8,923 research outputs found
Geometry of intensive scalar dissipation events in turbulence
Maxima of the scalar dissipation rate in turbulence appear in form of sheets
and correspond to the potentially most intensive scalar mixing events. Their
cross-section extension determines a locally varying diffusion scale of the
mixing process and extends the classical Batchelor picture of one mean
diffusion scale. The distribution of the local diffusion scales is analysed for
different Reynolds and Schmidt numbers with a fast multiscale technique applied
to very high-resolution simulation data. The scales take always values across
the whole Batchelor range and beyond. Furthermore, their distribution is traced
back to the distribution of the contractive short-time Lyapunov exponent of the
flow.Comment: 4 pages, 5 Postscript figures (2 with reduced quality
Large optical gain from four-wave mixing instabilities in semiconductor quantum wells
Based on a microscopic many-particle theory, we predict large optical gain in
the probe and background-free four-wave mixing directions caused by excitonic
instabilities in semiconductor quantum wells. For a single quantum well with
radiative-decay limited dephasing in a typical pump-probe setup we discuss the
microscopic driving mechanisms and polarization and frequency dependence of
these instabilities
Directional optical switching and transistor functionality using optical parametric oscillation in a spinor polariton fluid
Over the past decade, spontaneously emerging patterns in the density of
polaritons in semiconductor microcavities were found to be a promising
candidate for all-optical switching. But recent approaches were mostly
restricted to scalar fields, did not benefit from the polariton's unique
spin-dependent properties, and utilized switching based on hexagon far-field
patterns with 60{\deg} beam switching (i.e. in the far field the beam
propagation direction is switched by 60{\deg}). Since hexagon far-field
patterns are challenging, we present here an approach for a linearly polarized
spinor field, that allows for a transistor-like (e.g., crucial for
cascadability) orthogonal beam switching, i.e. in the far field the beam is
switched by 90{\deg}. We show that switching specifications such as
amplification and speed can be adjusted using only optical means
Molecular beam epitaxy of high structural quality Bi2Se3 on lattice matched InP(111) substrates
Epitaxial layers of the topological insulator Bi2Se3 have been grown by
molecular beam epitaxy on laterally lattice-matched InP(111)B substrates. High
resolution X-ray diffraction shows a significant improvement of Bi2Se3 crystal
quality compared to layers deposited on other substrates. The measured full
width at half maximum of the rocking curve is Delta omega=13 arcsec, and the
(omega-2theta) scans exhibit clear layer thickness fringes. Atomic force
microscope images show triangular twin domains with sizes increasing with layer
thickness. The structural quality of the domains is confirmed on the
microscopic level by transmission electron microscopy.Comment: 4 pages, 4 figure
Entanglement transmission and generation under channel uncertainty: Universal quantum channel coding
We determine the optimal rates of universal quantum codes for entanglement
transmission and generation under channel uncertainty. In the simplest scenario
the sender and receiver are provided merely with the information that the
channel they use belongs to a given set of channels, so that they are forced to
use quantum codes that are reliable for the whole set of channels. This is
precisely the quantum analog of the compound channel coding problem. We
determine the entanglement transmission and entanglement-generating capacities
of compound quantum channels and show that they are equal. Moreover, we
investigate two variants of that basic scenario, namely the cases of informed
decoder or informed encoder, and derive corresponding capacity results.Comment: 45 pages, no figures. Section 6.2 rewritten due to an error in
equation (72) of the old version. Added table of contents, added section
'Conclusions and further remarks'. Accepted for publication in
'Communications in Mathematical Physics
The quantum capacity is properly defined without encodings
We show that no source encoding is needed in the definition of the capacity
of a quantum channel for carrying quantum information. This allows us to use
the coherent information maximized over all sources and and block sizes, but
not encodings, to bound the quantum capacity. We perform an explicit
calculation of this maximum coherent information for the quantum erasure
channel and apply the bound in order find the erasure channel's capacity
without relying on an unproven assumption as in an earlier paper.Comment: 19 pages revtex with two eps figures. Submitted to Phys. Rev. A.
Replaced with revised and simplified version, and improved references, etc.
Why can't the last line of the comments field end with a period using this
web submission form
Renormalization Group and Quantum Information
The renormalization group is a tool that allows one to obtain a reduced
description of systems with many degrees of freedom while preserving the
relevant features. In the case of quantum systems, in particular,
one-dimensional systems defined on a chain, an optimal formulation is given by
White's "density matrix renormalization group". This formulation can be shown
to rely on concepts of the developing theory of quantum information.
Furthermore, White's algorithm can be connected with a peculiar type of
quantization, namely, angular quantization. This type of quantization arose in
connection with quantum gravity problems, in particular, the Unruh effect in
the problem of black-hole entropy and Hawking radiation. This connection
highlights the importance of quantum system boundaries, regarding the
concentration of quantum states on them, and helps us to understand the optimal
nature of White's algorithm.Comment: 16 pages, 5 figures, accepted in Journal of Physics
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