Broadening of the Berezinskii-Kosterlitz-Thouless transition by correlated disorder

The Berezinskii-Kosterlitz-Thouless (BKT) transition in two-dimensional superconductors is usually expected to be protected against disorder. However, its fingerprints in a real system, such as, e.g., the universal superfluid- density jump, are often at odds with this expectation. Here, we show that the disorder-induced granularity of the superconducting state modifies the nucleation mechanism for vortex-antivortex pairs. This leads to a considerable smearing of the universal superfluid-density jump as compared to the paradigmatic clean case, in agreement with experimental observations

Secure Multiparty Sessions with Topics

Multiparty session calculi have been recently equipped with security requirements, in order to guarantee properties such as access control and leak freedom. However, the proposed security requirements seem to be overly restrictive in some cases. In particular, a party is not allowed to communicate any kind of public information after receiving a secret information. This does not seem justified in case the two pieces of information are totally unrelated. The aim of the present paper is to overcome this restriction, by designing a type discipline for a simple multiparty session calculus, which classifies messages according to their topics and allows unrestricted sequencing of messages on independent topics.Comment: In Proceedings PLACES 2016, arXiv:1606.0540

Self-Adaptation and Secure Information Flow in Multiparty Structured Communications: A Unified Perspective

We present initial results on a comprehensive model of structured communications, in which self- adaptation and security concerns are jointly addressed. More specifically, we propose a model of self-adaptive, multiparty communications with secure information flow guarantees. In this model, security violations occur when processes attempt to read or write messages of inappropriate security levels within directed exchanges. Such violations trigger adaptation mechanisms that prevent the violations to occur and/or to propagate their effect in the choreography. Our model is equipped with local and global mechanisms for reacting to security violations; type soundness results ensure that global protocols are still correctly executed, while the system adapts itself to preserve security.Comment: In Proceedings BEAT 2014, arXiv:1408.556

Reactive concurrent programming revisited

In this note we revisit the so-called reactive programming style, which evolves from the synchronous programming model of the Esterel language by weakening the assumption that the absence of an event can be detected instantaneously. We review some research directions that have been explored since the emergence of the reactive model ten years ago. We shall also outline some questions that remain to be investigated

An algebraic characterization of observational equivalence

AbstractWe show that observational equivalence can be characterized by saturating homomorphisms (with respect to Hennessy-Milner logic), thus bringing together results developed independently by Castellani and by Arnold and Dicky on characterizations of transition system equivalences. We take this opportunity to compare Castellani's abstraction homomorphisms and Arnold-Dicky's saturating homomorphisms. It turns out that they are very similar notions: their difference in formulation is partly due to the fact that abstraction homomorphisms were defined on a restricted class of transition systems

Report on WS25CCC Workshop "25 Years of Combining Compositionality and Concurrency"

International audienceIn the peak of the summer 2013, between the 7th and the 9th of August, the workshop "25 Years of Combining Compositionality and Concurrency" took place in Königswinter, a picturesque little town overlooking the Rhine river, in the outskirts of Bonn. The event, organised by Ursula Goltz, Rob van Glabbeek and Ernst-Rüdiger Olderog, was meant to celebrate and revisit, a quarter of a century later, the workshop "Combining Compositionality and Concurrency" (CCC88) that had been held in March 1988 in the same hotel, the Loreley, on the initiative of the same trio of researchers (the first two of which were still PhD students at the time). Both workshops were by invitation only, and each attracted 34 participants. Because of its timely character and its deliberate focus on bridging the gap be-tween process calculi and "true-concurrency" models, the original CCC88 workshop, targeting a group of active researchers from both fields, had generated much enthusiasm and discussion. It had therefore gradually acquired, at least in the memories of its participants, the mythical status of a "foundational event". It was then quite natural for the organisers, 25 years later, to envisage a kind of jubilee event, which could bring together a number of participants from the original workshop, as well as younger researchers who had joined in more recent years the field of concurrency theory, now much broader and well-established

Kane-Fisher weak link physics in the clean scratched-XY model

The nature of the superfluid-insulator transition in 1D has been much debated recently. In particular, to describe the strong disorder regime characterized by weak link proliferation, a scratched-XY model has been proposed [New J. Phys. \textbf{18}, 045018 (2016)], where the transport is dominated by a single anomalously weak link and is governed by Kane-Fisher weak link physics. In this article, we consider the simplest problem to which the scratched-XY model relates: a single weak link in an otherwise \textit{clean} system, with an intensity $J_W$ which decreases algebraically with the size of the system $J_W\sim L^{-\alpha}$. Using a renormalization group approach and a vortex energy argument, we describe the Kane-Fisher physics in this model and show that it leads to a transition from a transparent regime for $K>K_c$ to a perfect cut for $K<K_c$, with an adjustable $K_c=1/(1-\alpha)$ depending on $\alpha$. We check our theoretical predictions with Monte Carlo numerical simulations complemented by finite-size scaling. Our results clarify two important assumptions at the basis of the scratched-XY scenario, the behaviors of the crossover length scale from weak link physics to transparency and of the superfluid stiffness.Comment: 11 pages, 4 figure

Typing Noninterference for Reactive Programs

We propose a type system to enforce the security property of noninterference in a core reactive language, obtained by extending the imperative language of Volpano, Smith and Irvine with reactive primitives manipulating broadcast signals and with a form of ``scheduled'' parallelism. Due to the particular nature of reactive computations, the definition of noninterference has to be adapted. We give a formulation of noninterference based on bisimulation. Our type system is inspired by that introduced by Boudol and Castellani, and independently by Smith, to cope with timing leaks in a language for parallel programs with scheduling. We establish the soundness of this type system with respect to our notion of noninterference