. Experiments
with entangled pairs of particles have amply confirmed these quantum
predictions, thus rendering local realistic theories untenable. Maintaining
realism as a fundamental concept would therefore necessitate the introduction
of 'spooky' actions that defy locality. Here we show by both theory and
experiment that a broad and rather reasonable class of such non-local realistic
theories is incompatible with experimentally observable quantum correlations.
In the experiment, we measure previously untested correlations between two
entangled photons, and show that these correlations violate an inequality
proposed by Leggett for non-local realistic theories. Our result suggests that
giving up the concept of locality is not sufficient to be consistent with
quantum experiments, unless certain intuitive features of realism are
abandoned.Comment: Minor corrections to the manuscript, the final inequality and all its
conclusions do not change; description of corrections (Corrigendum) added as
new Appendix III; Appendix II replaced by a shorter derivatio
Event-based prospective memory performance in autism spectrum disorder
The purpose of the present study was to investigate event-based prospective memory performance in individuals with autism spectrum disorder and to explore possible relations between laboratory-based prospective memory performance and everyday performance. Nineteen children and adolescents with autism spectrum disorder and 19 matched neurotypical controls participated. The laboratory-based prospective memory test was embedded in a visuo-spatial working memory test and required participants to remember to respond to a cue-event. Everyday planning performance was assessed with proxy ratings. Although parents of the autism group rated their children’s everyday performance as significantly poorer than controls’ parents, no group differences were found in event-based prospective memory. Nevertheless, individual differences in laboratory-based and everyday performances were related. Clinical implications of these findings are discussed
Point massive particle in General Relativity
It is well known that the Schwarzschild solution describes the gravitational
field outside compact spherically symmetric mass distribution in General
Relativity. In particular, it describes the gravitational field outside a point
particle. Nevertheless, what is the exact solution of Einstein's equations with
-type source corresponding to a point particle is not known. In the
present paper, we prove that the Schwarzschild solution in isotropic
coordinates is the asymptotically flat static spherically symmetric solution of
Einstein's equations with -type energy-momentum tensor corresponding to
a point particle. Solution of Einstein's equations is understood in the
generalized sense after integration with a test function. Metric components are
locally integrable functions for which nonlinear Einstein's equations are
mathematically defined. The Schwarzschild solution in isotropic coordinates is
locally isometric to the Schwarzschild solution in Schwarzschild coordinates
but differs essentially globally. It is topologically trivial neglecting the
world line of a point particle. Gravity attraction at large distances is
replaced by repulsion at the particle neighbourhood.Comment: 15 pages, references added, 1 figur
Experimental delayed-choice entanglement swapping
Motivated by the question, which kind of physical interactions and processes
are needed for the production of quantum entanglement, Peres has put forward
the radical idea of delayed-choice entanglement swapping. There, entanglement
can be "produced a posteriori, after the entangled particles have been measured
and may no longer exist". In this work we report the first realization of
Peres' gedanken experiment. Using four photons, we can actively delay the
choice of measurement-implemented via a high-speed tunable bipartite state
analyzer and a quantum random number generator-on two of the photons into the
time-like future of the registration of the other two photons. This effectively
projects the two already registered photons onto one definite of two mutually
exclusive quantum states in which either the photons are entangled (quantum
correlations) or separable (classical correlations). This can also be viewed as
"quantum steering into the past"
Radiation from a D-dimensional collision of shock waves: first order perturbation theory
We study the spacetime obtained by superimposing two equal Aichelburg-Sexl
shock waves in D dimensions traveling, head-on, in opposite directions.
Considering the collision in a boosted frame, one shock becomes stronger than
the other, and a perturbative framework to compute the metric in the future of
the collision is setup. The geometry is given, in first order perturbation
theory, as an integral solution, in terms of initial data on the null surface
where the strong shock has support. We then extract the radiation emitted in
the collision by using a D-dimensional generalisation of the Landau-Lifschitz
pseudo-tensor and compute the percentage of the initial centre of mass energy
epsilon emitted as gravitational waves. In D=4 we find epsilon=25.0%, in
agreement with the result of D'Eath and Payne. As D increases, this percentage
increases monotonically, reaching 40.0% in D=10. Our result is always within
the bound obtained from apparent horizons by Penrose, in D=4, yielding 29.3%,
and Eardley and Giddings, in D> 4, which also increases monotonically with
dimension, reaching 41.2% in D=10. We also present the wave forms and provide a
physical interpretation for the observed peaks, in terms of the null generators
of the shocks.Comment: 27 pages, 11 figures; v2 some corrections, including D dependent
factor in epsilon; matches version accepted in JHE
Experimental GHZ Entanglement beyond Qubits
The Greenberger-Horne-Zeilinger (GHZ) argument provides an all-or-nothing
contradiction between quantum mechanics and local-realistic theories. In its
original formulation, GHZ investigated three and four particles entangled in
two dimensions only. Very recently, higher dimensional contradictions
especially in three dimensions and three particles have been discovered but it
has remained unclear how to produce such states. In this article we
experimentally show how to generate a three-dimensional GHZ state from
two-photon orbital-angular-momentum entanglement. The first suggestion for a
setup which generates three-dimensional GHZ entanglement from these entangled
pairs came from using the computer algorithm Melvin. The procedure employs
novel concepts significantly beyond the qubit case. Our experiment opens up the
possibility of a truly high-dimensional test of the GHZ-contradiction which,
interestingly, employs non-Hermitian operators.Comment: 6+6 pages, 8 figure
Observation of eight-photon entanglement
Using ultra-bright sources of pure-state entangled photons from parametric
down conversion, an eight-photon interferometer and post-selection detection,
we demonstrate the ability to experimentally manipulate eight individual
photons and report the creation of an eight-photon Schr\"odinger cat state with
an observed fidelity of .Comment: 6 pages, 4 figure
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