5,117 research outputs found
ScotPID - a model of collaboration
ScotPID is a national personal development initiative in Scotland, with thirteen higher education institutions taking part in the development of case studies which enhance personal development planning for students. As a model of collaboration, ScotPID involves all stakeholders: each core project group is composed of an academic, IT support manager, careers service adviser and undergraduate student, with support from QAA Scotland. The case study is developed by the contribution of all of the members of the team. The strength of the ScotPID collaboration is the varied background of the team members. However, collaboration between the ScotPID teams should also be encouraged, to strengthen the inter-institutional approach further
Numerical Methods for Stochastic Differential Equations
Stochastic differential equations (sdes) play an important role in physics
but existing numerical methods for solving such equations are of low accuracy
and poor stability. A general strategy for developing accurate and efficient
schemes for solving stochastic equations in outlined here. High order numerical
methods are developed for integration of stochastic differential equations with
strong solutions. We demonstrate the accuracy of the resulting integration
schemes by computing the errors in approximate solutions for sdes which have
known exact solutions
Estimation of drift and diffusion functions from time series data: A maximum likelihood framework
Complex systems are characterized by a huge number of degrees of freedom
often interacting in a non-linear manner. In many cases macroscopic states,
however, can be characterized by a small number of order parameters that obey
stochastic dynamics in time. Recently techniques for the estimation of the
corresponding stochastic differential equations from measured data have been
introduced. This contribution develops a framework for the estimation of the
functions and their respective (Bayesian posterior) confidence regions based on
likelihood estimators. In succession approximations are introduced that
significantly improve the efficiency of the estimation procedure. While being
consistent with standard approaches to the problem this contribution solves
important problems concerning the applicability and the accuracy of estimated
parameters.Comment: 18 pages, 2 figure
Positive-P phase space method simulation in superradiant emission from a cascade atomic ensemble
The superradiant emission properties from an atomic ensemble with cascade
level configuration is numerically simulated. The correlated spontaneous
emissions (signal then idler fields) are purely stochastic processes which are
initiated by quantum fluctuations. We utilize the positive-P phase space method
to investigate the dynamics of the atoms and counter-propagating emissions. The
light field intensities are calculated, and the signal-idler correlation
function is studied for different optical depths of the atomic ensemble.
Shorter correlation time scale for a denser atomic ensemble implies a broader
spectral window needed to store or retrieve the idler pulse.Comment: To be published in Phys. Rev.
Sympathetic cooling of trapped fermions by bosons in the presence of particle losses
We study the sympathetic cooling of a trapped Fermi gas interacting with an
ideal Bose gas below the critical temperature of the Bose-Einstein
condensation. We derive the quantum master equation, which describes the
dynamics of the fermionic component, and postulating the thermal distribution
for both gases we calculate analytically the rate at which fermions are cooled
by the bosonic atoms. The particle losses constitute an important source of
heating of the degenerate Fermi gas. We evaluate the rate of loss-induced
heating and derive analytical results for the final temperature of fermions,
which is limited in the presence of particle losses.Comment: 7 pages, 2 figures, EPL style; final versio
Opacity of electromagnetically induced transparency for quantum fluctuations
We analyze the propagation of a pair of quantized fields inside a medium of
three-level atoms in configuration. We calculate the stationary
quadrature noise spectrum of the field after propagating through the medium, in
the case where the probe field is in a squeezed state and the atoms show
electromagnetically induced transparency (EIT). We find an oscillatory transfer
of the initial quantum properties between the probe and pump fields which is
most strongly pronounced when both fields have comparable Rabi frequencies.
This implies that the quantum state measured after propagation can be
completely different from the initial state, even though the mean values of the
field are unaltered
Polariton Squeezing in Semiconductor Microcavities
We report squeezed polariton generation using parametric polariton four-wave
mixing in semiconductor microcavities in the strong coupling regime. The
geometry of the experiment corresponds to degenerate four-wave mixing, which
gives rise to a bistability threshold. Spatial effects in the nonlinear regime
are evidenced, and spatial filtering is required in order to optimize the
measured squeezing. By measuring the noise of the outgoing light, we infer a 9
percent squeezing on the polariton field close to the bistability turning
point
Exact and approximate many-body dynamics with stochastic one-body density matrix evolution
We show that the dynamics of interacting fermions can be exactly replaced by
a quantum jump theory in the many-body density matrix space. In this theory,
jumps occur between densities formed of pairs of Slater determinants, , where each state evolves according to the Stochastic
Schr\"odinger Equation (SSE) given in ref. \cite{Jul02}. A stochastic
Liouville-von Neumann equation is derived as well as the associated
Bogolyubov-Born-Green-Kirwood-Yvon (BBGKY) hierarchy. Due to the specific form
of the many-body density along the path, the presented theory is equivalent to
a stochastic theory in one-body density matrix space, in which each density
matrix evolves according to its own mean field augmented by a one-body noise.
Guided by the exact reformulation, a stochastic mean field dynamics valid in
the weak coupling approximation is proposed. This theory leads to an
approximate treatment of two-body effects similar to the extended
Time-Dependent Hartree-Fock (Extended TDHF) scheme. In this stochastic mean
field dynamics, statistical mixing can be directly considered and jumps occur
on a coarse-grained time scale. Accordingly, numerical effort is expected to be
significantly reduced for applications.Comment: 12 pages, 1 figur
A practical scheme for error control using feedback
We describe a scheme for quantum error correction that employs feedback and
weak measurement rather than the standard tools of projective measurement and
fast controlled unitary gates. The advantage of this scheme over previous
protocols (for example Ahn et. al, PRA, 65, 042301 (2001)), is that it requires
little side processing while remaining robust to measurement inefficiency, and
is therefore considerably more practical. We evaluate the performance of our
scheme by simulating the correction of bit-flips. We also consider
implementation in a solid-state quantum computation architecture and estimate
the maximal error rate which could be corrected with current technology.Comment: 12 pages, 3 figures. Minor typographic change
Sliding friction between an elastomer network and a grafted polymer layer: the role of cooperative effects
We study the friction between a flat solid surface where polymer chains have
been end-grafted and a cross-linked elastomer at low sliding velocity. The
contribution of isolated grafted chains' penetration in the sliding elastomer
has been early identified as a weakly velocity dependent pull-out force. Recent
experiments have shown that the interactions between the grafted chains at high
grafting density modify the friction force by grafted chain. We develop here a
simple model that takes into account those interactions and gives a limit
grafting density beyond which the friction no longer increases with the
grafting density, in good agreement with the experimental dataComment: Submitted to Europhys. Letter
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