35 research outputs found
Revealing the Vicious Circle of Disengaged User Acceptance: A SaaS Provider's Perspective
User acceptance tests (UAT) are an integral part of many different software engineering methodologies. In this paper, we examine the influence of UATs on the relationship between users and Software-as-a-Service (SaaS) applications, which are continuously delivered rather than rolled out during a one-off signoff process. Based on an exploratory qualitative field study at a multinational SaaS provider in Denmark, we show that UATs often address the wrong problem in that positive user acceptance may actually indicate a negative user experience. Hence, SaaS providers should be careful not to rest on what we term disengaged user acceptance. Instead, we outline an approach that purposefully queries users for ambivalent emotions that evoke constructive criticism, in order to facilitate a discourse that favors the continuous innovation of a SaaS system. We discuss theoretical and practical implications of our approach for the study of user engagement in testing SaaS applications
Topological Magnons and Edge States in Antiferromagnetic Skyrmion Crystals
Antiferromagnetic skyrmion crystals are magnetic phases predicted to exist in
antiferromagnets with Dzyaloshinskii-Moriya interactions. Their spatially
periodic noncollinear magnetic texture gives rise to topological bulk magnon
bands characterized by nonzero Chern numbers. We find topologically-protected
chiral magnonic edge states over a wide range of magnetic fields and
Dzyaloshinskii-Moriya interaction values. Moreover, and of particular
importance for experimental realizations, edge states appear at the lowest
possible energies, namely, within the first bulk magnon gap. Thus,
antiferromagnetic skyrmion crystals show great promise as novel platforms for
topological magnonics.Comment: 5 pages, 5 figure
Magnonic Quadrupole Topological Insulator in Antiskyrmion Crystals
When the crystalline symmetries that protect a higher-order topological phase
are not preserved at the boundaries of the sample, gapless hinge modes or
in-gap corner states cannot be stabilized. Therefore, careful engineering of
the sample termination is required. Similarly, magnetic textures, whose quantum
fluctuations determine the supported magnonic excitations, tend to relax to new
configurations that may also break crystalline symmetries when boundaries are
introduced. Here we uncover that antiskyrmion crystals provide an
experimentally accessible platform to realize a magnonic topological quadrupole
insulator, whose hallmark signature are robust magnonic corner states.
Furthermore, we show that tuning an applied magnetic field can trigger the
self-assembly of antiskyrmions carrying a fractional topological charge along
the sample edges. Crucially, these fractional antiskyrmions restore the
symmetries needed to enforce the emergence of the magnonic corner states. Using
the machinery of nested Wilson loops, adapted to magnonic systems supported by
noncollinear magnetic textures, we demonstrate the quantization of the bulk
quadrupole moment, edge dipole moments, and corner charges
Chiral Magnonic Edge States in Ferromagnetic Skyrmion Crystals Controlled by Magnetic Fields
Achieving control over magnon spin currents in insulating magnets - where
dissipation due to Joule heating is highly suppressed - is an active area of
research that could lead to energy-efficient spintronics applications. However,
magnon spin currents supported by conventional systems with uniform magnetic
order have proven hard to control. An alternative approach that relies on
topologically protected magnonic edge states of spatially periodic magnetic
textures has recently emerged. A prime example of such textures is the
ferromagnetic skyrmion crystal which hosts chiral edge states providing a
platform for magnon spin currents. Here, we show, for the first time, an
external magnetic field can drive a topological phase transition in the spin
wave spectrum of a ferromagnetic skyrmion crystal. The topological phase
transition is signaled by the closing of a low-energy bulk magnon gap at a
critical field. In the topological phase, below the critical field, two
topologically protected chiral magnonic edge states lie within this gap, but
they unravel in the trivial phase, above the critical field. Remarkably, the
topological phase transition involves an inversion of two magnon bands that at
the point correspond to the breathing and anticlockwise modes of the
skyrmions in the crystal. Our findings suggest that an external magnetic field
could be used as a knob to switch on and off magnon spin currents carried by
topologically protected chiral magnonic edge states
Efficient Algorithms for Broadcast and Consensus Based on Proofs of Work
Inspired by the astonishing success of cryptocurrencies, most notably the Bitcoin system, several recent works have focused on the design of robust blockchain-style protocols that work in a peer-to-peer setting such as the Internet. In contrast to the setting traditionally considered in multiparty computation (MPC), in these systems, honesty is measured by computing power instead of requiring that only a certain fraction of parties is controlled by the adversary. This provides a potential countermeasure against the so-called Sybil attack, where an adversary creates fake identities, thereby easily taking over the majority of parties in the system. In this work we design protocols for Broadcast and Byzantine agreement that are secure under the assumption that the majority of computing power is controlled by the honest parties and for the first time have expected constant round complexity. This is in contrast to earlier works (Crypto\u2715, ePrint\u2714) which have round complexities that scale linearly with the number n of parties; an undesirable feature in a P2P environment with potentially thousands of users. In addition, our main protocol which runs in quasi-constant rounds, introduces novel ideas that significantly decrease communication complexity. Concretely, this is achieved by using an appropriate time-locked encryption scheme and by structuring the parties into a network of so-called cliques.
Note: This article contains incorrect claims.
Some of its contributions were subsumed by eprint article 2022/82
BIP32-Compatible Threshold Wallets
Cryptographic wallets have become an essential tool to secure users\u27 secret keys and consequently their funds in Blockchain networks. The most prominent wallet standard that is widely adopted in practice is the BIP32 specification. This standard specifies so-called hierarchical deterministic wallets, which are organized in a tree-like structure such that each node in the tree represents a wallet instance and such that a parent node can derive a new child node in a deterministic fashion.
BIP32 considers two types of child nodes, namely non-hardened and hardened nodes, which differ in the security guarantees they provide. While the corruption of a hardened wallet does not affect the security of any other wallet instance in the tree, the corruption of a non-hardened node leads to a breach of the entire scheme.
In this work, we address this significant drawback of non-hardened nodes by laying out the design for the first hierarchical deterministic wallet scheme with thresholdized non-hardened nodes. We first provide a game-based notion of threshold signatures with rerandomizable keys and show an instantiation via the Gennaro and Goldfeder threshold ECDSA scheme (CCS\u2718). We further observe that the derivation of hardened child wallets according to the BIP32 specification does not translate easily to the threshold setting. Therefore, we devise a new and efficient derivation mechanism for hardened wallets in the threshold setting that satisfies the same properties as the original BIP32 derivation mechanism and therefore allows for efficient constructions of BIP32-compatible threshold wallets
Round Efficient Byzantine Agreement from VDFs
Byzantine agreement (BA) is a fundamental primitive in distributed systems and has received huge interest as an important building block for blockchain systems. Classical byzantine agreement considers a setting where parties with fixed, known identities want to agree on an output in the presence of an adversary. Motivated by blockchain systems, the assumption of fixed identities is weakened by using a \emph{resource-based model}. In such models, parties do not have fixed known identities but instead have to invest some expensive resources to participate in the protocol. Prominent examples for such resources are computation (measured by, e.g., proofs-of-work) or money (measured by proofs-of-stake). Unlike in the classical setting where BA without trusted setup (e.g., a PKI or an unpredictable beacon) is impossible for corruptions, in such resource-based models, BA can be constructed for the optimal threshold of . In this work, we investigate BA without a PKI in the model where parties have restricted computational resources. Concretely, we consider sequential computation modeled via computing a verifiable delay function (VDF) and establish the following results:
Positive Result: We present the first protocol for BA with expected constant round complexity and termination under adaptive corruption, honest majority and without a PKI. Earlier work achieved round complexity (CRYPTO\u2715) or (PKC\u2718), where is the security parameter.
Negative Result: We give the first lower bound on the communication complexity of BA in a model where parties have restricted computational resources. Concretely, we show that a multicast complexity of is necessary even if the parties have access to a VDF oracle
Interplay of Coulomb blockade and Aharonov-Bohm resonances in a Luttinger liquid
We consider a ring of strongly interacting electrons connected to two
external leads by tunnel junctions. By studying the positions of conductance
resonances as a function of gate voltage and magnetic flux the interaction
parameter can be determined experimentally. For a finite ring the minimum
conductance is strongly influenced by device geometry and electron-electron
interactions. In particular, if the tunnel junctions are close to one another
the interaction-related orthogonality catastrophe is suppressed and the valley
current is unexpectedly large.Comment: 10 page
Correlation Functions and Coulomb Blockade of Interacting Fermions at Finite Temperature and Size
We present explicit expressions for the correlation functions of interacting
fermions in one dimension which are valid for arbitrary system sizes and
temperatures. The result applies to a number of very different strongly
correlated systems, including mesoscopic quantum wires, quantum Hall edges,
spin chains and quasi-one-dimensional metals. It is for example possible to
calculate Coulomb blockade oscillations from our expression and determine their
dependence on interaction strength and temperature. Numerical simulations show
excellent agreement with the analytical results.Comment: 10 pages in revtex format including 2 embedded figures (using epsf).
The latest complete postscript file is available from
http://fy.chalmers.se/~eggert/papers/corrfcn.ps or by request from
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