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 $\eta$ is larger than $\eta \geq d^{1/2} 2^{-0.0035d}$ where $d$ 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

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 $\sim175 {pb}^{-1}$ (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