16,995 research outputs found
Dynamics of vortex dipoles in anisotropic Bose-Einstein condensates
We study the motion of a vortex dipole in a Bose-Einstein condensate confined
to an anisotropic trap. We focus on a system of ordinary differential equations
describing the vortices' motion, which is in turn a reduced model of the
Gross-Pitaevskii equation describing the condensate's motion. Using a sequence
of canonical changes of variables, we reduce the dimension and simplify the
equations of motion. We uncover two interesting regimes. Near a family of
periodic orbits known as guiding centers, we find that the dynamics is
essentially that of a pendulum coupled to a linear oscillator, leading to
stochastic reversals in the overall direction of rotation of the dipole. Near
the separatrix orbit in the isotropic system, we find other families of
periodic, quasi-periodic, and chaotic trajectories. In a neighborhood of the
guiding center orbits, we derive an explicit iterated map that simplifies the
problem further. Numerical calculations are used to illustrate the phenomena
discovered through the analysis. Using the results from the reduced system we
are able to construct complex periodic orbits in the original, partial
differential equation, mean-field model for Bose-Einstein condensates, which
corroborates the phenomenology observed in the reduced dynamical equations
Multigrid Monte Carlo with higher cycles in the Sine Gordon model
We study the dynamical critical behavior of multigrid Monte Carlo for the two
dimensional Sine Gordon model on lattices up to 128 x 128. Using piecewise
constant interpolation, we perform a W-cycle (gamma=2). We examine whether one
can reduce critical slowing down caused by decreasing acceptance rates on large
blocks by doing more work on coarser lattices. To this end, we choose a higher
cycle with gamma = 4. The results clearly demonstrate that critical slowing
down is not reduced in either case.Comment: 7 pages, 1 figure, whole paper including figure contained in ps-file,
DESY 93-00
Reflective Ghost Imaging through Turbulence
Recent work has indicated that ghost imaging may have applications in
standoff sensing. However, most theoretical work has addressed
transmission-based ghost imaging. To be a viable remote-sensing system, the
ghost imager needs to image rough-surfaced targets in reflection through long,
turbulent optical paths. We develop, within a Gaussian-state framework,
expressions for the spatial resolution, image contrast, and signal-to-noise
ratio of such a system. We consider rough-surfaced targets that create fully
developed speckle in their returns, and Kolmogorov-spectrum turbulence that is
uniformly distributed along all propagation paths. We address both classical
and nonclassical optical sources, as well as a computational ghost imager.Comment: 13 pages, 3 figure
Theoretical Analysis of Acceptance Rates in Multigrid Monte Carlo
We analyze the kinematics of multigrid Monte Carlo algorithms by
investigating acceptance rates for nonlocal Metropolis updates. With the help
of a simple criterion we can decide whether or not a multigrid algorithm will
have a chance to overcome critial slowing down for a given model. Our method is
introduced in the context of spin models. A multigrid Monte Carlo procedure for
nonabelian lattice gauge theory is described, and its kinematics is analyzed in
detail.Comment: 7 pages, no figures, (talk at LATTICE 92 in Amsterdam
An Extinction Study of the Taurus Dark Cloud Complex
We present a study of the detailed distribution of extinction in a region of
the Taurus dark cloud complex. Our study uses new BVR images of the region,
spectral classification data for 95 stars, and IRAS Sky Survey Atlas (ISSA) 60
and 100 micron images. We study the extinction of the region in four different
ways, and we present the first inter-comparison of all these methods, which
are: 1) using the color excess of background stars for which spectral types are
known; 2) using the ISSA 60 and 100 micron images; 3) using star counts; and 4)
using an optical (V and R) version of the average color excess method used by
Lada et al. (1994). We find that all four methods give generally similar
results, with important exceptions. To study the structure in the dust
distribution, we compare the ISSA extinction and the extinction measured for
individual stars. From the comparison, we conclude that in the relatively low
extinction regions studied, with 0.9 < A_V < 3.0 mag (away from filamentary
dark clouds and IRAS cores), there are no fluctuations in the dust column
density greater than 45% (at the 99.7% confidence level), on scales smaller
than 0.2 pc. We also report the discovery of a previously unknown stellar
cluster behind the Taurus dark cloud near R.A 4h19m00s, Dec. 27:30:00 (B1950)Comment: 49 pages (which include 6 pages of tables and 6 pages of figures
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Human gut Bacteroides capture vitamin B12 via cell surface-exposed lipoproteins.
Human gut Bacteroides use surface-exposed lipoproteins to bind and metabolize complex polysaccharides. Although vitamins and other nutrients are also essential for commensal fitness, much less is known about how commensal bacteria compete with each other or the host for these critical resources. Unlike in Escherichia coli, transport loci for vitamin B12 (cobalamin) and other corrinoids in human gut Bacteroides are replete with conserved genes encoding proteins whose functions are unknown. Here we report that one of these proteins, BtuG, is a surface-exposed lipoprotein that is essential for efficient B12 transport in B. thetaiotaomicron. BtuG binds B12 with femtomolar affinity and can remove B12 from intrinsic factor, a critical B12 transport protein in humans. Our studies suggest that Bacteroides use surface-exposed lipoproteins not only for capturing polysaccharides, but also to acquire key vitamins in the gut
Riemann's theorem for quantum tilted rotors
The angular momentum, angular velocity, Kelvin circulation, and vortex
velocity vectors of a quantum Riemann rotor are proven to be either (1) aligned
with a principal axis or (2) lie in a principal plane of the inertia ellipsoid.
In the second case, the ratios of the components of the Kelvin circulation to
the corresponding components of the angular momentum, and the ratios of the
components of the angular velocity to those of the vortex velocity are analytic
functions of the axes lengths.Comment: 8 pages, Phys. Rev.
Local Nature of Coset Models
The local algebras of the maximal Coset model C_max associated with a chiral
conformal subtheory A\subset B are shown to coincide with the local relative
commutants of A in B, provided A contains a stress energy tensor.
Making the same assumption, the adjoint action of the unique
inner-implementing representation U^A associated with A\subset B on the local
observables in B is found to define net-endomorphisms of B. This property is
exploited for constructing from B a conformally covariant holographic image in
1+1 dimensions which proves useful as a geometric picture for the joint
inclusion A\vee C_max \subset B.
Immediate applications to the analysis of current subalgebras are given and
the relation to normal canonical tensor product subfactors is clarified. A
natural converse of Borchers' theorem on half-sided translations is made
accessible.Comment: 33 pages, no figures; typos, minor improvement
Efficient Manipulation of Bose-Einstein Condensates in a Double-Well Potential
We pose the problem of transferring a Bose-Einstein Condensate (BEC) from one side of a double-well potential to the other as an optimal control problem for determining the time-dependent form of the potential. We derive a reduced dynamical system using a Galerkin truncation onto a finite set of eigenfunctions and find that including three modes suffices to effectively control the full dynamics, described by the Gross-Pitaevskii model of BEC. The functional form of the control is reduced to finite dimensions by using a Galerkin-type method called the chopped random basis (CRAB) method, which is then optimized by a genetic algorithm called differential evolution (DE). Finally, we discuss the the extent to which the reduction-based optimal control strategy can be refined by means of including more modes in the Galerkin reduction
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