115 research outputs found
Information theoretic approach to single-particle and two-particle interference in multi-path interferometers
We propose entropic measures for the strength of single-particle and
two-particle interference in interferometric experiments where each particle of
a pair traverses a multi-path interferometer. Optimal single-particle
interference excludes any two-particle interference, and vice versa. We report
an inequality that states the compromises allowed by quantum mechanics in
intermediate situations, and identify a class of two-particle states for which
the upper bound is reached. Our approach is applicable to symmetric two-partite
systems of any finite dimension.Comment: RevTex 4, 4 pages, 2 figure
Better detection of Multipartite Bound Entanglement with Three-Setting Bell Inequalities
It was shown in Phys. Rev. Lett., 87, 230402 (2001) that N (N >= 4) qubits
described by a certain one parameter family F of bound entangled states violate
Mermin-Klyshko inequality for N >= 8. In this paper we prove that the states
from the family F violate Bell inequalities derived in Phys. Rev. A, 56, R1682
(1997), in which each observer measures three non-commuting sets of orthogonal
projectors, for N >=7. We also derive a simple one parameter family of
entanglement witnesses that detect entanglement for all the states belonging to
F. It is possible that these new entanglement witnesses could be generated by
some Bell inequalities.Comment: Revtex4, 1 figur
Functional Bell inequalities can serve as a stronger entanglement witness
We consider a Bell inequality for a continuous range of settings of the
apparatus at each site. This "functional" Bell inequality gives a better range
of violation for generalized GHZ states. Also a family of N-qubit bound
entangled states violate this inequality for N>5.Comment: 4 pages, REVTeX
N-particle N-level singlet states: Some properties and applications
Three apparently unrelated problems which have no solution using classical
tools are described: the "N-strangers," "secret sharing," and "liar detection"
problems. A solution for each of them is proposed. Common to all three
solutions is the use of quantum states of total spin zero of N spin-(N-1)/2
particles.Comment: REVTeX4, 4 pages, 1 figur
Filtering of the absolute value of photon-number difference for two-mode macroscopic quantum superpositions
We discuss a device capable of filtering out two-mode states of light with
mode populations differing by more than a certain threshold, while not
revealing which mode is more populated. It would allow engineering of
macroscopic quantum states of light in a way which is preserving specific
superpositions. As a result, it would enhance optical phase estimation with
these states as well as distinguishability of "macroscopic" qubits. We propose
an optical scheme, which is a relatively simple, albeit non-ideal, operational
implementation of such a filter. It uses tapping of the original polarization
two-mode field, with a polarization neutral beam splitter of low reflectivity.
Next, the reflected beams are suitably interfered on a polarizing beam
splitter. It is oriented such that it selects unbiased polarization modes with
respect to the original ones. The more an incoming two-mode Fock state is
unequally populated, the more the polarizing beam splitter output modes are
equally populated. This effect is especially pronounced for highly populated
states. Additionally, for such states we expect strong population correlations
between the original fields and the tapped one. Thus, after a photon-number
measurement of the polarizing beam splitter outputs, a feed-forward loop can be
used to let through a shutter the field, which was transmitted by the tapping
beam splitter. This happens only if the counts at the outputs are roughly
equal. In such a case, the transmitted field differs strongly in occupation
number of the two modes, while information on which mode is more populated is
non-existent (a necessary condition for preserving superpositions).Comment: 11 pages, 12 figure
Four Photon Entanglement from Down Conversion
Double-pair emission from type-II parametric down conversion results in a
highly entangled 4-photon state. Due to interference, which is similar to
bunching from thermal emission, this state is not simply a product of two
pairs. The observation of this state can be achieved by splitting the two
emission modes at beam splitters and subsequent detection of a photon in each
output. Here we describe the features of this state and give a Bell theorem for
a 4-photon test of local realistic hidden variable theories.Comment: 5 pages, 1 figure, submitted to PR
A feasible "Kochen-Specker" experiment with single particles
We present a simple experimental scheme which can be used to demonstrate an
all-or-nothing type contradiction between non-contextual hidden variables and
quantum mechanics. The scheme, which is inspired by recent ideas by Cabello and
Garcia-Alcaine, shows that even for a single particle, path and spin
information cannot be predetermined in a non-contextual way.Comment: 4 pages, 3 figure
Violations of local realism with quNits up to N=16
Predictions for systems in entangled states cannot be described in local
realistic terms. However, after admixing some noise such a description is
possible. We show that for two quNits (quantum systems described by N
dimensional Hilbert spaces) in a maximally entangled state the minimal
admixture of noise increases monotonically with N. The results are a direct
extension of those of Kaszlikowski et. al., Phys. Rev. Lett. {\bf 85}, 4418
(2000), where results for were presented. The extension up to N=16 is
possible when one defines for each N a specially chosen set of observables. We
also present results concerning the critical detectors efficiency beyond which
a valid test of local realism for entangled quNits is possible.Comment: 5 pages, 3 ps picture
Minimum detection efficiency for a loophole-free atom-photon Bell experiment
In Bell experiments, one problem is to achieve high enough photodetection to
ensure that there is no possibility of describing the results via a local
hidden-variable model. Using the Clauser-Horne inequality and a two-photon
non-maximally entangled state, a photodetection efficiency higher than 0.67 is
necessary. Here we discuss atom-photon Bell experiments. We show that, assuming
perfect detection efficiency of the atom, it is possible to perform a
loophole-free atom-photon Bell experiment whenever the photodetection
efficiency exceeds 0.50.Comment: REVTeX4, 4 pages, 1 figur
A note on bound entanglement and local realism
We show using a numerical approach that gives necessary and sufficient
conditions for the existence of local realism, that the bound entangled state
presented in Bennett et. al. Phys. Rev. Lett. 82, 5385 (1999) admits a local
and realistic description. We also find the lowest possible amount of some
appropriate entangled state that must be ad-mixed to the bound entangled state
so that the resulting density operator has no local and realistic description
and as such can be useful in quantum communication and quantum computation.Comment: 5 page
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