6,968 research outputs found
Receptor-phosphoinositidase C coupling Multiple G-proteins?
AbstractRecent evidence has suggested that receptor-mediated phosphoinositide turnover, like that of the adenylate cyclase cAMP pathway, is regulated by guanine nucleotides. It is likely that one or more guanine nucleotide-binding proteins (G-proteins) couple calcium-mobilizing receptors to the activation of phosphoinositidase C. Recent studies utilizing various bacterial toxins have strongly suggested the presence of multiple G-proteins in the regulation of receptor-phosphoinositidase C coupling in a variety of cell types
Single photon continuous variable quantum key distribution based on energy-time uncertainty relation
In previous quantum key distribution (QKD) protocols, information is encoded
on either the discrete-variable of single-photon signal or continuous-variables
of multi-photon signal. Here, we propose a new QKD protocol by encoding
information on continuous-variables of a single photon. In this protocol, Alice
randomly encodes her information on either the central frequency of a
narrow-band single photon pulse or the time-delay of a broadband single photon
pulse, while Bob randomly chooses to do either frequency measurement or time
measurement. The security of this protocol rests on the energy-time uncertainty
relation, which prevents Eve from simultaneously determining both frequency and
time information with arbitrarily high resolution. In practice, this scheme may
be more robust against various channel noises, such as polarization and phase
fluctuations.Comment: 4 pages, 3 figure
New, efficient and robust, fiber-based quantum key distribution schemes
We present a new fiber based quantum key distribution (QKD) scheme which can
be regarded as a modification of an idea proposed by Inoue, Waks and Yamamoto
(IWY) [1]. The scheme described here uses a single phase modulator and two
differential delay elements in series at the transmitter that form an
interferometer when combined with a third differential delay element at the
receiver. The protocol is characterized by a high efficiency, reduced exposure
to an attack by an eavesdropper, and higher sensitivity to such an attack when
compared to other QKD schemes. For example, the efficiency with which
transmitted data contribute to the private key is 3/4 compared with 1/4 for
BB84 [2]. Moreover, an eavesdropper can aquire a maximum of 1/3 of the key
which leads to an error probability in the private key of 1/3. This can be
compared to 1/2 and 1/4 for these same parameters in both BB84 and IWY. The
combination of these considerations should lead to increased range and key
distribution rate over present fiber-based QKD schemes.Comment: 4 pages, 5 figures, 1 equatio
Passive decoy state quantum key distribution with practical light sources
Decoy states have been proven to be a very useful method for significantly
enhancing the performance of quantum key distribution systems with practical
light sources. While active modulation of the intensity of the laser pulses is
an effective way of preparing decoy states in principle, in practice passive
preparation might be desirable in some scenarios. Typical passive schemes
involve parametric down-conversion. More recently, it has been shown that phase
randomized weak coherent pulses (WCP) can also be used for the same purpose [M.
Curty {\it et al.}, Opt. Lett. {\bf 34}, 3238 (2009).] This proposal requires
only linear optics together with a simple threshold photon detector, which
shows the practical feasibility of the method. Most importantly, the resulting
secret key rate is comparable to the one delivered by an active decoy state
setup with an infinite number of decoy settings. In this paper we extend these
results, now showing specifically the analysis for other practical scenarios
with different light sources and photo-detectors. In particular, we consider
sources emitting thermal states, phase randomized WCP, and strong coherent
light in combination with several types of photo-detectors, like, for instance,
threshold photon detectors, photon number resolving detectors, and classical
photo-detectors. Our analysis includes as well the effect that detection
inefficiencies and noise in the form of dark counts shown by current threshold
detectors might have on the final secret ket rate. Moreover, we provide
estimations on the effects that statistical fluctuations due to a finite data
size can have in practical implementations.Comment: 17 pages, 14 figure
Exposed-key weakness of
The protocol given by Barbosa \emph{et al.}, PRL 90, 227901
(2003) claims to be a secure way of encrypting messages using mesoscopic
coherent states. We show that transmission under exposes
information about the secret key to an eavesdropper, and we estimate the rate
at which an eavesdropper can learn about the key. We also consider the
consequences of using further randomization to protect the key and how our
analysis applies to this case. We conclude that is not
informationally secure.Comment: 6 pg. Was originally written in May 2006 and has languished in
getting-approved-land for 7 months, but we've tried to keep current with
papers published since then. This version changed for publicatio
Rayleigh-Taylor Instabilities in Type Ia Supernova Remnants undergoing Cosmic-Ray Particle Acceleration - Low Adiabatic Index Solutions
This study investigates the evolution of Rayleigh-Taylor (R-T) instabilities
in Type Ia supernova remnants that are associated with a low adiabatic index
gamma, where gamma < 5/3, which reflects the expected change in the supernova
shock structure as a result of cosmic-ray particle acceleration. Extreme cases,
such as the case with the maximum compression ratio that corresponds to
gamma=1.1, are examined. As gamma decreases, the shock compression ratio rises,
and an increasingly narrow inter shock region with a more pronounced initial
mixture of R-T unstable gas is produced. Consequently, the remnant outline may
be perturbed by small-amplitude, small-wavelength bumps. However, as the
instability decays over time, the extent of convective mixing in terms of the
ratio of the radius of the R-T fingers to the blast wave does not strongly
depend on the value of gamma for gamma >= 1.2. As a result of the age of the
remnant, the unstable gas cannot extend sufficiently far to form metal-enriched
filaments of ejecta material close to the periphery of Tycho's supernova
remnant. The consistency of the dynamic properties of Tycho's remnant with the
adiabatic model gamma=5/3 reveals that the injection of cosmic rays is too weak
to alter the shock structure. Even with very efficient acceleration of cosmic
rays at the shock, significantly enhanced mixing is not expected in Type Ia
supernova remnants.Comment: 11 pages, 10 figures, MNRAS, accepte
Analysis of the incompressibility constraint in the Smoothed Particle Hydrodynamics method
Smoothed particle hydrodynamics is a particle-based, fully Lagrangian, method
for fluid-flow simulations. In this work, fundamental concepts of the method
are first briefly recalled. Then, we present a thorough comparison of three
different incompressibility treatments in SPH: the weakly compressible
approach, where a suitably-chosen equation of state is used; and two truly
incompressible methods, where the velocity field projection onto a
divergence-free space is performed. A noteworthy aspect of the study is that,
in each incompressibility treatment, the same boundary conditions are used (and
further developed) which allows a direct comparison to be made. Problems
associated with implementation are also discussed and an optimal choice of the
computational parameters has been proposed and verified. Numerical results show
that the present state-of-the-art truly incompressible method (based on a
velocity correction) suffer from density accumulation errors. To address this
issue, an algorithm, based on a correction for both particle velocities and
positions, is presented. The usefulness of this density correction is examined
and demonstrated in the last part of the paper
Educational action research as the quest for virtue in teaching
Curriculum aims often remain unrealised aspirations. This is because the values and principles implicit in them fail to get articulated in forms that can effectively inform and guide the practice of teaching. Ideas such as ‘learner-centred education’, ‘independent/autonomous learning’, ‘self-directed learning', ‘enquiry/discovery learning’, ‘collaborative learning’, ‘active learning’ and ‘learning with understanding’ refer to critical aspects of the learning process rather than its outcomes. While often enthusiastically embraced by teachers, they rarely get realised in appropriate forms of virtuous action. Such is the power of an outcomes-based model of teaching and learning to shape the practice of teaching. This paper cites examples of curriculum design that specify the pedagogical values and principles implicit in various educational aims, and shows how they can provide a basis for practical experiments by teachers in their classrooms and schools, in a quest to transform their teaching into concrete forms of virtuous action. Indeed, the paper depicts a number of actual action research projects in which teachers generated some common insights into how to transform their teaching into the practice of virtue in education. It also explores the role of theory-informed action research in developing teaching as a virtuous form of action
Eavesdropper's Optimal Information in Variations of Bennett-Brassard 1984 Quantum Key Distribution in the Coherent Attacks
We calculate eavesdropper's optimal information on raw bits in
Bennett-Brassard 1984 quantum key distribution (BB84 QKD) and six-state scheme
in coherent attacks, using a formula by Lo and Chau [Science 283 (1999) 2050]
with single photon assumption. We find that eavesdropper's optimal information
in QKD without public announcement of bases [Phys. Lett. A 244 (1998) 489] is
the same as that of a corresponding QKD WITH it in the coherent attack. We
observe a sum-rule concerning each party's information.Comment: no correction, 7 pages, 1 figure, RevTe
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