11,150 research outputs found
Sensitivity analysis and parameter estimation for distributed hydrological modeling: potential of variational methods
Variational methods are widely used for the analysis and control of computationally intensive spatially distributed systems. In particular, the adjoint state method enables a very efficient calculation of the derivatives of an objective function (response function to be analysed or cost function to be optimised) with respect to model inputs. In this contribution, it is shown that the potential of variational methods for distributed catchment scale hydrology should be considered. A distributed flash flood model, coupling kinematic wave overland flow and Green Ampt infiltration, is applied to a small catchment of the Thoré basin and used as a relatively simple (synthetic observations) but didactic application case. It is shown that forward and adjoint sensitivity analysis provide a local but extensive insight on the relation between the assigned model parameters and the simulated hydrological response. Spatially distributed parameter sensitivities can be obtained for a very modest calculation effort (~6 times the computing time of a single model run) and the singular value decomposition (SVD) of the Jacobian matrix provides an interesting perspective for the analysis of the rainfall-runoff relation. For the estimation of model parameters, adjoint-based derivatives were found exceedingly efficient in driving a bound-constrained quasi-Newton algorithm. The reference parameter set is retrieved independently from the optimization initial condition when the very common dimension reduction strategy (i.e. scalar multipliers) is adopted. Furthermore, the sensitivity analysis results suggest that most of the variability in this high-dimensional parameter space can be captured with a few orthogonal directions. A parametrization based on the SVD leading singular vectors was found very promising but should be combined with another regularization strategy in order to prevent overfitting
Experimental polarization encoded quantum key distribution over optical fibres with real-time continuous birefringence compensation
In this paper we demonstrate an active polarization drift compensation scheme
for optical fibres employed in a quantum key distribution experiment with
polarization encoded qubits. The quantum signals are wavelength multiplexed in
one fibre along with two classical optical side channels that provide the
control information for the polarization compensation scheme. This set-up
allows us to continuously track any polarization change without the need to
interrupt the key exchange. The results obtained show that fast polarization
rotations of the order of 40*pi rad/s are effectively compensated for. We
demonstrate that our set-up allows continuous quantum key distribution even in
a fibre stressed by random polarization fluctuations. Our results pave the way
for Bell-state measurements using only linear optics with parties separated by
long-distance optical fibres
Topological confinement in graphene bilayer quantum rings
We demonstrate the existence of localized electron and hole states in a
ring-shaped potential kink in biased bilayer graphene. Within the continuum
description, we show that for sharp potential steps the Dirac equation
describing carrier states close to the K (or K') point of the first Brillouin
zone can be solved analytically for a circular kink/anti-kink dot. The
solutions exhibit interfacial states which exhibit Aharonov-Bohm oscillations
as functions of the height of the potential step and/or the radius of the ring
Tunable entanglement distillation of spatially correlated down-converted photons
We report on a new technique for entanglement distillation of the bipartite
continuous variable state of spatially correlated photons generated in the
spontaneous parametric down-conversion process (SPDC), where tunable
non-Gaussian operations are implemented and the post-processed entanglement is
certified in real-time using a single-photon sensitive electron multiplying CCD
(EMCCD) camera. The local operations are performed using non-Gaussian filters
modulated into a programmable spatial light modulator and, by using the EMCCD
camera for actively recording the probability distributions of the
twin-photons, one has fine control of the Schmidt number of the distilled
state. We show that even simple non-Gaussian filters can be finely tuned to a
~67% net gain of the initial entanglement generated in the SPDC process.Comment: 12 pages, 6 figure
Grand unification through gravitational effects
We systematically study the unification of gauge couplings in the presence of
(one or more) effective dimension-5 operators cHGG/4MPl, induced into the grand
unified theory by gravitational interactions at the Planck scale MPl. These
operators alter the usual condition for gauge coupling unification, which can,
depending on the Higgs content H and vacuum expectation value, result in
unification at scales MX significantly different than naively expected. We find
non-supersymmetric models of SU(5) and SO(10) unification, with natural Wilson
coefficients c, that easily satisfy the constraints from proton decay.
Furthermore, gauge coupling unification at scales as high as the Planck scale
seems feasible, possibly hinting at simultaneous unification of gauge and
gravitational interactions. In the Appendix we work out the group theoretical
aspects of this scenario for SU(5) and SO(10) unified groups in detail; this
material is also relevant in the analysis of non-universal gaugino masses
obtained from supergravity.Comment: 27 pages, 5 figures, 8 tables, 1 appendix, revtex; v2: introduction
and conclusion expanded, references added, minor changes, version published
in PR
Long-distance distribution of genuine energy-time entanglement
Any practical realization of entanglement-based quantum communication must be
intrinsically secure and able to span long distances avoiding the need of a
straight line between the communicating parties. The violation of Bell's
inequality offers a method for the certification of quantum links without
knowing the inner workings of the devices. Energy-time entanglement quantum
communication satisfies all these requirements. However, currently there is a
fundamental obstacle with the standard configuration adopted: an intrinsic
geometrical loophole that can be exploited to break the security of the
communication, in addition to other loopholes. Here we show the first
experimental Bell violation with energy-time entanglement distributed over 1 km
of optical fibers that is free of this geometrical loophole. This is achieved
by adopting a new experimental design, and by using an actively stabilized
fiber-based long interferometer. Our results represent an important step
towards long-distance secure quantum communication in optical fibers.Comment: 6 pages, 3 figures. Matches published versio
Sonic Hedgehog Signaling and Development of the Dentition
Sonic hedgehog (Shh) is an essential signaling peptide required for normal embryonic development. It represents a highly-conserved marker of odontogenesis amongst the toothed vertebrates. Signal transduction is involved in early specification of the tooth-forming epithelium in the oral cavity, and, ultimately, in defining tooth number within the established dentition. Shh also promotes the morphogenetic movement of epithelial cells in the early tooth bud, and influences cell cycle regulation, morphogenesis, and differentiation in the tooth germ. More recently, Shh has been identified as a stem cell regulator in the continuously erupting incisors of mice. Here, we review contemporary data relating to the role of Shh in odontogenesis, focusing on tooth development in mammals and cartilaginous fishes. We also describe the multiple actions of this signaling protein at the cellular level
Anomalous transport in Charney-Hasegawa-Mima flows
Transport properties of particles evolving in a system governed by the
Charney-Hasegawa-Mima equation are investigated. Transport is found to be
anomalous with a non linear evolution of the second moments with time. The
origin of this anomaly is traced back to the presence of chaotic jets within
the flow. All characteristic transport exponents have a similar value around
, which is also the one found for simple point vortex flows in the
literature, indicating some kind of universality. Moreover the law
linking the trapping time exponent within jets to the transport
exponent is confirmed and an accumulation towards zero of the spectrum of
finite time Lyapunov exponent is observed. The localization of a jet is
performed, and its structure is analyzed. It is clearly shown that despite a
regular coarse grained picture of the jet, motion within the jet appears as
chaotic but chaos is bounded on successive small scales.Comment: revised versio
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