1,893 research outputs found
Transverse excitations of ultracold matter waves upon propagation past abrupt waveguide changes
The propagation of ultracold atomic gases through abruptly changing waveguide
potentials is examined in the limit of non-interacting atoms. Time-independent
scattering calculations of microstructured waveguides with discontinuous
changes in the transverse harmonic binding potentials are used to mimic
waveguide perturbations and imperfections. Three basic configurations are
examined: step-like, barrier-like and well-like with waves incident in the
ground mode. At low energies, the spectra rapidly depart from single-moded,
with significant transmission and reflection of excited modes. The high-energy
limit sees 100 percent transmission, with the distribution of the transmitted
modes determined simply by the overlap of the mode wave functions and
interference.Comment: 20 pages, 7 figures, under review PR
Developing evidence-based practice: the role of case-based research
How can practitioners engage in evidence-based practice when the evidence for effectiveness of psychological treatments comes from randomized controlled trials using patient populations different from those encountered in everyday settings and treatment manuals that seem oversimplified and inflexible? The authors argue that important evidence about best practice comes from case-based research, which builds knowledge in a clinically useful manner and complements what is achieved by multivariate research methods. A multidimensional model of the research process is provided that includes clinical practice and case-based research as significant contributors. The authors summarize the principles of case-based research and provide examples of recent technical advances. Finally, the authors suggest ways in which practitioners can apply the case-based approach in researching and publishing their own cases, perhaps in collaboration with university-based researchers
Kinematics of the swimming of Spiroplasma
\emph{Spiroplasma} swimming is studied with a simple model based on
resistive-force theory. Specifically, we consider a bacterium shaped in the
form of a helix that propagates traveling-wave distortions which flip the
handedness of the helical cell body. We treat cell length, pitch angle, kink
velocity, and distance between kinks as parameters and calculate the swimming
velocity that arises due to the distortions. We find that, for a fixed pitch
angle, scaling collapses the swimming velocity (and the swimming efficiency) to
a universal curve that depends only on the ratio of the distance between kinks
to the cell length. Simultaneously optimizing the swimming efficiency with
respect to inter-kink length and pitch angle, we find that the optimal pitch
angle is 35.5 and the optimal inter-kink length ratio is 0.338, values
in good agreement with experimental observations.Comment: 4 pages, 5 figure
Signature of the Simplicial Supermetric
We investigate the signature of the Lund-Regge metric on spaces of simplicial
three-geometries which are important in some formulations of quantum gravity.
Tetrahedra can be joined together to make a three-dimensional piecewise linear
manifold. A metric on this manifold is specified by assigning a flat metric to
the interior of the tetrahedra and values to their squared edge-lengths. The
subset of the space of squared edge-lengths obeying triangle and analogous
inequalities is simplicial configuration space. We derive the Lund-Regge metric
on simplicial configuration space and show how it provides the shortest
distance between simplicial three-geometries among all choices of gauge inside
the simplices for defining this metric (Regge gauge freedom). We show
analytically that there is always at least one physical timelike direction in
simplicial configuration space and provide a lower bound on the number of
spacelike directions. We show that in the neighborhood of points in this space
corresponding to flat metrics there are spacelike directions corresponding to
gauge freedom in assigning the edge-lengths. We evaluate the signature
numerically for the simplicial configuration spaces based on some simple
triangulations of the three-sphere (S^3) and three-torus (T^3). For the surface
of a four-simplex triangulation of S^3 we find one timelike direction and all
the rest spacelike over all of the simplicial configuration space. For the
triangulation of T^3 around flat space we find degeneracies in the simplicial
supermetric as well as a few gauge modes corresponding to a positive
eigenvalue. Moreover, we have determined that some of the negative eigenvalues
are physical, i.e. the corresponding eigenvectors are not generators of
diffeomorphisms. We compare our results with the known properties of continuum
superspace.Comment: 24 pages, RevTeX, 4 eps Figures. Submitted to Classical Quantum
Gravit
Frozen quantum coherence
We analyze under which dynamical conditions the coherence of an open quantum system is totally unaffected by noise. For a single qubit, specific measures of coherence are found to freeze under different conditions, with no general agreement between them. Conversely, for an N-qubit system with even N, we identify universal conditions in terms of initial states and local incoherent channels such that all bona fide distance-based coherence monotones are left invariant during the entire evolution. This finding also provides an insightful physical interpretation for the freezing phenomenon of quantum correlations beyond entanglement. We further obtain analytical results for distance-based measures of coherence in two-qubit states with maximally mixed marginals
Quantitative complementarity in two-path interferometry
The quantitative formulation of Bohr's complementarity proposed by
Greenberger and Yasin is applied to some physical situations for which
analytical expressions are available. This includes a variety of conventional
double-slit experiments, but also particle oscillations, as in the case of the
neutral-kaon system, and Mott scattering of identical nuclei. For all these
cases, a unified description can be achieved including a new parameter, ,
which quantifies the effective number of fringes one can observe in each
specific interferometric set-up.Comment: 11 RevTex pages, 5 figure
The Periodic Standing-Wave Approximation: Overview and Three Dimensional Scalar Models
The periodic standing-wave method for binary inspiral computes the exact
numerical solution for periodic binary motion with standing gravitational
waves, and uses it as an approximation to slow binary inspiral with outgoing
waves. Important features of this method presented here are: (i) the
mathematical nature of the ``mixed'' partial differential equations to be
solved, (ii) the meaning of standing waves in the method, (iii) computational
difficulties, and (iv) the ``effective linearity'' that ultimately justifies
the approximation. The method is applied to three dimensional nonlinear scalar
model problems, and the numerical results are used to demonstrate extraction of
the outgoing solution from the standing-wave solution, and the role of
effective linearity.Comment: 13 pages RevTeX, 5 figures. New version. A revised form of the
nonlinearity produces better result
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