130 research outputs found
Conserving Local Magnetic Helicity in Numerical Simulations
Magnetic helicity is robustly conserved in systems with large magnetic
Reynolds numbers, including most systems of astrophysical interest. This plays
a major role in suppressing the kinematic large scale dynamo and driving the
large scale dynamo through the magnetic helicity flux. Numerical simulations of
astrophysical systems typically lack sufficient resolution to enforce global
magnetic helicity over several dynamical times. Errors in the internal
distribution of magnetic helicity are equally serious and possibly larger. Here
we propose an algorithm for enforcing strict local conservation of magnetic
helicity in the Coulomb gauge in numerical simulations.Comment: Comments are welcom
Classical and quantum partition bound and detector inefficiency
We study randomized and quantum efficiency lower bounds in communication
complexity. These arise from the study of zero-communication protocols in which
players are allowed to abort. Our scenario is inspired by the physics setup of
Bell experiments, where two players share a predefined entangled state but are
not allowed to communicate. Each is given a measurement as input, which they
perform on their share of the system. The outcomes of the measurements should
follow a distribution predicted by quantum mechanics; however, in practice, the
detectors may fail to produce an output in some of the runs. The efficiency of
the experiment is the probability that the experiment succeeds (neither of the
detectors fails).
When the players share a quantum state, this gives rise to a new bound on
quantum communication complexity (eff*) that subsumes the factorization norm.
When players share randomness instead of a quantum state, the efficiency bound
(eff), coincides with the partition bound of Jain and Klauck. This is one of
the strongest lower bounds known for randomized communication complexity, which
subsumes all the known combinatorial and algebraic methods including the
rectangle (corruption) bound, the factorization norm, and discrepancy.
The lower bound is formulated as a convex optimization problem. In practice,
the dual form is more feasible to use, and we show that it amounts to
constructing an explicit Bell inequality (for eff) or Tsirelson inequality (for
eff*). We give an example of a quantum distribution where the violation can be
exponentially bigger than the previously studied class of normalized Bell
inequalities.
For one-way communication, we show that the quantum one-way partition bound
is tight for classical communication with shared entanglement up to arbitrarily
small error.Comment: 21 pages, extended versio
Inter-arrival times of message propagation on directed networks
One of the challenges in fighting cybercrime is to understand the dynamics of
message propagation on botnets, networks of infected computers used to send
viruses, unsolicited commercial emails (SPAM) or denial of service attacks. We
map this problem to the propagation of multiple random walkers on directed
networks and we evaluate the inter-arrival time distribution between successive
walkers arriving at a target. We show that the temporal organization of this
process, which models information propagation on unstructured peer to peer
networks, has the same features as SPAM arriving to a single user. We study the
behavior of the message inter-arrival time distribution on three different
network topologies using two different rules for sending messages. In all
networks the propagation is not a pure Poisson process. It shows universal
features on Poissonian networks and a more complex behavior on scale free
networks. Results open the possibility to indirectly learn about the process of
sending messages on networks with unknown topologies, by studying inter-arrival
times at any node of the network.Comment: 9 pages, 12 figure
Discovery of a proto-white dwarf with a massive unseen companion
We report the discovery of SDSS~J022932.28+713002.7, a nascent extremely
low-mass (ELM) white dwarf (WD) orbiting a massive ( at 2
confidence) companion with a period of 36 hours. We use a combination of
spectroscopy, including data from the ongoing SDSS-V survey, and photometry to
measure the stellar parameters for the primary pre-ELM white dwarf. The
lightcurve of the primary WD exhibits ellipsoidal variation, which we combine
with radial velocity data and binary simulations to estimate the
mass of the invisible companion. We find that the primary WD has mass =
M and the unseen secondary has mass =
M. The mass of the companion suggests that it is
most likely a near-Chandrasekhar mass white dwarf or a neutron star. It is
likely that the system recently went through a Roche lobe overflow from the
visible primary onto the invisible secondary. The dynamical configuration of
the binary is consistent with the theoretical evolutionary tracks for such
objects, and the primary is currently in its contraction phase. The measured
orbital period puts this system on a stable evolutionary path which, within a
few Gyrs, will lead to a contracted ELM white dwarf orbiting a massive compact
companion.Comment: 21 Pages, 8 Figure
Non-locality and Communication Complexity
Quantum information processing is the emerging field that defines and
realizes computing devices that make use of quantum mechanical principles, like
the superposition principle, entanglement, and interference. In this review we
study the information counterpart of computing. The abstract form of the
distributed computing setting is called communication complexity. It studies
the amount of information, in terms of bits or in our case qubits, that two
spatially separated computing devices need to exchange in order to perform some
computational task. Surprisingly, quantum mechanics can be used to obtain
dramatic advantages for such tasks.
We review the area of quantum communication complexity, and show how it
connects the foundational physics questions regarding non-locality with those
of communication complexity studied in theoretical computer science. The first
examples exhibiting the advantage of the use of qubits in distributed
information-processing tasks were based on non-locality tests. However, by now
the field has produced strong and interesting quantum protocols and algorithms
of its own that demonstrate that entanglement, although it cannot be used to
replace communication, can be used to reduce the communication exponentially.
In turn, these new advances yield a new outlook on the foundations of physics,
and could even yield new proposals for experiments that test the foundations of
physics.Comment: Survey paper, 63 pages LaTeX. A reformatted version will appear in
Reviews of Modern Physic
Sublinear Algorithms for Approximating String Compressibility
We raise the question of approximating the compressibility of a string with respect to a fixed compression scheme, in sublinear time. We study this question in detail for two popular lossless compression schemes: run-length encoding (RLE) and a variant of Lempel-Ziv (LZ77), and present sublinear algorithms for approximating compressibility with respect to both schemes. We also give several lower bounds that show that our algorithms for both schemes cannot be improved significantly.
Our investigation of LZ77 yields results whose interest goes beyond the initial questions we set out to study. In particular, we prove combinatorial structural lemmas that relate the compressibility of a string with respect to LZ77 to the number of distinct short substrings contained in it (its ℓth subword complexity , for small ℓ). In addition, we show that approximating the compressibility with respect to LZ77 is related to approximating the support size of a distribution.National Science Foundation (U.S.) (Award CCF-1065125)National Science Foundation (U.S.) (Award CCF-0728645)Marie Curie International Reintegration Grant PIRG03-GA-2008-231077Israel Science Foundation (Grant 1147/09)Israel Science Foundation (Grant 1675/09
Politische Dimensionen von Militärübungen und Manövern – ein Projektbericht
Die virtuellen Kriege und Operationen, die in Militärübungen gespielt und geprobt werden, können entweder der Abschreckung dienen oder aber Angriffe vorbereiten bzw. zur Maskierung tatsächlicher Angriffe dienen. Für Beobachter ist es vielfach nicht offensichtlich, um welche Art von Militärübung es sich handelt. Die Ergebnisse eines vierjährigen internationalen Projektes zu politischen Dimensionen von Militärübungen richten das Schlaglicht insbesondere auf Missverständnisse und deren ungewollte politische Auswirkungen, die im Extremfall unbeabsichtigt zum Krieg führen können
Genome Erosion in a Nitrogen-Fixing Vertically Transmitted Endosymbiotic Multicellular Cyanobacterium
Background: An ancient cyanobacterial incorporation into a eukaryotic organism led to the evolution of plastids (chloroplasts) and subsequently to the origin of the plant kingdom. The underlying mechanism and the identities of the partners in this monophyletic event remain elusive. Methodology/Principal Findings: To shed light on this evolutionary process, we sequenced the genome of a cyanobacterium residing extracellularly in an endosymbiosis with a plant, the water-fern Azolla filiculoides Lam. This symbiosis was selected as it has characters which make it unique among extant cyanobacterial plant symbioses: the cyanobacterium lacks autonomous growth and is vertically transmitted between plant generations. Our results reveal features of evolutionary significance. The genome is in an eroding state, evidenced by a large proportion of pseudogenes (31.2%) and a high frequency of transposable elements (,600) scattered throughout the genome. Pseudogenization is found in genes such as the replication initiator dnaA and DNA repair genes, considered essential to free-living cyanobacteria. For some functional categories of genes pseudogenes are more prevalent than functional genes. Loss of function is apparent even within the ‘core’ gene categories of bacteria, such as genes involved in glycolysis and nutrient uptake. In contrast, serving as a critical source of nitrogen for the host, genes related to metabolic processes such as cell differentiation and nitrogen-fixation are well preserved. Conclusions/Significance: This is the first finding of genome degradation in a plant symbiont and phenotypically complex cyanobacterium and one of only a few extracellular endosymbionts described showing signs of reductive genome evolution. Our findings suggest an ongoing selective streamlining of this cyanobacterial genome which has resulted in an organism devoted to nitrogen fixation and devoid of autonomous growth. The cyanobacterial symbiont of Azolla can thus be considered at the initial phase of a transition from free-living organism to a nitrogen-fixing plant entity, a transition process which may mimic what drove the evolution of chloroplasts from a cyanobacterial ancestor
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