2,838 research outputs found
Evaluating the impact of a longitudinal patient case on the development of professionalism and professional identity
This article explores the student outcomes of a progressive case on the development of professional identity and professionalism within first-year student pharmacists
Characteristics of Two-Dimensional Quantum Turbulence in a Compressible Superfluid
Under suitable forcing a fluid exhibits turbulence, with characteristics
strongly affected by the fluid's confining geometry. Here we study
two-dimensional quantum turbulence in a highly oblate Bose-Einstein condensate
in an annular trap. As a compressible quantum fluid, this system affords a rich
phenomenology, allowing coupling between vortex and acoustic energy.
Small-scale stirring generates an experimentally observed disordered vortex
distribution that evolves into large-scale flow in the form of a persistent
current. Numerical simulation of the experiment reveals additional
characteristics of two-dimensional quantum turbulence: spontaneous clustering
of same-circulation vortices, and an incompressible energy spectrum with
dependence for low wavenumbers and dependence for high
.Comment: 7 pages, 7 figures. Reference [29] updated for v
The Precision Array for Probing the Epoch of Reionization: 8 Station Results
We are developing the Precision Array for Probing the Epoch of Reionization
(PAPER) to detect 21cm emission from the early Universe, when the first stars
and galaxies were forming. We describe the overall experiment strategy and
architecture and summarize two PAPER deployments: a 4-antenna array in the
low-RFI environment of Western Australia and an 8-antenna array at our
prototyping site in Green Bank, WV. From these activities we report on system
performance, including primary beam model verification, dependence of system
gain on ambient temperature, measurements of receiver and overall system
temperatures, and characterization of the RFI environment at each deployment
site.
We present an all-sky map synthesized between 139 MHz and 174 MHz using data
from both arrays that reaches down to 80 mJy (4.9 K, for a beam size of 2.15e-5
steradians at 154 MHz), with a 10 mJy (620 mK) thermal noise level that
indicates what would be achievable with better foreground subtraction. We
calculate angular power spectra () in a cold patch and determine them
to be dominated by point sources, but with contributions from galactic
synchrotron emission at lower radio frequencies and angular wavemodes. Although
the cosmic variance of foregrounds dominates errors in these power spectra, we
measure a thermal noise level of 310 mK at for a 1.46-MHz band
centered at 164.5 MHz. This sensitivity level is approximately three orders of
magnitude in temperature above the level of the fluctuations in 21cm emission
associated with reionization.Comment: 13 pages, 14 figures, submitted to AJ. Revision 2 corrects a scaling
error in the x axis of Fig. 12 that lowers the calculated power spectrum
temperatur
Quantum state of two trapped Bose-Einstein condensates with a Josephson coupling
We consider the precise quantum state of two trapped, coupled Bose Einstein
condensates in the two-mode approximation. We seek a representation of the
state in terms of a Wigner-like distribution on the two-mode Bloch sphere. The
problem is solved using a self-consistent rotation of the unknown state to the
south pole of the sphere. The two-mode Hamiltonian is projected onto the
harmonic oscillator phase plane, where it can be solved by standard techniques.
Our results show how the number of atoms in each trap and the squeezing in the
number difference depend on the physical parameters. Considering negative
scattering lengths, we show that there is a regime of squeezing in the relative
phase of the condensates which occurs for weaker interactions than the
superposition states found by Cirac et al% (quant-ph/9706034, 13 June 1997).
The phase squeezing is also apparent in mildly asymmetric trap configurations.Comment: 26 pages, 11 figure
Finite-temperature dynamics of a single vortex in a Bose-Einstein condensate: Equilibrium precession and rotational symmetry breaking
We consider a finite-temperature Bose-Einstein condensate in a
quasi-two-dimensional trap containing a single precessing vortex. We find that
such a configuration arises naturally as an ergodic equilibrium of the
projected Gross-Pitaevskii equation, when constrained to a finite conserved
angular momentum. In an isotropic trapping potential the condensation of the
classical field into an off-axis vortex state breaks the rotational symmetry of
the system. We present a methodology to identify the condensate and the
Goldstone mode associated with the broken rotational symmetry in the
classical-field model. We also examine the variation in vortex trajectories and
thermodynamic parameters of the field as the energy of the microcanonical field
simulation is varied.Comment: 21 pages, 10 figures. v2: Minor changes and corrections to figures
and text. To appear in PR
Dynamical thermalization and vortex formation in stirred 2D Bose-Einstein condensates
We present a quantum mechanical treatment of the mechanical stirring of
Bose-Einstein condensates using classical field techniques. In our approach the
condensate and excited modes are described using a Hamiltonian classical field
method in which the atom number and (rotating frame) energy are strictly
conserved. We simulate a T = 0 quasi-2D condensate perturbed by a rotating
anisotropic trapping potential. Vacuum fluctuations in the initial state
provide an irreducible mechanism for breaking the initial symmetries of the
condensate and seeding the subsequent dynamical instability. Highly turbulent
motion develops and we quantify the emergence of a rotating thermal component
that provides the dissipation necessary for the nucleation and motional-damping
of vortices in the condensate. Vortex lattice formation is not observed, rather
the vortices assemble into a spatially disordered vortex liquid state. We
discuss methods we have developed to identify the condensate in the presence of
an irregular distribution of vortices, determine the thermodynamic parameters
of the thermal component, and extract damping rates from the classical field
trajectories.Comment: 22 pages, 15 figures. v2: Minor refinements made at suggestion of
referee. Discussion of other treatments revised. To appear in Phys. Rev.
Quantum Dynamics of Three Coupled Atomic Bose-Einstein Condensates
The simplest model of three coupled Bose-Einstein Condensates (BEC) is
investigated using a group theoretical method. The stationary solutions are
determined using the SU(3) group under the mean field approximation. This
semiclassical analysis using the system symmetries shows a transition in the
dynamics of the system from self trapping to delocalization at a critical value
for the coupling between the condensates. The global dynamics are investigated
by examination of the stable points and our analysis shows the structure of the
stable points depends on the ratio of the condensate coupling to the
particle-particle interaction, undergoes bifurcations as this ratio is varied.
This semiclassical model is compared to a full quantum treatment, which also
displays the dynamical transition. The quantum case has collapse and revival
sequences superposed on the semiclassical dynamics reflecting the underlying
discreteness of the spectrum. Non-zero circular current states are also
demonstrated as one of the higher dimensional effects displayed in this system.Comment: Accepted to PR
Measurements of Relative Phase in Binary Mixtures of Bose-Einstein Condensates
We have measured the relative phase of two Bose-Einstein condensates (BEC)
using a time-domain separated-oscillatory-field condensate interferometer. A
single two-photon coupling pulse prepares the double condensate system with a
well-defined relative phase; at a later time, a second pulse reads out the
phase difference accumulated between the two condensates. We find that the
accumulated phase difference reproduces from realization to realization of the
experiment, even after the individual components have spatially separated and
their relative center-of-mass motion has damped.Comment: 12 pages, 3 figure
Spatial coherence and density correlations of trapped Bose gases
We study first and second order coherence of trapped dilute Bose gases using
appropriate correlation functions. Special attention is given to the discussion
of second order or density correlations. Except for a small region around the
surface of a Bose-Einstein condensate the correlations can be accurately
described as those of a locally homogeneous gas with a spatially varying
chemical potential. The degrees of first and second order coherence are
therefore functions of temperature, chemical potential, and position. The
second order correlation function is governed both by the tendency of bosonic
atoms to cluster and by a strong repulsion at small distances due to atomic
interactions. In present experiments both effects are of comparable magnitude.
Below the critical temperature the range of the bosonic correlation is affected
by the presence of collective quasi-particle excitations. The results of some
recent experiments on second and third order coherence are discussed. It is
shown that the relation between the measured quantities and the correlation
functions is much weaker than previously assumed.Comment: RevTeX, 25 pages with 7 Postscript figure
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