107 research outputs found
On a recent proposal of faster than light quantum communication
In a recent paper, A.Y. Shiekh has discussed an experimental set-up which, in
his opinion, should make possible faster-than-light communication using the
collapse of the quantum wave function. Contrary to the many proposals which
have been presented in the past, he does not resort to an entangled state of
two systems but he works with a single particle in a superposition of two
states - corresponding to its propagation in opposite directions - one of which
goes through an appropriate interferometer. The possibility for an observer
near the interferometer to introduce or not, at his free will, a phase shifter
along one of the paths should allow to change instantaneously the probability
of finding the particle in the far-away region corresponding to the other state
of the superposition and, correspondingly, to change the intensity of a beam of
particles reaching a distant observer. In this paper we show a flaw in the
argument: once more, as it has been proved in full generality a long time ago,
the process of wave packet reduction cannot be used for superluminal
communication.Comment: 9 pages, LaTeX. Minor changes mad
Non locality, closing the detection loophole and communication complexity
It is shown that the detection loophole which arises when trying to rule out
local realistic theories as alternatives for quantum mechanics can be closed if
the detection efficiency is larger than
where is the dimension of the entangled system. Furthermore it is argued
that this exponential decrease of the detector efficiency required to close the
detection loophole is almost optimal. This argument is based on a close
connection that exists between closing the detection loophole and the amount of
classical communication required to simulate quantum correlation when the
detectors are perfect.Comment: 4 pages Latex, minor typos correcte
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TEST OF A LIQUID ARGON CHAMBER WITH 20-u m RMS RESOLUTION
A measurement of the spatial resolution of a liquid-argon filled chamber was performed with minimum ionizing particles. Two multi-strip chambers with 20-{micro}m strip spacing operating in the ionization mode were used in the experiment. They perform in accordance with a simple model based on electron diffusion. An estimate of the amount of electron diffusion in liquid argon is given and the time jitter distribution has a FWHM of 200 ns. Under best conditions, the spatial resolution is better than 20 {micro}m rms with an efficiency of nearly 100%
A Zoology of Bell inequalities resistant to detector inefficiency
We derive both numerically and analytically Bell inequalities and quantum
measurements that present enhanced resistance to detector inefficiency. In
particular we describe several Bell inequalities which appear to be optimal
with respect to inefficient detectors for small dimensionality d=2,3,4 and 2 or
more measurement settings at each side. We also generalize the family of Bell
inequalities described in Collins et all (Phys. Rev. Lett. 88, 040404) to take
into account the inefficiency of detectors. In addition we consider the
possibility for pairs of entangled particles to be produced with probability
less than one. We show that when the pair production probability is small, one
must in general use different Bell inequalities than when the pair production
probability is high.Comment: 12 pages, revtex. Appendix completed, minor revision
Limits on Production of Magnetic Monopoles Utilizing Samples from the DO and CDF Detectors at the Tevatron
We present 90% confidence level limits on magnetic monopole production at the
Fermilab Tevatron from three sets of samples obtained from the D0 and CDF
detectors each exposed to a proton-antiproton luminosity of
(experiment E-882). Limits are obtained for the production cross-sections and
masses for low-mass accelerator-produced pointlike Dirac monopoles trapped and
bound in material surrounding the D0 and CDF collision regions. In the absence
of a complete quantum field theory of magnetic charge, we estimate these limits
on the basis of a Drell-Yan model. These results (for magnetic charge values of
1, 2, 3, and 6 times the minimum Dirac charge) extend and improve previously
published bounds.Comment: 18 pages, 17 figures, REVTeX
Violation of local realism vs detection efficiency
We put bounds on the minimum detection efficiency necessary to violate local
realism in Bell experiments. These bounds depends of simple parameters like the
number of measurement settings or the dimensionality of the entangled quantum
state. We derive them by constructing explicit local-hidden variable models
which reproduce the quantum correlations for sufficiently small detectors
efficiency.Comment: 6 pages, revtex. Modifications in the discussion for many parties in
section 3, small erros and typos corrected, conclusions unchange
About entanglement properties of kaons and tests of hidden variables models
In this letter we discuss entanglement properties of neutral kaons systems
and their use for testing local realism. In particular we show that, as
previous proposals, also a scheme recently suggested for performing a test of
hidden variable theories against standard quantum mechanics cannot be
conclusive
The Conway-Kochen argument and relativistic GRW models
In a recent paper, Conway and Kochen proposed what is now known as the "Free
Will theorem" which, among other things, should prove the impossibility of
combining GRW models with special relativity, i.e., of formulating
relativistically invariant models of spontaneous wavefunction collapse. Since
their argument basically amounts to a non-locality proof for any theory aiming
at reproducing quantum correlations, and since it was clear since very a long
time that any relativistic collapse model must be non-local in some way, we
discuss why the theorem of Conway and Kochen does not affect the program of
formulating relativistic GRW models.Comment: 16 pages, RevTe
Quantum mechanics, Furry's hypothesis and a measure of decoherence in the K^0 \bar{K}^0 system
We consider strangeness correlations of the EPR type in K^0 \bar{K}^0 pairs
created in a J^{PC} = 1^{--} state as a function of time under the hypothesis
that spontaneous decoherence takes place. We parameterize the degree of
decoherence by a factor (1-\zeta) which multiplies the quantum-mechanical
interference terms occurring in the amplitudes for like and unlike strangeness
events and discuss the dependence of this procedure on the basis chosen in the
K^0--\bar{K}^0 space to which the interference terms correspond. Consequently,
all statements about the ``decoherence parameter'' \zeta inferred from
experimental data are basis-dependent as well. We illustrate this point by
estimating the value of \zeta for the two bases {K_L, K_S} and {K^0, \bar{K}^0}
with the help of recent data of the CPLEAR experiment.Comment: 12 pages, 1 figure, revte
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