5,642 research outputs found
De-Quantising the Solution of Deutsch's Problem
Probably the simplest and most frequently used way to illustrate the power of
quantum computing is to solve the so-called {\it Deutsch's problem}. Consider a
Boolean function and suppose that we have a
(classical) black box to compute it. The problem asks whether is constant
(that is, ) or balanced (). Classically, to solve
the problem seems to require the computation of and , and then
the comparison of results. Is it possible to solve the problem with {\em only
one} query on ? In a famous paper published in 1985, Deutsch posed the
problem and obtained a ``quantum'' {\em partial affirmative answer}. In 1998 a
complete, probability-one solution was presented by Cleve, Ekert, Macchiavello,
and Mosca. Here we will show that the quantum solution can be {\it
de-quantised} to a deterministic simpler solution which is as efficient as the
quantum one. The use of ``superposition'', a key ingredient of quantum
algorithm, is--in this specific case--classically available.Comment: 8 page
Persistent accelerations disentangle Lagrangian turbulence
Particles in turbulence frequently encounter extreme accelerations between
extended periods of quiescence. The occurrence of extreme events is closely
related to the intermittent spatial distribution of intense flow structures
such as vorticity filaments. This mixed history of flow conditions leads to
very complex particle statistics with a pronounced scale dependence, which
presents one of the major challenges on the way to a non-equilibrium
statistical mechanics of turbulence. Here, we introduce the notion of
persistent Lagrangian acceleration, quantified by the squared particle
acceleration coarse-grained over a viscous time scale. Conditioning Lagrangian
particle data from simulations on this coarse-grained acceleration, we find
remarkably simple, close-to-Gaussian statistics for a range of Reynolds
numbers. This opens the possibility to decompose the complex particle
statistics into much simpler sub-ensembles. Based on this observation, we
develop a comprehensive theoretical framework for Lagrangian single-particle
statistics that captures the acceleration, velocity increments as well as
single-particle dispersion
On the Thermodynamics of NUT charged spaces
We discuss and compare at length the results of two methods used recently to
describe the thermodynamics of Taub-NUT solutions in a deSitter background. In
the first approach (\mathbb{% C}-approach), one deals with an analytically
continued version of the metric while in the second approach
(-approach), the discussion is carried out using the unmodified
metric with Lorentzian signature. No analytic continuation is performed on the
coordinates and/or the parameters that appear in the metric. We find that the
results of both these approaches are completely equivalent modulo analytic
continuation and we provide the exact prescription that relates the results in
both methods. The extension of these results to the AdS/flat cases aims to give
a physical interpretation of the thermodynamics of nut-charged spacetimes in
the Lorentzian sector. We also briefly discuss the higher dimensional spaces
and note that, analogous with the absence of hyperbolic nuts in AdS
backgrounds, there are no spherical Taub-Nut-dS solutions.Comment: 35pages, 4 figures. v.4 references added,few typos corrected, to
appear in Phys. Rev.
Natural Halting Probabilities, Partial Randomness, and Zeta Functions
We introduce the zeta number, natural halting probability and natural
complexity of a Turing machine and we relate them to Chaitin's Omega number,
halting probability, and program-size complexity. A classification of Turing
machines according to their zeta numbers is proposed: divergent, convergent and
tuatara. We prove the existence of universal convergent and tuatara machines.
Various results on (algorithmic) randomness and partial randomness are proved.
For example, we show that the zeta number of a universal tuatara machine is
c.e. and random. A new type of partial randomness, asymptotic randomness, is
introduced. Finally we show that in contrast to classical (algorithmic)
randomness--which cannot be naturally characterised in terms of plain
complexity--asymptotic randomness admits such a characterisation.Comment: Accepted for publication in Information and Computin
Massive stealth scalar fields from field redefinition method
We propose an uni-parametric deformation method of action principles of
scalar fields coupled to gravity which generates new models with massive
stealth field configurations, i.e. with vanishing energy-momentum tensor. The
method applies to a wide class of models and we provide three examples. In
particular we observe that in the case of the standard massive scalar action
principle, the respective deformed action contains the stealth configurations
and it preserves the massive ones of the undeformed model. We also observe
that, in this latter example, the effect of the energy-momentum tensor of the
massive (non-stealth) field can be amplified or damped by the deformation
parameter, alternatively the mass of the stealth field.Comment: 12 page
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