45,692 research outputs found
Marginal distributions in -dimensional phase space and the quantum marginal theorem
We study the problem of constructing a probability density in 2N-dimensional
phase space which reproduces a given collection of joint probability
distributions as marginals. Only distributions authorized by quantum mechanics,
i.e. depending on a (complete) commuting set of variables, are considered.
A diagrammatic or graph theoretic formulation of the problem is developed. We
then exactly determine the set of ``admissible'' data, i.e. those types of data
for which the problem always admits solutions. This is done in the case where
the joint distributions originate from quantum mechanics as well as in the case
where this constraint is not imposed. In particular, it is shown that a
necessary (but not sufficient) condition for the existence of solutions is
. When the data are admissible and the quantum constraint is not
imposed, the general solution for the phase space density is determined
explicitly. For admissible data of a quantum origin, the general solution is
given in certain (but not all) cases. In the remaining cases, only a subset of
solutions is obtained.Comment: 29 pages (Work supported by the Indo-French Centre for the Promotion
of Advanced Research, Project Nb 1501-02). v2 to add a report-n
Grid infrastructures for the electronics domain: requirements and early prototypes from an EPSRC pilot project
The fundamental challenges facing future electronics design is to address the decreasing â atomistic - scale of transistor devices and to understand and predict the impact and statistical variability these have on design of circuits and systems. The EPSRC pilot project âMeeting the Design Challenges of nanoCMOS Electronicsâ (nanoCMOS) which began in October 2006 has been funded to explore this space. This paper outlines the key requirements that need to be addressed for Grid technology to support the various research strands in this domain, and shows early prototypes demonstrating how these requirements are being addressed
Integrated atomistic process and device simulation of decananometre MOSFETs
In this paper we present a methodology for the integrated atomistic process and device simulation of decananometre MOSFETs. The atomistic process simulations were carried out using the kinetic Monte Carlo process simulator DADOS, which is now integrated into the Synopsys 3D process and device simulation suite Taurus. The device simulations were performed using the Glasgow 3D statistical atomistic simulator, which incorporates density gradient quantum corrections. The overall methodology is illustrated in the atomistic process and device simulation of a well behaved 35 nm physical gate length MOSFET reported by Toshiba
Helium nuclei around the neutron drip line
Neutron rich He nuclei have been investigated using relativistic mean field
approach in co-ordinate space. Elastic partial scattering cross sections for
proton scattering in inverse kinematics have been calculated using the
theoretically obtained density for He and compared with experiment. The
energies of the low-lying resonance states in the neutron unstable nuclei
He have also been calculated and compared with experimental
observations.Comment: To appear in Physical Review
Towards a grid-enabled simulation framework for nano-CMOS electronics
The electronics design industry is facing major challenges as transistors continue to decrease in size. The next generation of devices will be so small that the position of individual atoms will affect their behaviour. This will cause the transistors on a chip to have highly variable characteristics, which in turn will impact circuit and system design tools. The EPSRC project "Meeting the Design Challenges of Nano-CMOS Electronics" (Nana-CMOS) has been funded to explore this area. In this paper, we describe the distributed data-management and computing framework under development within Nano-CMOS. A key aspect of this framework is the need for robust and reliable security mechanisms that support distributed electronics design groups who wish to collaborate by sharing designs, simulations, workflows, datasets and computation resources. This paper presents the system design, and an early prototype of the project which has been useful in helping us to understand the benefits of such a grid infrastructure. In particular, we also present two typical use cases: user authentication, and execution of large-scale device simulations
Competition, quality and contract compliance: evidence from compulsory competitive tendering in local government in Great Britain, 1987-2000
The introduction of competition has frequently been found to cause costs to fall. There has, however, been a question as to whether this was partly achieved at the cost of quality. Auction theory predicts prices would fall more the greater the competition to provide the service. There has been some debate about whether the smaller budgets would make contract compliance more difficult. Evidence is found in support of this hypothesis. We also find some evidence that the better recorded performance of the in-house direct service organisations (DSOs) during this period was due to the information advantage they had from being incumbents
Strong associations between microbe phenotypes and their network architecture
Understanding the dependence and interplay between architecture and function
in biological networks has great relevance to disease progression, biological
fabrication and biological systems in general. We propose methods to assess the
association of various microbe characteristics and phenotypes with the topology
of their networks. We adopt an automated approach to characterize metabolic
networks of 32 microbial species using 11 topological metrics from complex
networks. Clustering allows us to extract the indispensable, independent and
informative metrics. Using hierarchical linear modeling, we identify relevant
subgroups of these metrics and establish that they associate with microbial
phenotypes surprisingly well. This work can serve as a stepping stone to
cataloging biologically relevant topological properties of networks and towards
better modeling of phenotypes. The methods we use can also be applied to
networks from other disciplines.Comment: Replaced by the version scheduled to appear in Phys. Rev. E (Rapid
Comm.
Bell Inequalities in Four Dimensional Phase Space and the Three Marginal Theorem
We address the classical and quantum marginal problems, namely the question
of simultaneous realizability through a common probability density in phase
space of a given set of compatible probability distributions. We consider only
distributions authorized by quantum mechanics, i.e. those corresponding to
complete commuting sets of observables. For four-dimensional phase space with
position variables qi and momentum variables pj, we establish the two following
points: i) given four compatible probabilities for (q1,q2), (q1,p2), (p1,q2)
and (p1,p2), there does not always exist a positive phase space density
rho({qi},{pj}) reproducing them as marginals; this settles a long standing
conjecture; it is achieved by first deriving Bell-like inequalities in phase
space which have their own theoretical and experimental interest. ii) given
instead at most three compatible probabilities, there always exist an
associated phase space density rho({qi},{pj}); the solution is not unique and
its general form is worked out. These two points constitute our ``three
marginal theorem''.Comment: 21 pages, Latex, no figure
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