100 research outputs found
A simple block representation of reversible cellular automata with time-symmetry
Reversible Cellular Automata (RCA) are a physics-like model of computation
consisting of an array of identical cells, evolving in discrete time steps by
iterating a global evolution G. Further, G is required to be shift-invariant
(it acts the same everywhere), causal (information cannot be transmitted faster
than some fixed number of cells per time step), and reversible (it has an
inverse which verifies the same requirements). An important, though only
recently studied special case is that of Time-symmetric Cellular Automata
(TSCA), for which G and its inverse are related via a local operation. In this
note we revisit the question of the Block representation of RCA, i.e. we
provide a very simple proof of the existence of a reversible circuit
description implementing G. This operational, bottom-up description of G turns
out to be time-symmetric, suggesting interesting connections with TSCA. Indeed
we prove, using a similar technique, that a wide class of them admit an Exact
block representation (EBR), i.e. one which does not increase the state space.Comment: 6 pages, 3 figures, Automata 201
Unitarity plus causality implies localizability
We consider a graph with a single quantum system at each node. The entire
compound system evolves in discrete time steps by iterating a global evolution
. We require that this global evolution be unitary, in accordance with
quantum theory, and that this global evolution be causal, in accordance
with special relativity. By causal we mean that information can only ever be
transmitted at a bounded speed, the speed bound being quite naturally that of
one edge of the underlying graph per iteration of . We show that under these
conditions the operator can be implemented locally; i.e. it can be put into
the form of a quantum circuit made up with more elementary operators -- each
acting solely upon neighbouring nodes. We take quantum cellular automata as an
example application of this representation theorem: this analysis bridges the
gap between the axiomatic and the constructive approaches to defining QCA.
KEYWORDS: Quantum cellular automata, Unitary causal operators, Quantum walks,
Quantum computation, Axiomatic quantum field theory, Algebraic quantum field
theory, Discrete space-time.Comment: V1: 5 pages, revtex. V2: Generalizes V1. V3: More precisions and
reference
A simple KPFM-based approach for electrostatic-free topographic measurements: the case of MoS on SiO
A simple implementation of Kelvin probe force microscopy is reported that
enables recording topographic images in the absence of any component of the
electrostatic force. Our approach is based on a close loop z-spectroscopy
operated in data cube mode. Curves of the tip-sample distance as a function of
time are recorded onto a 2D grid. A dedicated circuit holds the KPFM
compensation bias and subsequently cut off the modulation voltage during
well-defined time-windows within the spectroscopic acquisition. Topographic
images are recalculated from the matrix of spectroscopic curves. This approach
is applied to the case of transition metal dichalcogenides (TMD) monolayers
grown by chemical vapour deposition on silicon oxide substrates. In addition,
we check to what extent a proper stacking height estimation can also be
performed by recording series of images for decreasing values of the bias
modulation amplitude. The outputs of both approaches are shown to be fully
consistent. The results exemplify how in the operating conditions of
non-contact AFM under ultra-high vacuum, the stacking height values can
dramatically be overestimated due to variations in the tip-surface capacitive
gradient, even though the KPFM controller nullifies the potential difference.
We show that the number of atomic layers of a TMD can be safely assessed, only
if the KPFM measurement is performed with a modulated bias amplitude reduced at
its strict minimum or, even better, without any modulated bias. Last, the
spectroscopic data reveal that defects at the TMD/oxide interface can have a
counterintuitive impact on the electrostatic landscape, resulting in an
apparent decrease of the measured stacking height by conventional nc-AFM/KPFM
compared to non-defective sample areas. Hence, electrostatic free z-imaging
proves to be a promising tool to assess the existence of defects in atomically
thin TMD layers grown on oxide
DC-SIGN–mediated Infectious Synapse Formation Enhances X4 HIV-1 Transmission from Dendritic Cells to T Cells
Dendritic cells (DCs) are essential for the early events of human immunodeficiency virus (HIV) infection. Model systems of HIV sexual transmission have shown that DCs expressing the DC-specific C-type lectin DC-SIGN capture and internalize HIV at mucosal surfaces and efficiently transfer HIV to CD4+ T cells in lymph nodes, where viral replication occurs. Upon DC–T cell clustering, internalized HIV accumulates on the DC side at the contact zone (infectious synapse), between DCs and T cells, whereas HIV receptors and coreceptors are enriched on the T cell side. Viral concentration at the infectious synapse may explain, at least in part, why DC transmission of HIV to T cells is so efficient
Intervention and the ordering of the modern world
I am lead editor of a special issue of the Review of International Studies, which is the house journal of the British International Studies Association. The special issue arose from a competitive process. I am scheduled to have two pieces in this issue.This introductory discussion establishes the notion of intervention as a ‘social practice’ and carves out the contextual and conceptual space for the special issue as a whole. The first move is to recontextualise intervention in terms of ‘modernity’ as distinct from the sovereign states system. This shift enables a better appreciation of the dynamic and evolutionary context that generates variation in the practice of intervention over time and space and which is more analytically sensitive to the economic and cultural (as well as Great Power) hierarchies that generate rationales for intervention. The second move is to reconceptualise intervention as a specific modality of coercion relatively well-suited to the regulation or mediation of conflict between territorially bounded political communities and transnational social forces. Third is to ‘historicise’ the practice of intervention through showing how it has changed in relation to a range of international orders’ that have defined the modern world and which are each characterised by a different notion of the relationship between social and territorial space. Fourth and finally is a brief consideration of the possibility of intervention’s demise as a social practice.ESRC funded seminar series, ‘Rethinking Intervention: Intervention in the Modern World’, grant reference RES-451-26-066
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