On the Executability of Interactive Computation

The model of interactive Turing machines (ITMs) has been proposed to characterise which stream translations are interactively computable; the model of reactive Turing machines (RTMs) has been proposed to characterise which behaviours are reactively executable. In this article we provide a comparison of the two models. We show, on the one hand, that the behaviour exhibited by ITMs is reactively executable, and, on the other hand, that the stream translations naturally associated with RTMs are interactively computable. We conclude from these results that the theory of reactive executability subsumes the theory of interactive computability. Inspired by the existing model of ITMs with advice, which provides a model of evolving computation, we also consider RTMs with advice and we establish that a facility of advice considerably upgrades the behavioural expressiveness of RTMs: every countable transition system can be simulated by some RTM with advice up to a fine notion of behavioural equivalence.Comment: 15 pages, 0 figure

Orbital-Peierls State in NaTiSi2O6

Does the quasi one-dimensional titanium pyroxene NaTiSi2O6 exhibit the novel {\it orbital-Peierls} state? We calculate its groundstate properties by three methods: Monte Carlo simulations, a spin-orbital decoupling scheme and a mapping onto a classical model. The results show univocally that for the spin and orbital ordering to occur at the same temperature --an experimental observation-- the crystal field needs to be small and the orbitals are active. We also find that quantum fluctuations in the spin-orbital sector drive the transition, explaining why canonical bandstructure methods fail to find it. The conclusion that NaTiSi2O6 shows an orbital-Peierls transition is therefore inevitable.Comment: 4 pages, 3 figure

Quantifying the resource of sharing a reference frame

We define a new quantity called refbit, which allows one to quantify the resource of sharing a reference frame in quantum communication protocols. By considering both asymptotic and nonasymptotic protocols we find relations between refbits and other communication resources. We also consider the same resources in encoded, reference-frame independent, form. This allows one to rephrase and unify previous work on phase references, reference frames, and superselection rules.Comment: Updated title as PRA did not accept the word "refbit" in the title: PRA accepts neither neologisms (="a meaningless word coined by a psychotic", according to Webster), nor novophasm

Observation of anomalous Hanle spin precession lineshapes resulting from interaction with localized states

It has been shown recently that in spin precession experiments, the interaction of spins with localized states can change the response to a magnetic field, leading to a modified, effective spin relaxation time and precession frequency. Here, we show that also the shape of the Hanle curve can change, so that it cannot be fitted with the solutions of the conventional Bloch equation. We present experimental data that shows such an effect arising at low temperatures in epitaxial graphene on silicon carbide with localized states in the carbon buffer layer. We compare the strength of the effect between materials with different growth methods, epitaxial growth by sublimation and by chemical vapor deposition. The presented analysis gives information about the density of localized states and their coupling to the graphene states, which is inaccessible by charge transport measurements and can be applied to any spin transport channel that is coupled to localized states.Comment: 6 pages, 6 figure

A contagious living fluid: objectification and assemblage in the history of virology

This article deals with the birth of `the virus' as an object of technoscientific analysis. The aim is to discuss the process of objectification of pathogen virulence in virological and medical discourses. Through a short excursion into the history of modern virology, it will be argued that far from being a matter of fact, pathogen virulence had to be `produced', for example in petri-dishes, test-kits and hyper-real signification-practices. The now commonly accepted objective status of `the virus' has been an accomplishment of a complex ensemble of actors. Indeed, this illustrates why objectification rather than objectivity has become the main focus of science and technology studies. The objectification of `the' virus was by no means a smooth process. It involved more than five decades of highly speculative and fragmented research projects before it became actualized as a separate discipline under the heading of virology. The specific objectification of viruses took place through an inter-disciplinary de-differentiation of research questions, methodologies, techniques and technologies. The main argument of this article is that viruses only became intelligible after the establishment of a virology-assemblage. Its inauguration in the early 1950s was radical and sudden because only then could the various substrands of virological technoscience affect each other through deliberate enrolment, and engender a universal intelligibility

Skating on slippery ice

The friction of a stationary moving skate on smooth ice is investigated, in particular in relation to the formation of a thin layer of water between skate and ice. It is found that the combination of ploughing and sliding gives a friction force that is rather insensitive for parameters such as velocity and temperature. The weak dependence originates from the pressure adjustment inside the water layer. For instance, high velocities, which would give rise to high friction, also lead to large pressures, which, in turn, decrease the contact zone and so lower the friction. The theory is a combination and completion of two existing but conflicting theories on the formation of the water layer.Comment: 26 pages, 8 figures Posted at SciPos