406 research outputs found
A discontinuous Galerkin method for the Vlasov-Poisson system
A discontinuous Galerkin method for approximating the Vlasov-Poisson system
of equations describing the time evolution of a collisionless plasma is
proposed. The method is mass conservative and, in the case that piecewise
constant functions are used as a basis, the method preserves the positivity of
the electron distribution function and weakly enforces continuity of the
electric field through mesh interfaces and boundary conditions. The performance
of the method is investigated by computing several examples and error estimates
associated system's approximation are stated. In particular, computed results
are benchmarked against established theoretical results for linear advection
and the phenomenon of linear Landau damping for both the Maxwell and Lorentz
distributions. Moreover, two nonlinear problems are considered: nonlinear
Landau damping and a version of the two-stream instability are computed. For
the latter, fine scale details of the resulting long-time BGK-like state are
presented. Conservation laws are examined and various comparisons to theory are
made. The results obtained demonstrate that the discontinuous Galerkin method
is a viable option for integrating the Vlasov-Poisson system.Comment: To appear in Journal for Computational Physics, 2011. 63 pages, 86
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Quantum Nature of Light Measured With a Single Detector
We realized the most fundamental quantum optical experiment to prove the
non-classical character of light: Only a single quantum emitter and a single
superconducting nanowire detector were used. A particular appeal of our
experiment is its elegance and simplicity. Yet its results unambiguously
enforce a quantum theory for light. Previous experiments relied on more complex
setups, such as the Hanbury-Brown-Twiss configuration, where a beam splitter
directs light to two photodetectors, giving the false impression that the beam
splitter is required. Our work results in a major simplification of the widely
used photon-correlation techniques with applications ranging from quantum
information processing to single-molecule detection.Comment: 7 page
Micromechanical studies of styrene/butadiene block copolymer blends
The microhardness behavior of binary blends comprising a star block and a triblock copolymer, both consisting of polystyrene (PB) and polybutadiene (PB), was investigated over a wide composition range. In particular, a correlation between the microhardness H, the yield stress a y and the Young's modulus Ε was examined. The hardness was found to correlate with the
mechanical parameters obtained by uniaxial tensile testing as follows: H/ay ~ 2 and Ε/Η ~ 30. In agreement with the studies performed in case of the pure microphase separated block copolymers, the microhardness behavior was found to be strongly dependent on the morphology of the blends. The glass transition temperature of the hard phase (Tg.PS) is shown to remain nearly constant in the blends with lower LN4 content. The glass transition temperature of the soft phase (Tg.PB), which varies with blend composition, is not related to the microhardness variation in the blends.Peer reviewe
Relationship between Nanostructure and Deformation Behavior of Microphase-Separated Styrene/Butadiene Systems
The relationship between the morphology
and the mechanical properties of nanostructured blends,comprising an asymmetric styrene/butadiene star block copolymer,exhibiting cocontinuous-like morphology and low molar mass homopolymers, was studied by electron microscopy,
microindentation hardness, and tensile testing methods. Results show that the deformation behavior of these systems is significantly modified by the presence of unentangled homopolymer chains, leading partly to a decrease in mechanical properties. In contrast to common polymer
blends, in which usually, the hardness values do not markedly deviate from the additivity law, the H-values in the
system investigated show large deviations from the linear additivity behavior. The observed anomalous behavior is discussed in terms of the enhanced local flow processes
induced by the phase-separated morphology at a nanometer scale.Peer reviewe
Single-particle nonlocality and entanglement with the vacuum
We propose a single-particle experiment that is equivalent to the
conventional two-particle experiment used to demonstrate a violation of Bell's
inequalities. Hence, we argue that quantum mechanical nonlocality can be
demonstrated by single-particle states. The validity of such a claim has been
discussed in the literature, but without reaching a clear consensus. We show
that the disagreement can be traced to what part of the total state of the
experiment one assigns to the (macroscopic) measurement apparatus. However,
with a conventional and legitimate interpretation of the measurement process
one is led to the conclusion that even a single particle can show nonlocal
properties.Comment: 6 pages, 5 figure
Single Photons on Pseudo-Demand from Stored Parametric Down-Conversion
We describe the results of a parametric down-conversion experiment in which
the detection of one photon of a pair causes the other photon to be switched
into a storage loop. The stored photon can then be switched out of the loop at
a later time chosen by the user, providing a single photon for potential use in
a variety of quantum information processing applications. Although the stored
single photon is only available at periodic time intervals, those times can be
chosen to match the cycle time of a quantum computer by using pulsed
down-conversion. The potential use of the storage loop as a photonic quantum
memory device is also discussed.Comment: 8 pages, 7 Figs., RevTe
3D printed microchannels for sub-nL NMR spectroscopy
Nuclear magnetic resonance (NMR) experiments on subnanoliter (sub-nL) volumes are hindered by the limited sensitivity of the detector and the difficulties in positioning and holding such small samples in proximity of the detector. In this work, we report on NMR experiments on liquid and biological entities immersed in liquids having volumes down to 100 pL. These measurements are enabled by the fabrication of high spatial resolution 3D printed microfluidic structures, specifically conceived to guide and confine sub-nL samples in the sub-nL most sensitive volume of a single-chip integrated NMR probe. The microfluidic structures are fabricated using a two-photon polymerization 3D printing technique having a resolution better than 1 \u3bcm3. The high spatial resolution 3D printing approach adopted here allows to rapidly fabricate complex microfluidic structures tailored to position, hold, and feed biological samples, with a design that maximizes the NMR signals amplitude and minimizes the static magnetic field inhomogeneities. The layer separating the sample from the microcoil, crucial to exploit the volume of maximum sensitivity of the detector, has a thickness of 10 \u3bcm. To demonstrate the potential of this approach, we report NMR experiments on sub-nL intact biological entities in liquid media, specifically ova of the tardigrade Richtersius coronifer and sections of Caenorhabditis elegans nematodes. We show a sensitivity of 2.5x1013spins/ Hz1/2on1H nuclei at 7 T, sufficient to detect 6 pmol of1H nuclei of endogenous compounds in active volumes down to 100 pL and in a measurement time of 3 hours. Spectral resolutions of 0.01 ppm in liquid samples and of 0.1 ppm in the investigated biological entities are also demonstrated. The obtained results may indicate a route for NMR studies at the single unit level of important biological entities having sub-nL volumes, such as living microscopic organisms and eggs of several mammalians, humans included
Suitability versus fidelity for rating single-photon guns
The creation of specified quantum states is important for most, if not all,
applications in quantum computation and communication. The quality of the state
preparation is therefore an essential ingredient in any assessment of a
quantum-state gun. We show that the fidelity, under the standard definitions is
not sufficient to assess quantum sources, and we propose a new measure of
suitability that necessarily depends on the application for the source. We
consider the performance of single-photon guns in the context of quantum key
distribution (QKD) and linear optical quantum computation. Single-photon
sources for QKD need radically different properties than sources for quantum
computing. Furthermore, the suitability for single-photon guns is discussed
explicitly in terms of experimentally accessible criteria.Comment: 4 pages, 2 figures Revised per referee suggestion
Self-interference of a single Bose-Einstein condensate due to boundary effects
A simple model wavefunction, consisting of a linear combination of two
free-particle Gaussians, describes many of the observed features seen in the
interactions of two isolated Bose-Einstein condensates as they expand, overlap,
and interfere. We show that a simple extension of this idea can be used to
predict the qualitative time-development of a single expanding BEC condensate
produced near an infinite wall boundary, giving similar interference phenomena.
We also briefly discuss other possible time-dependent behaviors of single BEC
condensates in restricted geometries,such as wave packet revivals.Comment: 8 pages, no figures, to appear in Physica Script
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