110,832 research outputs found
Entanglement swapping of noisy states: A kind of superadditivity in nonclassicality
We address the question as to whether an entangled state that satisfies local
realism will give a violation of the same, after entanglement swapping in a
suitable scenario. We consider such possibility as a kind of superadditivity in
nonclassicality. Importantly, it will indicate that checking for violation of
local realism, in the state obtained after entanglement swapping, can be a
method for detecting entanglement in the input state of the swapping procedure.
We investigate various entanglement swapping schemes, which involve mixed
initial states. The strength of violation of local realism by the state
obtained after entanglement swapping, is compared with the one for the input
states. We obtain a kind of superadditivity of violation of local realism for
Werner states, consequent upon entanglement swapping involving
Greenberger-Horne-Zeilinger state measurements. We also discuss whether
entanglement swapping of specific states may be used in quantum repeaters with
a substantially reduced need to perform the entanglement distillation step.Comment: 11 pages, 6 figures, RevTeX4; v2: new discussions added, published
versio
Optimal Gaussian Entanglement Swapping
We consider entanglement swapping with general mixed two-mode Gaussian states
and calculate the optimal gains for a broad class of such states including
those states most relevant in communication scenarios. We show that for this
class of states, entanglement swapping adds no additional mixedness, that is
the ensemble average output state has the same purity as the input states. This
implies that, by using intermediate entanglement swapping steps, it is, in
principle, possible to distribute entangled two-mode Gaussian states of higher
purity as compared to direct transmission. We then apply the general results on
optimal Gaussian swapping to the problem of quantum communication over a lossy
fiber and demonstrate that, contrary to negative conclusions in the literature,
swapping-based schemes in fact often perform better than direct transmission
for high input squeezing. However, an effective transmission analysis reveals
that the hope for improved performance based on optimal Gaussian entanglement
swapping is spurious since the swapping does not lead to an enhancement of the
effective transmission. This implies that the same or better results can always
be obtained using direct transmission in combination with, in general, less
squeezing.Comment: 10 pages, 2 figures, minor corrections in version 2 with one
reference added (ref.9
On pole-swapping algorithms for the eigenvalue problem
Pole-swapping algorithms, which are generalizations of the QZ algorithm for
the generalized eigenvalue problem, are studied. A new modular (and therefore
more flexible) convergence theory that applies to all pole-swapping algorithms
is developed. A key component of all such algorithms is a procedure that swaps
two adjacent eigenvalues in a triangular pencil. An improved swapping routine
is developed, and its superiority over existing methods is demonstrated by a
backward error analysis and numerical tests. The modularity of the new
convergence theory and the generality of the pole-swapping approach shed new
light on bi-directional chasing algorithms, optimally packed shifts, and bulge
pencils, and allow the design of novel algorithms
Unconditional teleportation of continuous-variable entanglement
We give a protocol and criteria for demonstrating unconditional teleportation
of continuous-variable entanglement (i.e., entanglement swapping). The initial
entangled states are produced with squeezed light and linear optics. We show
that any nonzero entanglement (any nonzero squeezing) in both of two
entanglement sources is sufficient for entanglement swapping to occur. In fact,
realization of continuous-variable entanglement swapping is possible using only
{\it two} single-mode squeezed states.Comment: 4 pages, 2 figures, published version, title change
Complexity of Token Swapping and its Variants
In the Token Swapping problem we are given a graph with a token placed on
each vertex. Each token has exactly one destination vertex, and we try to move
all the tokens to their destinations, using the minimum number of swaps, i.e.,
operations of exchanging the tokens on two adjacent vertices. As the main
result of this paper, we show that Token Swapping is -hard parameterized
by the length of a shortest sequence of swaps. In fact, we prove that, for
any computable function , it cannot be solved in time where is the number of vertices of the input graph, unless the ETH
fails. This lower bound almost matches the trivial -time algorithm.
We also consider two generalizations of the Token Swapping, namely Colored
Token Swapping (where the tokens have different colors and tokens of the same
color are indistinguishable), and Subset Token Swapping (where each token has a
set of possible destinations). To complement the hardness result, we prove that
even the most general variant, Subset Token Swapping, is FPT in nowhere-dense
graph classes.
Finally, we consider the complexities of all three problems in very
restricted classes of graphs: graphs of bounded treewidth and diameter, stars,
cliques, and paths, trying to identify the borderlines between polynomial and
NP-hard cases.Comment: 23 pages, 7 Figure
On Face Segmentation, Face Swapping, and Face Perception
We show that even when face images are unconstrained and arbitrarily paired,
face swapping between them is actually quite simple. To this end, we make the
following contributions. (a) Instead of tailoring systems for face
segmentation, as others previously proposed, we show that a standard fully
convolutional network (FCN) can achieve remarkably fast and accurate
segmentations, provided that it is trained on a rich enough example set. For
this purpose, we describe novel data collection and generation routines which
provide challenging segmented face examples. (b) We use our segmentations to
enable robust face swapping under unprecedented conditions. (c) Unlike previous
work, our swapping is robust enough to allow for extensive quantitative tests.
To this end, we use the Labeled Faces in the Wild (LFW) benchmark and measure
the effect of intra- and inter-subject face swapping on recognition. We show
that our intra-subject swapped faces remain as recognizable as their sources,
testifying to the effectiveness of our method. In line with well known
perceptual studies, we show that better face swapping produces less
recognizable inter-subject results. This is the first time this effect was
quantitatively demonstrated for machine vision systems
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