3,331 research outputs found
Inverse targeting -- an effective immunization strategy
We propose a new method to immunize populations or computer networks against
epidemics which is more efficient than any method considered before. The
novelty of our method resides in the way of determining the immunization
targets. First we identify those individuals or computers that contribute the
least to the disease spreading measured through their contribution to the size
of the largest connected cluster in the social or a computer network. The
immunization process follows the list of identified individuals or computers in
inverse order, immunizing first those which are most relevant for the epidemic
spreading. We have applied our immunization strategy to several model networks
and two real networks, the Internet and the collaboration network of high
energy physicists. We find that our new immunization strategy is in the case of
model networks up to 14%, and for real networks up to 33% more efficient than
immunizing dynamically the most connected nodes in a network. Our strategy is
also numerically efficient and can therefore be applied to large systems
Efficient algorithm to study interconnected networks
Interconnected networks have been shown to be much more vulnerable to random
and targeted failures than isolated ones, raising several interesting questions
regarding the identification and mitigation of their risk. The paradigm to
address these questions is the percolation model, where the resilience of the
system is quantified by the dependence of the size of the largest cluster on
the number of failures. Numerically, the major challenge is the identification
of this cluster and the calculation of its size. Here, we propose an efficient
algorithm to tackle this problem. We show that the algorithm scales as O(N log
N), where N is the number of nodes in the network, a significant improvement
compared to O(N^2) for a greedy algorithm, what permits studying much larger
networks. Our new strategy can be applied to any network topology and
distribution of interdependencies, as well as any sequence of failures.Comment: 5 pages, 6 figure
Achieving diffraction-limited resolution in soft-X-ray Fourier-transform holography
The spatial resolution of microscopic images acquired via X-ray Fourier-transform holography is limited by the source size of the reference wave and by the numerical aperture of the detector. We analyze the interplay between both influences and show how they are matched in practice. We further identify, how high spatial frequencies translate to imaging artifacts in holographic reconstructions where mainly the reference beam limits the spatial resolution. As a solution, three methods are introduced based on numerical post-processing of the reconstruction. The methods comprise apodization of the hologram, refocusing via wave propagation, and deconvolution using the transfer function of the imaging system. In particular for the latter two, we demonstrate that image details smaller than the source size of the reference beam can be recovered up to the diffraction limit of the hologram. Our findings motivate the intentional application of a large reference-wave source enhancing the image contrast in applications with low photon numbers such as single-shot experiments at free-electron lasers or imaging at laboratory sources.BMBF, 05K10KTB, Verbundprojekt: FSP 301 - FLASH: Nanoskopische Systeme. Teilprojekt 1.1: Universelle Experimentierkammer für Streuexperimente mit kohärenten Femtosekunden-Röntgenpulsen Multi Purpose Coherent Scattering Chamber for FLASH and XFEL 'MPscatt
Dynamics and decoherence in the central spin model using exact methods
The dynamics and decoherence of an electronic spin-1/2 qubit coupled to a
bath of nuclear spins via hyperfine interactions in a quantum dot is studied.
We show how exact results from the integrable solution can be used to
understand the dynamic behavior of the qubit. It is possible to predict the
main frequency contributions and their broadening for relatively general
initial states analytically, leading to an estimate of the corresponding decay
times. Furthermore, for a small bath polarization, a new low-frequency time
scale is observed.Comment: 4 pages, 2 figures. Published version. See also
http://www.physik.uni-kl.de/eggert/papers/index.htm
Coupling Human Mobility and Social Ties
Studies using massive, passively data collected from communication
technologies have revealed many ubiquitous aspects of social networks, helping
us understand and model social media, information diffusion, and organizational
dynamics. More recently, these data have come tagged with geographic
information, enabling studies of human mobility patterns and the science of
cities. We combine these two pursuits and uncover reproducible mobility
patterns amongst social contacts. First, we introduce measures of mobility
similarity and predictability and measure them for populations of users in
three large urban areas. We find individuals' visitations patterns are far more
similar to and predictable by social contacts than strangers and that these
measures are positively correlated with tie strength. Unsupervised clustering
of hourly variations in mobility similarity identifies three categories of
social ties and suggests geography is an important feature to contextualize
social relationships. We find that the composition of a user's ego network in
terms of the type of contacts they keep is correlated with mobility behavior.
Finally, we extend a popular mobility model to include movement choices based
on social contacts and compare it's ability to reproduce empirical measurements
with two additional models of mobility
Chameleon: A Hybrid Secure Computation Framework for Machine Learning Applications
We present Chameleon, a novel hybrid (mixed-protocol) framework for secure
function evaluation (SFE) which enables two parties to jointly compute a
function without disclosing their private inputs. Chameleon combines the best
aspects of generic SFE protocols with the ones that are based upon additive
secret sharing. In particular, the framework performs linear operations in the
ring using additively secret shared values and nonlinear
operations using Yao's Garbled Circuits or the Goldreich-Micali-Wigderson
protocol. Chameleon departs from the common assumption of additive or linear
secret sharing models where three or more parties need to communicate in the
online phase: the framework allows two parties with private inputs to
communicate in the online phase under the assumption of a third node generating
correlated randomness in an offline phase. Almost all of the heavy
cryptographic operations are precomputed in an offline phase which
substantially reduces the communication overhead. Chameleon is both scalable
and significantly more efficient than the ABY framework (NDSS'15) it is based
on. Our framework supports signed fixed-point numbers. In particular,
Chameleon's vector dot product of signed fixed-point numbers improves the
efficiency of mining and classification of encrypted data for algorithms based
upon heavy matrix multiplications. Our evaluation of Chameleon on a 5 layer
convolutional deep neural network shows 133x and 4.2x faster executions than
Microsoft CryptoNets (ICML'16) and MiniONN (CCS'17), respectively
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