198,973 research outputs found
Spectral universality of strong shocks accelerating charged particles
As a rule, the shock compression controls the spectrum of diffusively
accelerated particles. We argue that this is not so if the backreaction of
these particles on the shock structure is significant. We present a
self-similar solution in which the accelerated particles change the flow
structure near the shock so strongly that the total shock compression may
become arbitrarily large. Despite this, the energy spectrum behind the shock is
close to E^{-3/2} independently of anything at all.Comment: Submitted to ApJL, 4 pages, 1 figure, uses revtex and boxedep
Information content in Gaussian noise: optimal compression rates
We approach the theoretical problem of compressing a signal dominated by
Gaussian noise. We present expressions for the compression ratio which can be
reached, under the light of Shannon's noiseless coding theorem, for a linearly
quantized stochastic Gaussian signal (noise). The compression ratio decreases
logarithmically with the amplitude of the frequency spectrum of the
noise. Entropy values and compression rates are shown to depend on the shape of
this power spectrum, given different normalizations. The cases of white noise
(w.n.), power-law noise ---including noise---, (w.n.)
noise, and piecewise (w.n.+ w.n.) noise are discussed, while
quantitative behaviours and useful approximations are provided.Comment: 28 LateX pages and 6 Fig, replaced with minor changes to match
published versio
Linear Redshift Distortions and Power in the PSCz Survey
We present a state-of-the-art linear redshift distortion analysis of the
recently published IRAS Point Source Catalog Redshift Survey (PSCz). The
procedure involves linear compression into 4096 Karhunen-Loeve modes culled
from a potential pool of about 3 x 10^5 modes, followed by quadratic
compression into three separate power spectra, the galaxy-galaxy,
galaxy-velocity, and velocity-velocity power spectra. Least squares fitting to
the decorrelated power spectra yields a linear redshift distortion parameter
beta = Omega_m^0.6/b = 0.41(+0.13,-0.12).Comment: Minor changes to agree with accepted version. Slight changes to power
spectrum, including one more point added at large scales, from binning points
formerly discarded as too noisy. 5 pages, including 4 embedded PostScript
figures. Accepted for publication in MNRAS Letters (pink pages). Power
spectrum data available at http://casa.colorado.edu/~ajsh/pscz
Spectrum of the free rod under tension and compression
In this paper, we study the spectrum of the one-dimensional vibrating free
rod equation under tension or compression
. The eigenvalues as functions of the tension/compression
parameter exhibit three distinct types of behavior. In particular,
eigenvalue branches in the lower half-plane exhibit a cascading pattern of
barely-avoided crossings.
We provide a complete description of the eigenfunctions and eigenvalues by
implicitly parameterizing the eigenvalue curves. We also establish properties
of the eigenvalue curves such as monotonicity, crossings, asymptotic growth,
cascading and phantom spectral lines.Comment: 38 pages with 17 figure
Modulational-instability-free pulse compression in anti-resonant hollow-core photonic crystal fiber
Gas-filled hollow-core photonic crystal fiber (PCF) is used for efficient
nonlinear temporal compression of femtosecond laser pulses, two main schemes
being direct soliton-effect self-compression, and spectral broadening followed
by phase compensation. To obtain stable compressed pulses, it is crucial to
avoid decoherence through modulational instability (MI) during spectral
broadening. Here we show that changes in dispersion due to spectral
anti-crossings between the fundamental core mode and core wall resonances in
anti-resonant-guiding hollow-core PCF can strongly alter the MI gain spectrum,
enabling MI-free pulse compression for optimized fiber designs. In addition,
higher-order dispersion can introduce MI even when the pump pulses lie in the
normal dispersion region
Efficient Quantum Compression for Ensembles of Identically Prepared Mixed States
We present one-shot compression protocols that optimally encode ensembles of
identically prepared mixed states into qubits. In contrast to
the case of pure-state ensembles, we find that the number of encoding qubits
drops down discontinuously as soon as a nonzero error is tolerated and the
spectrum of the states is known with sufficient precision. For qubit ensembles,
this feature leads to a 25% saving of memory space. Our compression protocols
can be implemented efficiently on a quantum computer.Comment: 5+19 pages, 2 figures. Published versio
- …