760 research outputs found
Magneto-optics in pure and defective Ga_{1-x}Mn_xAs from first-principles
The magneto-optical properties of GaMnAs including their most
common defects were investigated with precise first--principles
density-functional FLAPW calculations in order to: {\em i}) elucidate the
origin of the features in the Kerr spectra in terms of the underlying
electronic structure; {\em ii}) perform an accurate comparison with
experiments; and {\em iii}) understand the role of the Mn concentration and
occupied sites in shaping the spectra. In the substitutional case, our results
show that most of the features have an interband origin and are only slightly
affected by Drude--like contributions, even at low photon energies. While not
strongly affected by the Mn concentration for the intermediately diluted range
( 10%), the Kerr factor shows a marked minimum (up to 1.5) occurring
at a photon energy of 0.5 eV. For interstitial Mn, the calculated
results bear a striking resemblance to the experimental spectra, pointing to
the comparison between simulated and experimental Kerr angles as a valid tool
to distinguish different defects in the diluted magnetic semiconductors
framework.Comment: 10 pages including 2 figures, submitted to Phys. Rev.
Cbl-Associated Protein CAP contributes to correct formation and robust function of the Drosophila heart tube
The formation of a tube-like structure is a basic step in the making of functional hearts in vertebrates and invertebrates and therefore, its understanding provides important information on heart development and function. In Drosophila, the cardiac tube originates from two bilateral rows of dorsally migrating cells. On meeting at the dorsal midline, coordinated changes in cell shape and adhesive properties transform the two sheets of cells into a linear tube. ECM and transmembrane proteins linked to the cytoskeleton play an important role during these dynamic processes. Here we characterize the requirement of Cbl-Associated Protein (CAP) in Drosophila heart formation. In embryos, CAP is expressed in late migrating cardioblasts and is located preferentially at their luminal and abluminal periphery. CAP mutations result in irregular cardioblast alignment and imprecisely controlled cardioblast numbers. Furthermore, CAP mutant embryos show a strongly reduced heart lumen and an aberrant shape of lumen forming cardioblasts. Analysis of double heterozygous animals reveals a genetic interaction of CAP with Integrin- and Talin-encoding genes. In post-embryonic stages, CAP closely colocalizes with Integrin near Z-bands and at cell-cell contact sites. CAP mutants exhibit a reduced contractility in larval hearts and show a locally disrupted morphology, which correlates with a reduced pumping efficiency. Our observations imply a function of CAP in linking Integrin signaling with the actin cytoskeleton. As a modulator of the cytoskeleton, CAP is involved in the establishment of proper cell shapes during cardioblast alignment and cardiac lumen formation in the Drosophila embryo. Furthermore, CAP is required for correct heart function throughout development
Multi-pulse addressing of a Raman quantum memory: configurable beam splitting and efficient readout
Quantum memories are vital to the scalability of photonic quantum information
processing (PQIP), since the storage of photons enables repeat-until-success
strategies. On the other hand the key element of all PQIP architectures is the
beam splitter, which allows to coherently couple optical modes. Here we show
how to combine these crucial functionalities by addressing a Raman quantum
memory with multiple control pulses. The result is a coherent optical storage
device with an extremely large time-bandwidth product, that functions as an
array of dynamically configurable beam splitters, and that can be read out with
arbitrarily high efficiency. Networks of such devices would allow fully
scalable PQIP, with applications in quantum computation, long-distance quantum
communications and quantum metrology.Comment: 4 pages, 3 figure
Multimode Memories in Atomic Ensembles
The ability to store multiple optical modes in a quantum memory allows for
increased efficiency of quantum communication and computation. Here we compute
the multimode capacity of a variety of quantum memory protocols based on light
storage in ensembles of atoms. We find that adding a controlled inhomogeneous
broadening improves this capacity significantly.Comment: Published version. Many thanks are due to Christoph Simon for his
help and suggestions. (This acknowledgement is missing from the final draft:
apologies!
Efficient spatially-resolved multimode quantum memory
We propose a method that enables efficient storage and retrieval of a
photonic excitation stored in an ensemble quantum memory consisting of
Lambda-type absorbers with non-zero Stokes shift. We show that this can be used
to implement a multimode quantum memory storing multiple frequency-encoded
qubits in a single ensemble, and allowing their selective retrieval. The
read-out scheme applies to memory setups based on both
electromagnetically-induced transparency and stimulated Raman scattering, and
spatially separates the output signal field from the control fields
Regulation of releasable vesicle pool sizes by protein kinase A-dependent phosphorylation of SNAP-25
AbstractProtein kinase A (PKA) is a key regulator of neurosecretion, but the molecular targets remain elusive. We combined pharmacological manipulations of kinase and phosphatase activities with mutational studies on the exocytotic machinery driving fusion of catecholamine-containing vesicles from chromaffin cells. We found that constitutive PKA activity was necessary to maintain a large number of vesicles in the release-ready, so-called primed, state, whereas calcineurin (protein phosphatase 2B) activity antagonized this effect. Overexpression of the SNARE protein SNAP-25a mutated in a PKA phosphorylation site (Thr-138) eliminated the effect of PKA inhibitors on the vesicle priming process. Another, unidentified, PKA target regulated the relative size of two different primed vesicle pools that are distinguished by their release kinetics. Overexpression of the SNAP-25b isoform increased the size of both primed vesicle pools by a factor of two, and mutations in the conserved Thr-138 site had similar effects as in the a isoform
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