558 research outputs found
Identical particles and entanglement
We review two general criteria for deciding whether a pure bipartite quantum
state describing a system of two identical particles is entangled or not. The
first one considers the possibility of attributing a complete set of objective
properties to each particle belonging to the composed system, while the second
is based both on the consideration of the Slater-Schmidt number of the
fermionic and bosonic analog of the Schmidt decomposition and on the evaluation
of the von Neumann entropy of the one-particle reduced statistical operators.Comment: 8 pages; Latex; Talk delivered at the International Conference on
Quantum Optics 2004, Minsk, Belaru
Dynamical Reduction Models with General Gaussian Noises
We consider the effect of replacing in stochastic differential equations
leading to the dynamical collapse of the statevector, white noise stochastic
processes with non white ones. We prove that such a modification can be
consistently performed without altering the most interesting features of the
previous models. One of the reasons to discuss this matter derives from the
desire of being allowed to deal with physical stochastic fields, such as the
gravitational one, which cannot give rise to white noises. From our point of
view the most relevant motivation for the approach we propose here derives from
the fact that in relativistic models the occurrence of white noises is the main
responsible for the appearance of untractable divergences. Therefore, one can
hope that resorting to non white noises one can overcome such a difficulty. We
investigate stochastic equations with non white noises, we discuss their
reduction properties and their physical implications. Our analysis has a
precise interest not only for the above mentioned subject but also for the
general study of dissipative systems and decoherence.Comment: 22 pages, Late
Selective cloning of Gaussian states by linear optics
We investigate the performances of a selective cloning machine based on
linear optical elements and Gaussian measurements, which allows to clone at
will one of the two incoming input states. This machine is a complete
generalization of a 1 to 2 cloning scheme demonstrated by U. L. Andersen et al.
[Phys. Rev. Lett. vol. 94, 240503 (2005)]. The input-output fidelity is studied
for generic Gaussian input state and the effect of non-unit quantum efficiency
is also taken into account. We show that if the states to be cloned are
squeezed states with known squeezing parameter, then the fidelity can be
enhanced using a third suitable squeezed state during the final stage of the
cloning process. A binary communication protocol based on the selective cloning
machne is also discussed.Comment: 6 pages, 6 figure
A critical analysis of Popper's experiment
An experiment which could decide against the Copenhagen interpretation of
quantum mechanics has been proposed by K. Popper and, subsequently, it has been
criticized by M.J. Collett and R. Loudon. Here we show that both the above
mentioned arguments are not correct because they are based on a misuse of basic
quantum rules.Comment: 12 pages, 3 figures, RevTex; to be published on PR
Conservation laws, uncertainty relations, and quantum limits of measurements
The uncertainty relation between the noise operator and the conserved
quantity leads to a bound for the accuracy of general measurements. The bound
extends the assertion by Wigner, Araki, and Yanase that conservation laws limit
the accuracy of ``repeatable'', or ``nondisturbing'', measurements to general
measurements, and improves the one previously obtained by Yanase for spin
measurements. The bound also sets an obstacle to making a small quantum
computer.Comment: 4 pages, RevTex, to appear in PR
Active and Passive Quantum Erasers for Neutral Kaons
Quantum marking and quantum erasure are discussed for the neutral kaon
system. Contrary to other two-level systems, strangeness and lifetime of a
neutral kaon state can be alternatively measured via an "active" or a "passive"
procedure. This offers new quantum erasure possibilities. In particular, the
operation of a quantum eraser in the "delayed choice" mode is clearly
illustrated.Comment: 17 pages, LaTeX, 1 figure, references added, accepted by Phys. Rev.
Remarks on a Proposed Super-Kamiokande Test for Quantum Gravity Induced Decoherence Effects
Lisi, Marrone, and Montanino have recently proposed a test for quantum
gravity induced decoherence effects in neutrino oscillations observed at
Super-Kamiokande. We comment here that their equations have the same
qualitative form as the energy conserving objective state vector reduction
equations discussed by a number of authors. However, using the Planckian
parameter value proposed to explain state vector reduction leads to a neutrino
oscillation effect many orders of magnitude smaller than would be detectable at
Super-Kamiokande. Similar estimates hold for the Ghirardi, Rimini, and Weber
spontaneous localization approach to state vector reduction, and our remarks
are relevant as well to proposed meson and meson tests of gravity
induced decoherence.Comment: 10 pages, plain Tex, no figure
A test of Local Realism with entangled kaon pairs and without inequalities
We propose the use of entangled pairs of neutral kaons, considered as a
promising tool to close the well known loopholes affecting generic Bell's
inequality tests, in a specific Hardy-type experiment. Hardy's contradiction
without inequalities between Local Realism and Quantum Mechanics can be
translated into a feasible experiment by requiring ideal detection efficiencies
for only one of the observables to be alternatively measured. Neutral kaons are
near to fulfil this requirement and therefore to close the efficiency loophole.Comment: 4 RevTeX page
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