Ideal hierarchical secret sharing schemes

Hierarchical secret sharing is among the most natural generalizations of threshold secret sharing, and it has attracted a lot of attention from the invention of secret sharing until nowadays. Several constructions of ideal hierarchical secret sharing schemes have been proposed, but it was not known what access structures admit such a scheme. We solve this problem by providing a natural definition for the family of the hierarchical access structures and, more importantly, by presenting a complete characterization of the ideal hierarchical access structures, that is, the ones admitting an ideal secret sharing scheme. Our characterization deals with the properties of the hierarchically minimal sets of the access structure, which are the minimal qualified sets whose participants are in the lowest possible levels in the hierarchy. By using our characterization, it can be efficiently checked whether any given hierarchical access structure that is defined by its hierarchically minimal sets is ideal. We use the well known connection between ideal secret sharing and matroids and, in particular, the fact that every ideal access structure is a matroid port. In addition, we use recent results on ideal multipartite access structures and the connection between multipartite matroids and integer polymatroids. We prove that every ideal hierarchical access structure is the port of a representable matroid and, more specifically, we prove that every ideal structure in this family admits ideal linear secret sharing schemes over fields of all characteristics. In addition, methods to construct such ideal schemes can be derived from the results in this paper and the aforementioned ones on ideal multipartite secret sharing. Finally, we use our results to find a new proof for the characterization of the ideal weighted threshold access structures that is simpler than the existing one.Peer ReviewedPostprint (author's final draft

A measure of tripartite entanglement in bosonic and fermionic systems

We describe an efficient theoretical criterion suitable for the evaluation of the tripartite entanglement of any mixed three-boson or -fermion state, based on the notion of the entanglement of particles for bipartite systems of identical particles. Our approach allows one to quantify the accessible amount of quantum correlations in the systems without any violation of the local particle number superselection rule. A generalization of the tripartite negativity is here applied to some correlated systems including the continuous-time quantum walks of identical particles (both for bosons and fermions) and compared with other criteria recently proposed in the literature. Our results show the dependence of the entanglement dynamics upon the quantum statistics: the bosonic bunching results into a low amount of quantum correlations while Fermi-Dirac statistics allows for higher values of the entanglement.Comment: 19 pages, 3 figure

Photonic multipartite entanglement conversion using nonlocal operations

We propose a simple setup for the conversion of multipartite entangled states in a quantum network with restricted access. The scheme uses nonlocal operations to enable the preparation of states that are inequivalent under local operations and classical communication, but most importantly does not require full access to the states. It is based on a flexible linear optical conversion gate that uses photons, which are ideally suited for distributed quantum computation and quantum communication in extended networks. In order to show the basic working principles of the gate, we focus on converting a four-qubit entangled cluster state to other locally inequivalent four-qubit states, such as the GHZ and symmetric Dicke state. We also show how the gate can be incorporated into extended graph state networks, and can be used to generate variable entanglement and quantum correlations without entanglement but nonvanishing quantum discord.Comment: 10 pages, 6 figures, correction of reference list, add Journal ref. and DO

Enhancing the Effectiveness of Social Dialogue Articulation in Europe (EESDA) Project No. VS/2017/0434 Stakeholders’ views on and experiences with the articulation of social dialogue and its effectiveness

This report studies the articulation and effectiveness of social dialogue at the European and national levels. It provides an overview of the existing social dialogue structures describing the main actors involved. The overall approach of the study is actor-centred in the sense that the interactions and perception of actors are in the core of this research. The analysis also takes into account the multilevel governance structure in Europe by considering the interaction and vertical/horizontal articulation of social dialogue between the EU and national levels. The study also benefits from original data collection comprised of several layers including both the EU and national levels and through at least two methods. First, semi-structured interviews were conducted with European social partners as well as with national social stakeholders in a selection of six Member States. Second, an EU-wide online survey collected responses from national social partners in 27 Member States. The remainder of the analysis is complemented with desk research

Matroids and Quantum Secret Sharing Schemes

A secret sharing scheme is a cryptographic protocol to distribute a secret state in an encoded form among a group of players such that only authorized subsets of the players can reconstruct the secret. Classically, efficient secret sharing schemes have been shown to be induced by matroids. Furthermore, access structures of such schemes can be characterized by an excluded minor relation. No such relations are known for quantum secret sharing schemes. In this paper we take the first steps toward a matroidal characterization of quantum secret sharing schemes. In addition to providing a new perspective on quantum secret sharing schemes, this characterization has important benefits. While previous work has shown how to construct quantum secret sharing schemes for general access structures, these schemes are not claimed to be efficient. In this context the present results prove to be useful; they enable us to construct efficient quantum secret sharing schemes for many general access structures. More precisely, we show that an identically self-dual matroid that is representable over a finite field induces a pure state quantum secret sharing scheme with information rate one

The challenges of participatory research with 'tech-savvy' youth

This paper focuses on participatory research and how it can be understood and employed when researching children and youth. The aim of this paper is to provide a theoretically and empirically grounded discussion of participatory research methodologies with respect to investigating the dynamic and evolving phenomenon of young people growing up in networked societies. Initially, we review the nature of participatory research and how other researchers have endeavoured to involve young people (children and youth) in their research projects. Our review of these approaches aims to elucidate what we see as recurring and emerging issues with respect to the methodological design of involving young people as co-researchers. In the light of these issues and in keeping with our aim, we offer a case study of our own research project that seeks to understand the ways in which high school students use new media and network ICT systems (Internet, mobile phone applications, social networking sites) to construct identities, form social relations, and engage in creative practices as part of their everyday lives. The article concludes by offering an assessment of our tripartite model of participatory research that may benefit other researchers who share a similar interest in youth and new media

1. Strengthening International Regulation Through Transnational New Governance: Overcoming the Orchestration Deficit. 2. International Regulation without International Government: Improving IO Performance through Orchestration

1. A new kind of international regulatory system is spontaneously arising out of the failure of international 'Old Governance' (i.e., treaties and intergovernmental organizations) to adequately regulate international business. Nongovernmental organizations, business firms, and other actors, singly and in novel combinations, are creating innovative institutions to apply transnational norms to business. These institutions are predominantly private and operate through voluntary standards. The Authors depict the diversity of these new regulatory institutions on the 'Governance Triangle,' according to the roles of different actors in their operations. To analyze this complex system, we adapt the domestic 'New Governance' model of regulation to the international setting. 'Transnational New Governance' potentially provides many benefits of New Governance and is particularly suitable for international regulation because it demands less of states and intergovernmental organizations (IGOs). However, Transnational New Governance does require states and IGOs to act as orchestrators of the international regulatory system, and that system currently suffers from a significant orchestration deficit. If states and IGOs expanded 'directive' and especially 'facilitative' orchestration of the Transnational New Governance system, they could strengthen high-quality private regulatory standards, improve the international regulatory system, and better achieve their own regulatory goals. 2. International organizations (IOs) have been widely criticized as ineffective. Yet scholars and practitioners assessing IO performance frequently focus on traditional modes of governance such as treaties and inter-state dispute-resolution mechanisms. When they observe poor performance, moreover, they often prescribe a strengthening of those same activities. We call this reliance on traditional state-based mechanisms 'International Old Governance' (IOG). A better way to understand and improve IO performance is to consider the full range of ways in which IOs can and do operate - including, increasingly, by reaching out to private actors and institutions, collaborating with them, and supporting and shaping their activities. Such actions are helping to develop an intricate global network of public, private and mixed institutions and norms, partially orchestrated by IOs, that we call 'Transnational New Governance' (TNG). With proper orchestration by IOs, TNG can ameliorate both 'state failure' - the inadequacies of IOG - and 'market failure' - the problems that result when the creation and evolution of norm-setting institutions is highly decentralized. Orchestration thus provides a significant way for IOs to improve their regulatory performance. Some IOs already engage actively with private actors and institutions - we provide a range of illustrations, highlighting the activities of the UN Environment Programme (UNEP). Yet there remains a significant 'orchestration deficit' that provides real opportunities for IOs. We draw on the lessons of existing IO activities to suggest additional possibilities for improving IO performance. -- 1. Ein internationales regulatorisches System neuer Art entsteht derzeit spontan aus dem Versagen der internationalen 'Old Governance' - also der bestehenden internationalen Verträge und Organisationen - dabei, den internationalen Handel angemessen zu regulieren. Nichtregierungsorganisationen, Unternehmen und andere Akteure - allein und in ganz neuen Kombinationen - schaffen sich neue internationale Einrichtungen, um transnationale Normen auf internationale Geschäftstätigkeit anzuwenden. Es geht dabei vornehmlich um private Einrichtungen, die vor allem über freiwillige Standardbefolgung wirken. In diesem Beitrag werden die unterschiedlichen Regulierungseinrichtungen als Teil eines 'Governance Dreiecks' beschreiben und das geschieht vor allem in Blick darauf, welche Rollen die unterschiedlichen Akteure in ihrer Tätigkeit spielen. Um dieses komplexe System zu untersuchen passen wir das innenpolitische Regulierungsmodell der 'New Governance' an den internationalen Rahmen an. Die 'Transnational New Governance' enthält viele Vorteile der 'New Governance' und sie ist für die internationale Regulierung besonders angemessen, weil sie geringere Anforderungen an die Staatenwelt und an die intergouvernementalen Organisationen, die IGOs, stellen. Allerdings ist es für eine wirksame 'Transnational New Governance' weiterhin erforderlich, dass die Staaten und die IGOs das internationale regulatorische System 'orchestrieren'. Das heutige internationale regulatorische System leidet allerdings unter einem Orchestrierungsdefizit. Würden die Staaten und die IGOs die 'anweisende' und 'ermöglichende' Orchestrierungsfunktion des 'Transnational New Governance System' ausbauen, dann stärkten sie die privaten regulatorischen Standards von hoher Qualität, verbesserten das internationale regulatorische System und könnten ihre eigenen regulatorischen Ziele besser verwirklichen. 2. Internationale Organisationen (IOs) werden vielfach ob Ihrer Ineffizienz kritisiert. Allerdings konzentrieren sich Wissenschaftler und Praktiker bei dieser Bewertung der Leistungsfähigkeit von IOs häufig auf Maßstäbe, die den herkömmlichen Formen von Governance - wie internationalen Verträgen und den Mechanismen für die Schlichtung zwischenstaatlicher Konflikte - entlehnt sind. Soweit eine ungenügende Leistung festgestellt wird, empfehlen sie zudem meist, Tätigkeiten der traditionellen Art zu verstärken. Wir bezeichnen dies als ein Sich-Verlassen auf die herkömmlichen staats-basierten Mechanismen, als 'International Old Governance' (IOG). Man versteht die IOs besser und verbessert ihre Leistungsfähigkeit aber der Situation weitaus angemessener, wenn man die gesamte Vielfalt wirklicher und möglicher IO-Tätigkeiten in den Blick nimmt: Das umfasst vor allem, dass die IOs private Akteure und Organisationen einbeziehen, mit ihnen zusammenarbeiten, ihre Aktivitäten stützen, sie formen und ihnen Richtung geben. Diese Tätigkeiten schaffen ein fein gesponnenes globales Netzwerk von öffentlichen, privaten und gemischten Organisationen und Normen, das seinerseits teilweise durch die IOs orchestriert wird. Dieses Netzwerk bezeichnen wir als 'Transnational New Governance' (TNG). Wenn die Orchestrierung durch IOs gut funktioniert kann sie auch Staatsversagen (state failure) abmildern - also hier die Inadäquanzen von 'International Organization Governance' - und ebenso kann sie Marktversagen ausgleichen, also die Probleme, die sich ergeben wenn Herausbildung und Schaffung normgebender Einrichtungen höchst dezentralisiert verläuft. Über die Orchestrierung können die IOs auch die Leistungsfähigkeit ihrer Regulierungen erhöhen. Einige IOs sind schon jetzt stark zusammen mit privaten Akteuren und Einrichtungen unterwegs; in diesem Beitrag mustern wir das Spektrum solcher Aktivitäten. Wir betonen dabei die Unternehmungen des UN Environment Programme (UNEP), des Umweltprogramms der VN. Allerdings verbleibt ein erhebliches 'Orchestrierungsdefizit', das für die IOs zugleich Herausforderung und Chance ist. Wir stützten uns auf den Erfahrungsschatz der vorgefundenen IO-Aktivitäten, um den Möglichkeitsraum der IOs für die Zukunft auszumessen und Maßnahmen zur Verbesserung der Leistungsfähigkeit von IOs anzuregen.