8,962 research outputs found
Bragg Spectroscopy of Cold Atomic Fermi Gases
We propose a Bragg spectroscopy experiment to measure the onset of superfluid
pairing in ultracold trapped Fermi gases. In particular, we study two component
Fermi gases in the weak coupling BCS and BEC limits as well as in the strong
coupling unitarity limit. The low temperature Bragg spectrum exhibits a gap
directly related to the pair-breaking energy. Furthermore, the Bragg spectrum
has a large maximum just below the critical temperature when the gas is
superfluid in the BCS limit. In the unitarity regime, we show how the pseudogap
in the normal phase leads to a significant suppression of the low frequency
Bragg spectrum.Comment: 8 pages, 9 figures. Typos corrected. Reference update
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Centralized content delivery infrastructure exploiting resource pools : performance models and asymptotics
textWe consider a centralized content delivery infrastructure where a large number of storage-intensive files are replicated across several collocated servers. To achieve scalable delays in file downloads under stochastic loads, we allow multiple servers to work together as a pooled resource to meet individual download requests. In such systems basic questions include: How and where to replicate files? How significant are the gains of resource pooling over policies which use single server per request? What are the tradeoffs among conflicting metrics such as delays, reliability and recovery costs, and power? How robust is performance to heterogeneity and choice of fairness criterion? In this thesis we provide a simple performance model for large systems towards addressing these basic questions. For large systems where the overall system load is proportional to the number of servers, we establish scaling laws among delays, system load, number of file replicas, demand heterogeneity, power, and network capacity.Electrical and Computer Engineerin
Particle Filtering for Large Dimensional State Spaces with Multimodal Observation Likelihoods
We study efficient importance sampling techniques for particle filtering (PF)
when either (a) the observation likelihood (OL) is frequently multimodal or
heavy-tailed, or (b) the state space dimension is large or both. When the OL is
multimodal, but the state transition pdf (STP) is narrow enough, the optimal
importance density is usually unimodal. Under this assumption, many techniques
have been proposed. But when the STP is broad, this assumption does not hold.
We study how existing techniques can be generalized to situations where the
optimal importance density is multimodal, but is unimodal conditioned on a part
of the state vector. Sufficient conditions to test for the unimodality of this
conditional posterior are derived. The number of particles, N, to accurately
track using a PF increases with state space dimension, thus making any regular
PF impractical for large dimensional tracking problems. We propose a solution
that partially addresses this problem. An important class of large dimensional
problems with multimodal OL is tracking spatially varying physical quantities
such as temperature or pressure in a large area using a network of sensors
which may be nonlinear and/or may have non-negligible failure probabilities.Comment: To appear in IEEE Trans. Signal Processin
Harnessing entropy to enhance toughness in reversibly crosslinked polymer networks
Reversible crosslinking is a design paradigm for polymeric materials, wherein
they are microscopically reinforced with chemical species that form transient
crosslinks between the polymer chains. Besides the potential for self-healing,
recent experimental work suggests that freely diffusing reversible crosslinks
in polymer networks, such as gels, can enhance the toughness of the material
without substantial change in elasticity. This presents the opportunity for
making highly elastic materials that can be strained to a large extent before
rupturing. Here, we employ Gaussian chain theory, molecular simulation, and
polymer self-consistent field theory for networks to construct an equilibrium
picture for how reversible crosslinks can toughen a polymer network without
affecting its linear elasticity. Maximisation of polymer entropy drives the
reversible crosslinks to bind preferentially near the permanent crosslinks in
the network, leading to local molecular reinforcement without significant
alteration of the network topology. In equilibrium conditions, permanent
crosslinks share effectively the load with neighbouring reversible crosslinks,
forming multi-functional crosslink points. The network is thereby globally
toughened, while the linear elasticity is left largely unaltered. Practical
guidelines are proposed to optimise this design in experiment, along with a
discussion of key kinetic and timescale considerations
Simultaneous Exoplanet Characterization and deep wide-field imaging with a diffractive pupil telescope
High-precision astrometry can identify exoplanets and measure their orbits
and masses, while coronagraphic imaging enables detailed characterization of
their physical properties and atmospheric compositions through spectroscopy. In
a previous paper, we showed that a diffractive pupil telescope (DPT) in space
can enable sub-microarcsecond accuracy astrometric measurements from wide-field
images by creating faint but sharp diffraction spikes around the bright target
star. The DPT allows simultaneous astrometric measurement and coronagraphic
imaging, and we discuss and quantify in this paper the scientific benefits of
this combination for exoplanet science investigations: identification of
exoplanets with increased sensitivity and robustness, and ability to measure
planetary masses to high accuracy. We show how using both measurements to
identify planets and measure their masses offers greater sensitivity and
provides more reliable measurements than possible with separate missions, and
therefore results in a large gain in mission efficiency. The combined
measurements reliably identify potentially habitable planets in multiple
systems with a few observations, while astrometry or imaging alone would
require many measurements over a long time baseline. In addition, the combined
measurement allows direct determination of stellar masses to percent-level
accuracy, using planets as test particles. We also show that the DPT maintains
the full sensitivity of the telescope for deep wide-field imaging, and is
therefore compatible with simultaneous scientific observations unrelated to
exoplanets. We conclude that astrometry, coronagraphy, and deep wide-field
imaging can be performed simultaneously on a single telescope without
significant negative impact on the performance of any of the three techniques.Comment: 15 pages, 6 figures. This second paper, following the paper
describing the diffractive pupil telescope (DPT) astrometric technique, shows
how simultaneous astrometry and coronagraphy observations, enabled by the DPT
concept, constrain the orbital parameters and mass of exoplanet
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