945 research outputs found
A Short Counterexample Property for Safety and Liveness Verification of Fault-tolerant Distributed Algorithms
Distributed algorithms have many mission-critical applications ranging from
embedded systems and replicated databases to cloud computing. Due to
asynchronous communication, process faults, or network failures, these
algorithms are difficult to design and verify. Many algorithms achieve fault
tolerance by using threshold guards that, for instance, ensure that a process
waits until it has received an acknowledgment from a majority of its peers.
Consequently, domain-specific languages for fault-tolerant distributed systems
offer language support for threshold guards.
We introduce an automated method for model checking of safety and liveness of
threshold-guarded distributed algorithms in systems where the number of
processes and the fraction of faulty processes are parameters. Our method is
based on a short counterexample property: if a distributed algorithm violates a
temporal specification (in a fragment of LTL), then there is a counterexample
whose length is bounded and independent of the parameters. We prove this
property by (i) characterizing executions depending on the structure of the
temporal formula, and (ii) using commutativity of transitions to accelerate and
shorten executions. We extended the ByMC toolset (Byzantine Model Checker) with
our technique, and verified liveness and safety of 10 prominent fault-tolerant
distributed algorithms, most of which were out of reach for existing
techniques.Comment: 16 pages, 11 pages appendi
Approximate Consensus in Highly Dynamic Networks: The Role of Averaging Algorithms
In this paper, we investigate the approximate consensus problem in highly
dynamic networks in which topology may change continually and unpredictably. We
prove that in both synchronous and partially synchronous systems, approximate
consensus is solvable if and only if the communication graph in each round has
a rooted spanning tree, i.e., there is a coordinator at each time. The striking
point in this result is that the coordinator is not required to be unique and
can change arbitrarily from round to round. Interestingly, the class of
averaging algorithms, which are memoryless and require no process identifiers,
entirely captures the solvability issue of approximate consensus in that the
problem is solvable if and only if it can be solved using any averaging
algorithm. Concerning the time complexity of averaging algorithms, we show that
approximate consensus can be achieved with precision of in a
coordinated network model in synchronous
rounds, and in rounds when
the maximum round delay for a message to be delivered is . While in
general, an upper bound on the time complexity of averaging algorithms has to
be exponential, we investigate various network models in which this exponential
bound in the number of nodes reduces to a polynomial bound. We apply our
results to networked systems with a fixed topology and classical benign fault
models, and deduce both known and new results for approximate consensus in
these systems. In particular, we show that for solving approximate consensus, a
complete network can tolerate up to 2n-3 arbitrarily located link faults at
every round, in contrast with the impossibility result established by Santoro
and Widmayer (STACS '89) showing that exact consensus is not solvable with n-1
link faults per round originating from the same node
Iterative Approximate Consensus in the presence of Byzantine Link Failures
This paper explores the problem of reaching approximate consensus in
synchronous point-to-point networks, where each directed link of the underlying
communication graph represents a communication channel between a pair of nodes.
We adopt the transient Byzantine link failure model [15, 16], where an
omniscient adversary controls a subset of the directed communication links, but
the nodes are assumed to be fault-free.
Recent work has addressed the problem of reaching approximate consen- sus in
incomplete graphs with Byzantine nodes using a restricted class of iterative
algorithms that maintain only a small amount of memory across iterations [22,
21, 23, 12]. However, to the best of our knowledge, we are the first to
consider approximate consensus in the presence of Byzan- tine links. We extend
our past work that provided exact characterization of graphs in which the
iterative approximate consensus problem in the presence of Byzantine node
failures is solvable [22, 21]. In particular, we prove a tight necessary and
sufficient condition on the underlying com- munication graph for the existence
of iterative approximate consensus algorithms under transient Byzantine link
model. The condition answers (part of) the open problem stated in [16].Comment: arXiv admin note: text overlap with arXiv:1202.609
Combined Untargeted and Targeted Fingerprinting by Comprehensive Two-Dimensional Gas Chromatography to Track Compositional Changes on Hazelnut Primary Metabolome during Roasting
This study focuses on the detectable metabolome of high-quality raw hazelnuts (Cory- lus avellana L.) and on its changes after dry-roasting. Informative fingerprinting was obtained by comprehensive two-dimensional gas chromatography with fast-scanning quadrupole mass spectrom- etry (GCĂGC-qMS) combined with dedicated data processing. In particular, combined untargeted and targeted (UT) fingerprinting, based on pattern recognition by template matching, is applied to chromatograms from raw and roasted samples of Tonda Gentile Trilobata and Anakliuri hazelnuts harvested in Italy and Georgia. Lab-scale roasting was designed to develop a desirable organoleptic profile matching industrial standards. Results, based on 430 peak features, reveal that phenotype expression is markedly correlated to cultivar and pedoclimatic conditions. Discriminant components between cultivars are amino acids (valine, alanine, glycine, and proline); organic acids (citric, aspartic, malic, gluconic, threonic, and 4-aminobutanoic acids); and sugars and polyols (maltose, xylulose, xylitol, turanose, mannitol, scyllo-inositol, and pinitol). Of these, alanine, glycine, and proline have a high informational role as precursors of 2-acetyl- and 2-propionylpyrroline, two key-aroma com- pounds of roasted hazelnuts. Roasting has a decisive impact on metabolite patternsâit caused a marked decrease (â90%) of alanine, proline, leucine and valine, and aspartic and pyroglutamic acid and a â50% reduction of saccharose and galactose
Quantum computing in optical microtraps based on the motional states of neutral atoms
We investigate quantum computation with neutral atoms in optical microtraps
where the qubit is implemented in the motional states of the atoms, i.e., in
the two lowest vibrational states of each trap. The quantum gate operation is
performed by adiabatically approaching two traps and allowing tunneling and
cold collisions to take place. We demonstrate the capability of this scheme to
realize a square-root of swap gate, and address the problem of double
occupation and excitation to other unwanted states. We expand the two-particle
wavefunction in an orthonormal basis and analyze quantum correlations
throughout the whole gate process. Fidelity of the gate operation is evaluated
as a function of the degree of adiabaticity in moving the traps. Simulations
are based on rubidium atoms in state-of-the-art optical microtraps with quantum
gate realizations in the few tens of milliseconds duration range.Comment: 11 pages, 7 figures, for animations of the gate operation, see
http://www.itp.uni-hannover.de/~eckert/na/index.htm
Ubiquitination and degradation of the hominoid- specific oncoprotein TBC1D3 Is mediated by CUL7 E3 ligase
Expression of the hominoid-specific TBC1D3 oncoprotein enhances growth factor receptor signaling and subsequently promotes cellular proliferation and survival. Here we report that TBC1D3 is degraded in response to growth factor signaling, suggesting that TBC1D3 expression is regulated by a growth factor-driven negative feedback loop. To gain a better understanding of how TBC1D3 is regulated, we studied the effects of growth factor receptor signaling on TBC1D3 post-translational processing and turnover. Using a yeast two-hybrid screen, we identified CUL7, the scaffolding subunit of the CUL7 E3 ligase complex, as a TBC1D3-interacting protein. We show that CUL7 E3 ligase ubiquitinates TBC1D3 in response to serum stimulation. Moreover, TBC1D3 recruits F-box 8 (Fbw8), the substrate recognition domain of CUL7 E3 ligase, in pull-down experiments and in an in vitro assay. Importantly, alkaline phosphatase treatment of TBC1D3 suppresses its ability to recruit Fbw8, indicating that TBC1D3 phosphorylation is critical for its ubiquitination and degradation. We conclude that serum- and growth factor-stimulated TBC1D3 ubiquitination and degradation are regulated by its interaction with CUL7-Fbw8
Single Atom Cooling by Superfluid Immersion: A Non-Destructive Method for Qubits
We present a scheme to cool the motional state of neutral atoms confined in
sites of an optical lattice by immersing the system in a superfluid. The motion
of the atoms is damped by the generation of excitations in the superfluid, and
under appropriate conditions the internal state of the atom remains unchanged.
This scheme can thus be used to cool atoms used to encode a series of entangled
qubits non-destructively. Within realisable parameter ranges, the rate of
cooling to the ground state is found to be sufficiently large to be useful in
experiments.Comment: 14 pages, 9 figures, RevTeX
Quantum computing implementations with neutral particles
We review quantum information processing with cold neutral particles, that
is, atoms or polar molecules. First, we analyze the best suited degrees of
freedom of these particles for storing quantum information, and then we discuss
both single- and two-qubit gate implementations. We focus our discussion mainly
on collisional quantum gates, which are best suited for atom-chip-like devices,
as well as on gate proposals conceived for optical lattices. Additionally, we
analyze schemes both for cold atoms confined in optical cavities and hybrid
approaches to entanglement generation, and we show how optimal control theory
might be a powerful tool to enhance the speed up of the gate operations as well
as to achieve high fidelities required for fault tolerant quantum computation.Comment: 19 pages, 12 figures; From the issue entitled "Special Issue on
Neutral Particles
Paxos Consensus, Deconstructed and Abstracted (Extended Version)
Lamport's Paxos algorithm is a classic consensus protocol for state machine
replication in environments that admit crash failures. Many versions of Paxos
exploit the protocol's intrinsic properties for the sake of gaining better
run-time performance, thus widening the gap between the original description of
the algorithm, which was proven correct, and its real-world implementations. In
this work, we address the challenge of specifying and verifying complex
Paxos-based systems by (a) devising composable specifications for
implementations of Paxos's single-decree version, and (b) engineering
disciplines to reason about protocol-aware, semantics-preserving optimisations
to single-decree Paxos. In a nutshell, our approach elaborates on the
deconstruction of single-decree Paxos by Boichat et al. We provide novel
non-deterministic specifications for each module in the deconstruction and
prove that the implementations refine the corresponding specifications, such
that the proofs of the modules that remain unchanged can be reused across
different implementations. We further reuse this result and show how to obtain
a verified implementation of Multi-Paxos from a verified implementation of
single-decree Paxos, by a series of novel protocol-aware transformations of the
network semantics, which we prove to be behaviour-preserving.Comment: Accepted for publication in the 27th European Symposium on
Programming (ESOP'18
Matter-wave interferometry in a double well on an atom chip
Matter-wave interference experiments enable us to study matter at its most
basic, quantum level and form the basis of high-precision sensors for
applications such as inertial and gravitational field sensing. Success in both
of these pursuits requires the development of atom-optical elements that can
manipulate matter waves at the same time as preserving their coherence and
phase. Here, we present an integrated interferometer based on a simple,
coherent matter-wave beam splitter constructed on an atom chip. Through the use
of radio-frequency-induced adiabatic double-well potentials, we demonstrate the
splitting of Bose-Einstein condensates into two clouds separated by distances
ranging from 3 to 80 microns, enabling access to both tunnelling and isolated
regimes. Moreover, by analysing the interference patterns formed by combining
two clouds of ultracold atoms originating from a single condensate, we measure
the deterministic phase evolution throughout the splitting process. We show
that we can control the relative phase between the two fully separated samples
and that our beam splitter is phase-preserving
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