475 research outputs found
First-principles calculation of intrinsic defect formation volumes in silicon
We present an extensive first-principles study of the pressure dependence of
the formation enthalpies of all the know vacancy and self-interstitial
configurations in silicon, in each charge state from -2 through +2. The neutral
vacancy is found to have a formation volume that varies markedly with pressure,
leading to a remarkably large negative value (-0.68 atomic volumes) for the
zero-pressure formation volume of a Frenkel pair (V + I). The interaction of
volume and charge was examined, leading to pressure--Fermi level stability
diagrams of the defects. Finally, we quantify the anisotropic nature of the
lattice relaxation around the neutral defects.Comment: 9 pages, 9 figure
Optical properties of metallic (III,Mn)V ferromagnetic semiconductors in the infrared to visible range
We report on a study of the ac conductivity and magneto-optical properties of
metallic ferromagnetic (III,Mn)V semiconductors in the infrared to visible
spectrum. Our analysis is based on the successful kinetic exchange model for
(III,Mn)V ferromagnetic semiconductors. We perform the calculations within the
Kubo formalism and treat the disorder effects pertubatively within the Born
approximation, valid for the metallic regime. We consider an eight-band
Kohn-Luttinger model (six valence bands plus two conduction bands) as well as a
ten-band model with additional dispersionless bands simulating
phenomenologically the upper-mid-gap states induced by antisite and
interstitial impurities. These models qualitatively account for
optical-absorption experiments and predict new features in the mid-infrared
Kerr angle and magnetic-circular-dichroism properties as a function of Mn
concentration and free carrier density.Comment: 10 pages, 7 figures, some typos correcte
Entanglement over global distances via quantum repeaters with satellite links
We study entanglement creation over global distances based on a quantum
repeater architecture that uses low-earth orbit satellites equipped with
entangled photon sources, as well as ground stations equipped with quantum
non-demolition detectors and quantum memories. We show that this approach
allows entanglement creation at viable rates over distances that are
inaccessible via direct transmission through optical fibers or even from very
distant satellites.Comment: 5+3 pages, 3+2 figure
Studying Free-Space Transmission Statistics and Improving Free-Space QKD in the Turbulent Atmosphere
The statistical fluctuations in free-space links in the turbulent atmosphere
are important for the distribution of quantum signals. To that end, we first
study statistics generated by the turbulent atmosphere in an entanglement based
free-space quantum key distribution (QKD) system. Using the insights gained
from this analysis, we study the effect of link fluctuations on the security
and key generation rate of decoy state QKD concluding that it has minimal
effect in the typical operating regimes. We then investigate the novel idea of
using these turbulent fluctuations to our advantage in QKD experiments. We
implement a signal-to-noise ratio filter (SNRF) in our QKD system which rejects
measurements during periods of low transmission efficiency, where the measured
quantum bit error rate (QBER) is temporarily elevated. Using this, we increase
the total secret key generated by the system from 78,009 bits to 97,678 bits,
representing an increase of 25.2% in the final secure key rate, generated from
the same raw signals. Lastly, we present simulations of a QKD exchange with an
orbiting LEO satellite and show that an SNRF will be extremely useful in such a
situation, allowing many more passes to extract a secret key than would
otherwise be possible.Comment: 9 pages, 6 figure
Protein kinase CĪ¶ regulates phospholipase D activity in rat-1 fibroblasts expressing the Ī±(1A )adrenergic receptor
BACKGROUND: Phenylephrine (PHE), an Ī±(1 )adrenergic receptor agonist, increases phospholipase D (PLD) activity, independent of classical and novel protein kinase C (PKC) isoforms, in rat-1 fibroblasts expressing Ī±(1A )adrenergic receptors. The aim of this study was to determine the contribution of atypical PKCĪ¶ to PLD activation in response to PHE in these cells. RESULTS: PHE stimulated a PLD activity as demonstrated by phosphatidylethanol production. PHE increased PKCĪ¶ translocation to the particulate cell fraction in parallel with a time-dependent decrease in its activity. PKCĪ¶ activity was reduced at 2 and 5 min and returned to a sub-basal level within 10ā15 min. Ectopic expression of kinase-dead PKCĪ¶, but not constitutively active PKCĪ¶, potentiated PLD activation elicited by PHE. A cell-permeable pseudosubstrate inhibitor of PKCĪ¶ reduced basal PKCĪ¶ activity and abolished PHE-induced PLD activation. CONCLUSION: Ī±(1A )adrenergic receptor stimulation promotes the activation of a PLD activity by a mechanism dependent on PKCĪ¶; Our data also suggest that catalytic activation of PKCĪ¶ is not required for PLD stimulation
ARFGAP1 promotes the formation of COPI vesicles, suggesting function as a component of the coat
The role of GTPase-activating protein (GAP) that deactivates ADP-ribosylation factor 1 (ARF1) during the formation of coat protein I (COPI) vesicles has been unclear. GAP is originally thought to antagonize vesicle formation by triggering uncoating, but later studies suggest that GAP promotes cargo sorting, a process that occurs during vesicle formation. Recent models have attempted to reconcile these seemingly contradictory roles by suggesting that cargo proteins suppress GAP activity during vesicle formation, but whether GAP truly antagonizes coat recruitment in this process has not been assessed directly. We have reconstituted the formation of COPI vesicles by incubating Golgi membrane with purified soluble components, and find that ARFGAP1 in the presence of GTP promotes vesicle formation and cargo sorting. Moreover, the presence of GTPĪ³S not only blocks vesicle uncoating but also vesicle formation by preventing the proper recruitment of GAP to nascent vesicles. Elucidating how GAP functions in vesicle formation, we find that the level of GAP on the reconstituted vesicles is at least as abundant as COPI and that GAP binds directly to the dilysine motif of cargo proteins. Collectively, these findings suggest that ARFGAP1 promotes vesicle formation by functioning as a component of the COPI coat
Eavesdropping and countermeasures for backflash side channel in quantum cryptography
Quantum key distribution (QKD) promises information theoretic secure key as
long as the device performs as assumed in the theoretical model. One of the
assumptions is an absence of information leakage about individual photon
detection outcomes of the receiver unit. Here we investigate the information
leakage from a QKD receiver due to photon emission caused by detection events
in single-photon detectors (backflash). We test commercial silicon avalanche
photodiodes and a photomultiplier tube, and find that the former emit
backflashes. We study the spectral, timing and polarization characteristics of
these backflash photons. We experimentally demonstrate on a free-space QKD
receiver that an eavesdropper can distinguish which detector has clicked inside
it, and thus acquire secret information. A set of countermeasures both in
theory and on the physical devices are discussed.Comment: 9 pages, 7 figure
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