850 research outputs found
Implementation of a Quantum Search Algorithm on a Nuclear Magnetic Resonance Quantum Computer
We demonstrate an implementation of a quantum search algorithm on a two qubit
NMR quantum computer based on cytosine.Comment: Six pages, three figure
Geometric quantum computation with NMR
The experimental realisation of the basic constituents of quantum information
processing devices, namely fault-tolerant quantum logic gates, requires
conditional quantum dynamics, in which one subsystem undergoes a coherent
evolution that depends on the quantum state of another subsystem. In
particular, the subsystem may acquire a conditional phase shift. Here we
consider a novel scenario in which this phase is of geometric rather than
dynamical origin. As the conditional geometric (Berry) phase depends only on
the geometry of the path executed it is resilient to certain types of errors,
and offers the potential of an intrinsically fault-tolerant way of performing
quantum gates. Nuclear Magnetic Resonance (NMR) has already been used to
demonstrate both simple quantum information processing and Berry's phase. Here
we report an NMR experiment which implements a conditional Berry phase, and
thus a controlled phase shift gate. This constitutes the first elementary
geometric quantum computation.Comment: Minor additions at request of referees. 4 pages revtex including 2
figures (1 eps). Nature in pres
Entropic Uncertainty Relations in Quantum Physics
Uncertainty relations have become the trademark of quantum theory since they
were formulated by Bohr and Heisenberg. This review covers various
generalizations and extensions of the uncertainty relations in quantum theory
that involve the R\'enyi and the Shannon entropies. The advantages of these
entropic uncertainty relations are pointed out and their more direct connection
to the observed phenomena is emphasized. Several remaining open problems are
mentionedComment: 35 pages, review pape
Experimental simulation of quantum tunneling in small systems
It is well known that quantum computers are superior to classical computers
in efficiently simulating quantum systems. Here we report the first
experimental simulation of quantum tunneling through potential barriers, a
widespread phenomenon of a unique quantum nature, via NMR techniques. Our
experiment is based on a digital particle simulation algorithm and requires
very few spin-1/2 nuclei without the need of ancillary qubits. The occurrence
of quantum tunneling through a barrier, together with the oscillation of the
state in potential wells, are clearly observed through the experimental
results. This experiment has clearly demonstrated the possibility to observe
and study profound physical phenomena within even the reach of small quantum
computers.Comment: 17 pages and 8 figure
Emergent global patterns of ecosystem structure and function from a mechanistic general ecosystem model
Anthropogenic activities are causing widespread degradation of ecosystems worldwide, threatening the ecosystem services upon which all human life depends. Improved understanding of this degradation is urgently needed to improve avoidance and mitigation measures. One tool to assist these efforts is predictive models of ecosystem structure and function that are mechanistic: based on fundamental ecological principles. Here we present the first mechanistic General Ecosystem Model (GEM) of ecosystem structure and function that is both global and applies in all terrestrial and marine environments. Functional forms and parameter values were derived from the theoretical and empirical literature where possible. Simulations of the fate of all organisms with body masses between 10 µg and 150,000 kg (a range of 14 orders of magnitude) across the globe led to emergent properties at individual (e.g., growth rate), community (e.g., biomass turnover rates), ecosystem (e.g., trophic pyramids), and macroecological scales (e.g., global patterns of trophic structure) that are in general agreement with current data and theory. These properties emerged from our encoding of the biology of, and interactions among, individual organisms without any direct constraints on the properties themselves. Our results indicate that ecologists have gathered sufficient information to begin to build realistic, global, and mechanistic models of ecosystems, capable of predicting a diverse range of ecosystem properties and their response to human pressures
Strongly magnetized pulsars: explosive events and evolution
Well before the radio discovery of pulsars offered the first observational
confirmation for their existence (Hewish et al., 1968), it had been suggested
that neutron stars might be endowed with very strong magnetic fields of
-G (Hoyle et al., 1964; Pacini, 1967). It is because of their
magnetic fields that these otherwise small ed inert, cooling dead stars emit
radio pulses and shine in various part of the electromagnetic spectrum. But the
presence of a strong magnetic field has more subtle and sometimes dramatic
consequences: In the last decades of observations indeed, evidence mounted that
it is likely the magnetic field that makes of an isolated neutron star what it
is among the different observational manifestations in which they come. The
contribution of the magnetic field to the energy budget of the neutron star can
be comparable or even exceed the available kinetic energy. The most magnetised
neutron stars in particular, the magnetars, exhibit an amazing assortment of
explosive events, underlining the importance of their magnetic field in their
lives. In this chapter we review the recent observational and theoretical
achievements, which not only confirmed the importance of the magnetic field in
the evolution of neutron stars, but also provide a promising unification scheme
for the different observational manifestations in which they appear. We focus
on the role of their magnetic field as an energy source behind their persistent
emission, but also its critical role in explosive events.Comment: Review commissioned for publication in the White Book of
"NewCompStar" European COST Action MP1304, 43 pages, 8 figure
Evaluating Effects of Divided Hemispheric Processing on Word Recognition in Foveal and Extrafoveal Displays: The Evidence from Arabic
Background: Previous studies have claimed that a precise split at the vertical midline of each fovea causes all words to the left and right of fixation to project to the opposite, contralateral hemisphere, and this division in hemispheric processing has considerable consequences for foveal word recognition. However, research in this area is dominated by the use of stimuli from Latinate languages, which may induce specific effects on performance. Consequently, we report two experiments using stimuli from a fundamentally different, non-Latinate language (Arabic) that offers an alternative way of revealing effects of split-foveal processing, if they exist. Methods and Findings: Words (and pseudowords) were presented to the left or right of fixation, either close to fixation and entirely within foveal vision, or further from fixation and entirely within extrafoveal vision. Fixation location and stimulus presentations were carefully controlled using an eye-tracker linked to a fixation-contingent display. To assess word recognition, Experiment 1 used the Reicher-Wheeler task and Experiment 2 used the lexical decision task. Results: Performance in both experiments indicated a functional division in hemispheric processing for words in extrafoveal locations (in recognition accuracy in Experiment 1 and in reaction times and error rates in Experiment 2) but no such division for words in foveal locations. Conclusions: These findings from a non-Latinate language provide new evidence that although a functional division i
The use of the Nintendo Wii in motor rehabilitation for virtual reality interventions:a literature review
Several review articles have been published on the use of Virtual Reality (VR) in motor rehabilitation. The majority of these focus on the effectiveness of VR on improving motor function using relatively expensive commercial tools and technologies including robotics, cybergloves, cybergrasps, joysticks, force sensors and motion capture systems. However, we present the case in this chapter that game sensors and VR technologies which can be customized and reconfigured, such as the Nintendo Wii, provide an alternative and affordable VR intervention for rehabilitation. While the performance of many of the Wii based interventions in motor rehabilitation are currently the focus of investigation by researchers, an extensive and holistic discussion on this subject does not yet exist. As such, the purpose of this chapter is to provide readers with an understanding of the advantages and limitations of the Nintendo Wii game sensor device (and its associated accessories) for motor rehabilitation and in addition, to outline the potential for incorporating these into clinical interventions for the benefit of patients and therapists
Non-stationary covariance function modelling in 2D least-squares collocation
Standard least-squares collocation (LSC) assumes 2D stationarity and 3D isotropy, and relies on a covariance function to account for spatial dependence in the ob-served data. However, the assumption that the spatial dependence is constant through-out the region of interest may sometimes be violated. Assuming a stationary covariance structure can result in over-smoothing of, e.g., the gravity field in mountains and under-smoothing in great plains. We introduce the kernel convolution method from spatial statistics for non-stationary covariance structures, and demonstrate its advantage fordealing with non-stationarity in geodetic data. We then compared stationary and non-stationary covariance functions in 2D LSC to the empirical example of gravity anomaly interpolation near the Darling Fault, Western Australia, where the field is anisotropic and non-stationary. The results with non-stationary covariance functions are better than standard LSC in terms of formal errors and cross-validation against data not used in the interpolation, demonstrating that the use of non-stationary covariance functions can improve upon standard (stationary) LSC
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