2,927 research outputs found
Fringe field simulations of a non-scaling FFAG accelerator
Fixed-field Alternating Gradient (FFAG) accelerators offer the potential of
high-quality, moderate energy ion beams at low cost. Modeling of these
structures is challenging with conventional beam tracking codes because of the
large radial excursions of the beam and the significance of fringe field
effects. Numerous tune resonances are crossed during the acceleration, which
would lead to beam instability and loss in a storage ring. In a non-scaling
FFAG, the hope is that these resonances can be crossed sufficiently rapidly to
prevent beam loss. Simulations are required to see if this is indeed the case.
Here we simulate a non-scaling FFAG which accelerates protons from 31 to 250
MeV. We assume only that the bending magnets have mid-plane symmetry, with
specified vertical bending field in the mid-plane (y=0). The magnetic field can
be obtained everywhere using a power series expansion, and we develop
mathematical tools for calculating this expansion to arbitrary order when the
longitudinal field profile is given by an Enge function. We compare the use of
a conventional hard-edge fringe with a more accurate, soft-edge fringe field
model. The tune 1/3 resonance is the strongest, and crossing it in the
hard-edge fringe model results in a 21% loss of the beam. Using the soft-edge
fringe model the beam loss is less than 6%.Comment: 12 pages; 12 figure
Computational power of correlations
We study the intrinsic computational power of correlations exploited in
measurement-based quantum computation. By defining a general framework the
meaning of the computational power of correlations is made precise. This leads
to a notion of resource states for measurement-based \textit{classical}
computation. Surprisingly, the Greenberger-Horne-Zeilinger and
Clauser-Horne-Shimony-Holt problems emerge as optimal examples. Our work
exposes an intriguing relationship between the violation of local realistic
models and the computational power of entangled resource states.Comment: 4 pages, 2 figures, 2 tables, v2: introduction revised and title
changed to highlight generality of established framework and results, v3:
published version with additional table I
How good are citizen weather stations? Addressing a biased opinion
The UK is home to a dense network of Citizen Weather Stations (CWS) primarily set up by members of the public. The majority of these stations record air temperature, relative humidity and precipitation, amongst other variables, at sub-hourly intervals. This high resolution network could have benefits in many applications, but only if the data quality is well characterised. Here we present results from an intercomparison field study, in which popular CWS models were tested against Met Office standard equipment. The study identifies some common instrumental biases and their dependencies, which will help us to quantify and correct such biases from the CWS network
Generalized Bell Inequality Experiments and Computation
We consider general settings of Bell inequality experiments with many
parties, where each party chooses from a finite number of measurement settings
each with a finite number of outcomes. We investigate the constraints that Bell
inequalities place upon the correlations possible in a local hidden variable
theories using a geometrical picture of correlations. We show that local hidden
variable theories can be characterized in terms of limited computational
expressiveness, which allows us to characterize families of Bell inequalities.
The limited computational expressiveness for many settings (each with many
outcomes) generalizes previous results about the many-party situation each with
a choice of two possible measurements (each with two outcomes). Using this
computational picture we present generalizations of the Popescu-Rohrlich
non-local box for many parties and non-binary inputs and outputs at each site.
Finally, we comment on the effect of pre-processing on measurement data in our
generalized setting and show that it becomes problematic outside of the binary
setting, in that it allows local hidden variable theories to simulate maximally
non-local correlations such as those of these generalised Popescu-Rohrlich
non-local boxes.Comment: 16 pages, 2 figures, supplemental material available upon request.
Typos corrected and references adde
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