439 research outputs found
Stabilization of dipole solitons in nonlocal nonlinear media
We address the stabilization of dipole solitons in nonlocal nonlinear
materials by two different approaches. First, we study the properties of such
solitons in thermal nonlinear media, where the refractive index landscapes
induced by laser beams strongly depend on the boundary conditions and on the
sample geometry. We show how the sample geometry impacts the stability of
higher-order solitons in thermal nonlinear media and reveal that dipole
solitons can be made dynami-cally stable in rectangular geometries in contrast
to their counterparts in thermal samples with square cross-section. Second, we
discuss the impact of the saturation of the nonlocal nonlinear response on the
properties of multipole solitons. We find that the saturable response also
stabi-lizes dipole solitons even in symmetric geometries, provided that the
input power exceeds a criti-cal value.Comment: 29 pages, 8 figures, to appear in Phys. Rev.
Equivalence between two-qubit entanglement and secure key distribution
We study the problem of secret key distillation from bipartite states in the
scenario where Alice and Bob can only perform measurements at the single-copy
level and classically process the obtained outcomes. Even with these
limitations, secret bits can be asymptotically distilled by the honest parties
from any two-qubit entangled state, under any individual attack. Our results
point out a complete equivalence between two-qubit entanglement and secure key
distribution: a key can be established through a one-qubit channel if and only
if it allows to distribute entanglement. These results can be generalized to
higher dimension for all those states that are one-copy distillable.Comment: 5 pages, REVTEX. Accepted version + added appendix. Proof of the main
result and discussion improved, conclusions unchange
Vortex soliton tori with multiple nested phase singularities in dissipative media
We show the existence of stable two- and three-dimensional vortex solitons
carrying multiple, spatially separated, single-charge topological dislocations
nested around a vortex-ring core. Such new nonlinear states are supported by
elliptical gain landscapes in focusing nonlinear media with two-photon
absorption. The separation between the phase dislocations is dictated mostly by
the geometry of gain landscape and it only slightly changes upon variation of
the gain or absorption strength.Comment: 17 pages, 5 figures, to appear in Physical Review
From Bell's Theorem to Secure Quantum Key Distribution
Any Quantum Key Distribution (QKD) protocol consists first of sequences of
measurements that produce some correlation between classical data. We show that
these correlation data must violate some Bell inequality in order to contain
distillable secrecy, if not they could be produced by quantum measurements
performed on a separable state of larger dimension. We introduce a new QKD
protocol and prove its security against any individual attack by an adversary
only limited by the no-signaling condition.Comment: 5 pages, 2 figures, REVTEX
Soliton topology versus discrete symmetry in optical lattices
We address the existence of vortex solitons supported by azimuthally
modulated lattices and reveal how the global lattice discrete symmetry has
fundamental implications on the possible topological charges of solitons. We
set a general ``charge rule'' using group-theory techniques, which holds for
all lattices belonging to a given symmetry group. Focusing in the case of
Bessel lattices allows us to derive also a overall stability rule for the
allowed vortex solitons.Comment: 4 pages, 3 figures. To appear in Phys. Rev. Let
Enhanced soliton interactions by inhomogeneous nonlocality and nonlinearity
We address the interactions between optical solitons in the system with
longitudinally varying nonlocality degree and nonlinearity strength. We
consider a physical model describing light propagation in nematic liquid
crystals featuring a strongly nonlocal nonlinear response. We reveal that the
variation of the nonlocality and nonlinearity along the propagation direction
can substantially enhance or weaken the interaction between out-of-phase
solitons. This phenomenon manifests itself as a slowdown or acceleration of the
soliton collision dynamics in one-dimensional geometries or of the soliton
spiraling rate in bulk media. Therefore, one finds that by engineering the
nonlocality and nonlinearity variation rate one can control the output soliton
location.Comment: 22 pages, 5 figures, to appear in Physical Review
A case study analysis of factors determining success or failure for participants in collaborative innovation projects in e-Government
In this paper we use the case study of a successful innovative e-government project, iSAC6+, to examine some of the key factors in the project's success with the aim of contributing to the general understanding of the challenges associated with managing e-government projects. We make observations, identify areas for deeper consideration and draw conclusions as to how lessons learned might be applied to other e-government projects. There has been considerable analysis into the success and failure of e-government projects. We analyse some of the literature to identify the unique features which might add additional challenge and risk to e-government projects and then focus on the case study, specifically on individual participants and stakeholders rather than on the project as a whole. The discussion looks at the vulnerability of e-gov projects resulting from one of their defining characteristics, their collaborative and multi-organisational nature. A collaborative project which meets its objectives will rightly be seen as a success, though this may not be the viewpoint of all participants, some of whom may have found that the hoped for benefits have not been realised. For these participants the project is at best a limited success, but for many, a failure. The high failure rate for e-government projects is researched, analysed and documented. One feature which is a consequence of the complex and unpredictable environment within which e-government initiatives take place is that they are inherently innovative in that their purpose is invariably aimed at establishing new IT enabled solutions to embedded and complex problems. Innovative projects operating in complex, unpredictable environments are at high risk of failure for some if not all participants. The aim of iSAC6+, was to implement of a semantic web based Citizens Advice Service (CAS) application in five pilots. Success was judged by the achievement of cost and quality benefits for all stakeholders, citizens and administrators (cost burden reduction), and by the impact on the pilot organisation and operation (service modification). The analysis will demonstrate how the development of a management and measurement framework based upon the strategic aims and objectives for each partner supported success. It will also show how absence or lack of clarity about aims and objectives adversely affected some partners. Finally the paper will make comments and suggestions based upon the lessons learned
Stable ring vortex solitons in Bessel optical lattices
Stable ring vortex solitons, featuring a bright-shape, appear to be very rare
in nature. However, here we show that they exist and can be made dynamically
stable in defocusing cubic nonlinear media with an imprinted Bessel optical
lattice. We find the families of vortex lattice solitons and reveal their
salient properties, including the conditions required for their stability. We
show that the higher the soliton topological charge, the deeper the lattice
modulation necessary for stabilization.Comment: 14 pages, 4 figures, submitted to Physical Review Letter
Topological light bullets supported by spatio-temporal gain
We reveal that the competition between diffraction, cubic nonlinearity,
two-photon absorption, and gain localized in both space and time results in
arrest of collapse, suppression of azimuthal modula-tion instabilities for
spatiotemporal wavepackets, and formation of stable three-dimensional light
bul-lets. We show that Gaussian spatiotemporal gain landscapes support bright,
fundamental light bullets, while gain landscapes featuring a ring-like spatial
and a Gaussian temporal shapes may support stable vortex bullets carrying
topological phase dislocations.Comment: 19 pages, 5 figures, to appear in Physical Review
Secrecy extraction from no-signalling correlations
Quantum cryptography shows that one can guarantee the secrecy of correlation
on the sole basis of the laws of physics, that is without limiting the
computational power of the eavesdropper. The usual security proofs suppose that
the authorized partners, Alice and Bob, have a perfect knowledge and control of
their quantum systems and devices; for instance, they must be sure that the
logical bits have been encoded in true qubits, and not in higher-dimensional
systems. In this paper, we present an approach that circumvents this strong
assumption. We define protocols, both for the case of bits and for generic
-dimensional outcomes, in which the security is guaranteed by the very
structure of the Alice-Bob correlations, under the no-signalling condition. The
idea is that, if the correlations cannot be produced by shared randomness, then
Eve has poor knowledge of Alice's and Bob's symbols. The present study assumes,
on the one hand that the eavesdropper Eve performs only individual attacks
(this is a limitation to be removed in further work), on the other hand that
Eve can distribute any correlation compatible with the no-signalling condition
(in this sense her power is greater than what quantum physics allows). Under
these assumptions, we prove that the protocols defined here allow extracting
secrecy from noisy correlations, when these correlations violate a Bell-type
inequality by a sufficiently large amount. The region, in which secrecy
extraction is possible, extends within the region of correlations achievable by
measurements on entangled quantum states.Comment: 23 pages, 4 figure
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